ATmega48A/PA/88A/PA/168A/PA/328/P

20. USART0

20.1 Features
• Full Duplex Operation (Independent Serial Receive and Transmit Registers)
• Asynchronous or Synchronous Operation
• Master or Slave Clocked Synchronous Operation
• High Resolution Baud Rate Generator
• Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits
• Odd or Even Parity Generation and Parity Check Supported by Hardware
• Data OverRun Detection
• Framing Error Detection
• Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter
• Three Separate Interrupts on TX Complete, TX Data Register Empty and RX Complete
• Multi-processor Communication Mode
• Double Speed Asynchronous Communication Mode

20.2 Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly flexible
serial communication device.
The USART0 can also be used in Master SPI mode, see “USART in SPI Mode” on page 205. The Power
Reduction USART bit, PRUSART0, in ”Minimizing Power Consumption” on page 51 must be disabled by writing
a logical zero to it.
A simplified block diagram of the USART Transmitter is shown in Figure 20-1 on page 180. CPU accessible I/O
Registers and I/O pins are shown in bold.
The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top): Clock
Generator, Transmitter and Receiver. Control Registers are shared by all units. The Clock Generation logic
consists of synchronization logic for external clock input used by synchronous slave operation, and the baud
rate generator. The XCKn (Transfer Clock) pin is only used by synchronous transfer mode. The Transmitter
consists of a single write buffer, a serial Shift Register, Parity Generator and Control logic for handling different
serial frame formats. The write buffer allows a continuous transfer of data without any delay between frames.
The Receiver is the most complex part of the USART module due to its clock and data recovery units. The
recovery units are used for asynchronous data reception. In addition to the recovery units, the Receiver includes
a Parity Checker, Control logic, a Shift Register and a two level receive buffer (UDRn). The Receiver supports
the same frame formats as the Transmitter, and can detect Frame Error, Data OverRun and Parity Errors.

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Figure 20-1. USART Block Diagram(1)

Clock Generator

UBRRn [H:L]
OSC

Transfer Clock
BAUD RATE GENERATOR

SYNC LOGIC PIN

XCKn
CONTROL

Transmitter
TX
UDRn(Transmit)
CONTROL
PARITY
GENERATOR
DATA BUS

PIN
TRANSMIT SHIFT REGISTER TxDn
CONTROL

Receiver
CLOCK RX
RECOVERY CONTROL

DATA PIN
RECEIVE SHIFT REGISTER RxDn
RECOVERY CONTROL

PARITY
UDRn (Receive)
CHECKER

UCSRnA UCSRnB UCSRnC

Note: 1. Refer to Figure 1-1 on page 12 and Table 14-9 on page 97 for USART0 pin placement.

20.3 Clock Generation

The Clock Generation logic generates the base clock for the Transmitter and Receiver. The USART supports
four modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master synchronous and
Slave synchronous mode. The UMSELn bit in USART Control and Status Register C (UCSRnC) selects
between asynchronous and synchronous operation. Double Speed (asynchronous mode only) is controlled by
the U2Xn found in the UCSRnA Register. When using synchronous mode (UMSELn = 1), the Data Direction
Register for the XCKn pin (DDR_XCKn) controls whether the clock source is internal (Master mode) or external
(Slave mode). The XCKn pin is only active when using synchronous mode.

UCSRC P202

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Figure 20-2 shows a block diagram of the clock generation logic.

Figure 20-2. Clock Generation Logic, Block Diagram

UBRRn
U2Xn
foscn

Prescaling UBRRn+1
/2 /4 /2
Down-Counter 0
1
0
OSC txclk
1
DDR_XCKn

Sync Edge
xcki Register Detector 0
XCKn UMSELn
xcko 1
Pin

DDR_XCKn UCPOLn 1
rxclk
0

Signal description:
txclk Transmitter clock (Internal Signal).
rxclk Receiver base clock (Internal Signal).
xcki Input from XCK pin (internal Signal). Used for synchronous slave operation.
xcko Clock output to XCK pin (Internal Signal). Used for synchronous master
operation.
fosc System clock frequency.

20.3.1 Internal Clock Generation – The Baud Rate Generator

Internal clock generation is used for the asynchronous and the synchronous master modes of operation. The
description in this section refers to Figure 20-2.
The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a programmable
prescaler or baud rate generator. The down-counter, running at system clock (fosc), is loaded with the UBRRn
value each time the counter has counted down to zero or when the UBRRnL Register is written. A clock is
generated each time the counter reaches zero. This clock is the baud rate generator clock output (=
fosc/(UBRRn+1)). The Transmitter divides the baud rate generator clock output by 2, 8 or 16 depending on
mode. The baud rate generator output is used directly by the Receiver’s clock and data recovery units.
However, the recovery units use a state machine that uses 2, 8 or 16 states depending on mode set by the state
of the UMSELn, U2Xn and DDR_XCKn bits.

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Table 20-1 contains equations for calculating the baud rate (in bits per second) and for calculating the UBRRn
value for each mode of operation using an internally generated clock source.

Table 20-1. Equations for Calculating Baud Rate Register Setting

Equation for Calculating Baud Equation for Calculating
Operating Mode Rate(1) UBRRn Value

Asynchronous Normal mode f OSC f OSC EX 196

(U2Xn = 0) BAUD = ------------------------------------------ UBRRn = ------------------------ – 1
16  UBRRn + 1  16BAUD

Asynchronous Double Speed f OSC f OSC

mode (U2Xn = 1) BAUD = --------------------------------------- UBRRn = -------------------- – 1
8  UBRRn + 1  8BAUD

f OSC f OSC
Synchronous Master mode BAUD = --------------------------------------- UBRRn = -------------------- – 1
2  UBRRn + 1  2BAUD

Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps)
BAUDBaud rate (in bits per second, bps)
fOSCSystem Oscillator clock frequency
UBRRnContents of the UBRRnH and UBRRnL Registers, (0-4095)
Some examples of UBRRn values for some system clock frequencies are found in Table 20-4 (see page 196).

20.3.2 Double Speed Operation (U2Xn)

The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has effect for the
asynchronous operation. Set this bit to zero when using synchronous operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer rate
for asynchronous communication. Note however that the Receiver will in this case only use half the number of
samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate baud rate
setting and system clock are required when this mode is used. For the Transmitter, there are no downsides.

20.3.3 External Clock

External clocking is used by the synchronous slave modes of operation. The description in this section refers to
Figure 20-2 for details.

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External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance of meta-
stability. The output from the synchronization register must then pass through an edge detector before it can be
used by the Transmitter and Receiver. This process introduces a two CPU clock period delay and therefore the
maximum external XCKn clock frequency is limited by the following equation:
f OSC
f XCK  -----------
4

Note that fosc depends on the stability of the system clock source. It is therefore recommended to add some
margin to avoid possible loss of data due to frequency variations.

20.3.4 Synchronous Clock Operation

When synchronous mode is used (UMSELn = 1), the XCKn pin will be used as either clock input (Slave) or clock
output (Master). The dependency between the clock edges and data sampling or data change is the same. The
basic principle is that data input (on RxDn) is sampled at the opposite XCKn clock edge of the edge the data
output (TxDn) is changed.

Figure 20-3. Synchronous Mode XCKn Timing.

UCPOL = 1 XCK

RxD / TxD

Sample

UCPOL = 0 XCK

RxD / TxD

Sample

The UCPOLn bit UCRSC selects which XCKn clock edge is used for data sampling and which is used for data
change. As Figure 20-3 shows, when UCPOLn is zero the data will be changed at rising XCKn edge and
sampled at falling XCKn edge. If UCPOLn is set, the data will be changed at falling XCKn edge and sampled at
rising XCKn edge.

20.4 Frame Formats

A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits), and
optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as valid frame
formats:
 1 start bit
 5, 6, 7, 8, or 9 data bits
 no, even or odd parity bit
 1 or 2 stop bits
A frame starts with the start bit followed by the least significant data bit. Then the next data bits, up to a total of
nine, are succeeding, ending with the most significant bit. If enabled, the parity bit is inserted after the data bits,
before the stop bits. When a complete frame is transmitted, it can be directly followed by a new frame, or the
communication line can be set to an idle (high) state. Figure 20-4 illustrates the possible combinations of the
frame formats. Bits inside brackets are optional.

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Figure 20-4. Frame Formats

FRAME

(IDLE) St 0 1 2 3 4 [5] [6] [7] [8] [P] Sp1 [Sp2] (St / IDLE)

St Start bit, always low.

(n) Data bits (0 to 8).
P Parity bit. Can be odd or even.
Sp Stop bit, always high.
IDLE No transfers on the communication line (RxDn or TxDn). An IDLE line must be
high.
The frame format used by the USART is set by the UCSZn2:0, UPMn1:0 and USBSn bits in UCSRnB and
UCSRnC. The Receiver and Transmitter use the same setting. Note that changing the setting of any of these
bits will corrupt all ongoing communication for both the Receiver and Transmitter.
The USART Character SiZe (UCSZn2:0) bits select the number of data bits in the frame. The USART Parity
mode (UPMn1:0) bits enable and set the type of parity bit. The selection between one or two stop bits is done by
the USART Stop Bit Select (USBSn) bit. The Receiver ignores the second stop bit. An FE (Frame Error) will
therefore only be detected in the cases where the first stop bit is zero.

20.4.1 Parity Bit Calculation

The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of the
exclusive or is inverted. The relation between the parity bit and data bits is as follows:

P even = d n – 1    d 3  d 2  d 1  d 0  0
P odd = d n – 1    d 3  d 2  d 1  d 0  1

Peven Parity bit using even parity

Podd Parity bit using odd parity
dn Data bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.

20.5 USART Initialization

The USART has to be initialized before any communication can take place. The initialization process normally
consists of setting the baud rate, setting frame format and enabling the Transmitter or the Receiver depending
on the usage. For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and interrupts
globally disabled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing
transmissions during the period the registers are changed. The TXCn Flag can be used to check that the
Transmitter has completed all transfers, and the RXC Flag can be used to check that there are no unread data
in the receive buffer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is
written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are equal
in functionality. The examples assume asynchronous operation using polling (no interrupts enabled) and a fixed

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frame format. The baud rate is given as a function parameter. For the assembly code, the baud rate parameter
is assumed to be stored in the r17:r16 Registers.

Assembly Code Example(1)

USART_Init:
; Set baud rate
out UBRRnH, r17
out UBRRnL, r16
; Enable receiver and transmitter
ldi r16, (1<<RXENn)|(1<<TXENn)
out UCSRnB,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<USBSn)|(3<<UCSZn0)
out UCSRnC,r16
ret
C Code Example(1)
#define FOSC 1843200 // Clock Speed
#define BAUD 9600
#define MYUBRR FOSC/16/BAUD-1
void main( void )
{
...
USART_Init(MYUBRR) UART_bigin(9600);
...
}
void USART_Init( unsigned int ubrr)
{
/*Set baud rate */
UBRR0H = (unsigned char)(ubrr>>8);
UBRR0L = (unsigned char)ubrr;
Enable receiver and transmitter */
UCSR0B = (1<<RXEN0)|(1<<TXEN0);
/* Set frame format: 8data, 2stop bit */
UCSR0C = (1<<USBS0)|(3<<UCSZ00);
}

Note: 1. See ”About Code Examples” on page 17.

More advanced initialization routines can be made that include frame format as parameters, disable interrupts
and so on. However, many applications use a fixed setting of the baud and control registers, and for these types
of applications the initialization code can be placed directly in the main routine, or be combined with initialization
code for other I/O modules.

20.6 Data Transmission – The USART Transmitter

The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB Register. When
the Transmitter is enabled, the normal port operation of the TxDn pin is overridden by the USART and given the
function as the Transmitter’s serial output. The baud rate, mode of operation and frame format must be set up
once before doing any transmissions. If synchronous operation is used, the clock on the XCKn pin will be
overridden and used as transmission clock.

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20.6.1 Sending Frames with 5 to 8 Data Bit

A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU can load
the transmit buffer by writing to the UDRn I/O location. The buffered data in the transmit buffer will be moved to
the Shift Register when the Shift Register is ready to send a new frame. The Shift Register is loaded with new
data if it is in idle state (no ongoing transmission) or immediately after the last stop bit of the previous frame is
transmitted. When the Shift Register is loaded with new data, it will transfer one complete frame at the rate
given by the Baud Register, U2Xn bit or by XCKn depending on mode of operation.
The following code examples show a simple USART transmit function based on polling of the Data Register
Empty (UDREn) Flag. When using frames with less than eight bits, the most significant bits written to the UDRn
are ignored. The USART has to be initialized before the function can be used. For the assembly code, the data
to be sent is assumed to be stored in Register R16
Assembly Code Example(1)
USART_Transmit:
; Wait for empty transmit buffer
in r16, UCSRnA
sbrs r16, UDREn
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out UDRn,r16
ret
C Code Example(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) ) If UDREn=1?
;
/* Put data into buffer, sends the data */
UDRn = data;
}

Note: 1. See ”About Code Examples” on page 17.

The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with
new data to be transmitted. If the Data Register Empty interrupt is utilized, the interrupt routine writes the data
into the buffer.

20.6.2 Sending Frames with 9 Data Bit

If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before the low
byte of the character is written to UDRn. The following code examples show a transmit function that handles 9-
bit characters. For the assembly code, the data to be sent is assumed to be stored in registers R17:R16.

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Assembly Code Example(1)(2)

USART_Transmit:
; Wait for empty transmit buffer
in r16, UCSRnA
sbrs r16, UDREn
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi UCSRnB,TXB8
sbrc r17,0
sbi UCSRnB,TXB8
; Put LSB data (r16) into buffer, sends the data
out UDRn,r16
ret
C Code Example(1)(2)
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn))) )
;
/* Copy 9th bit to TXB8 */
UCSRnB &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSRnB |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDRn = data;
}

Notes: 1. These transmit functions are written to be general functions. They can be optimized if the contents of the
UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used after initialization.
2. See ”About Code Examples” on page 17.
The ninth bit can be used for indicating an address frame when using multi processor communication mode or
for other protocol handling as for example synchronization.

20.6.3 Transmitter Flags and Interrupts

The USART Transmitter has two flags that indicate its state: USART Data Register Empty (UDREn) and
Transmit Complete (TXCn). Both flags can be used for generating interrupts.
The Data Register Empty (UDREn) Flag indicates whether the transmit buffer is ready to receive new data. This
bit is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be transmitted
that has not yet been moved into the Shift Register. For compatibility with future devices, always write this bit to
zero when writing the UCSRnA Register.
When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the USART Data
Register Empty Interrupt will be executed as long as UDREn is set (provided that global interrupts are enabled).
UDREn is cleared by writing UDRn. When interrupt-driven data transmission is used, the Data Register Empty
interrupt routine must either write new data to UDRn in order to clear UDREn or disable the Data Register
Empty interrupt, otherwise a new interrupt will occur once the interrupt routine terminates.
The Transmit Complete (TXCn) Flag bit is set one when the entire frame in the Transmit Shift Register has been
shifted out and there are no new data currently present in the transmit buffer. The TXCn Flag bit is automatically
cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location.
The TXCn Flag is useful in half-duplex communication interfaces (like the RS-485 standard), where a

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transmitting application must enter receive mode and free the communication bus immediately after completing
the transmission.
When the Transmit Compete Interrupt Enable (TXCIEn) bit in UCSRnB is set, the USART Transmit Complete
Interrupt will be executed when the TXCn Flag becomes set (provided that global interrupts are enabled). When
the transmit complete interrupt is used, the interrupt handling routine does not have to clear the TXCn Flag, this
is done automatically when the interrupt is executed.

20.6.4 Parity Generator

The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled (UPMn1 = 1),
the transmitter control logic inserts the parity bit between the last data bit and the first stop bit of the frame that is
sent.

20.6.5 Disabling the Transmitter

The disabling of the Transmitter (setting the TXEN to zero) will not become effective until ongoing and pending
transmissions are completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain
data to be transmitted. When disabled, the Transmitter will no longer override the TxDn pin.

20.7 Data Reception – The USART Receiver

The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the
UCSRnB Register to one. When the Receiver is enabled, the normal pin operation of the RxDn pin is overridden
by the USART and given the function as the Receiver’s serial input. The baud rate, mode of operation and
frame format must be set up once before any serial reception can be done. If synchronous operation is used,
the clock on the XCKn pin will be used as transfer clock.

20.7.1 Receiving Frames with 5 to 8 Data Bits

The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be
sampled at the baud rate or XCKn clock, and shifted into the Receive Shift Register until the first stop bit of a
frame is received. A second stop bit will be ignored by the Receiver. When the first stop bit is received, i.e., a
complete serial frame is present in the Receive Shift Register, the contents of the Shift Register will be moved
into the receive buffer. The receive buffer can then be read by reading the UDRn I/O location.
The following code example shows a simple USART receive function based on polling of the Receive Complete
(RXCn) Flag. When using frames with less than eight bits the most significant bits of the data read from the
UDRn will be masked to zero. The USART has to be initialized before the function can be used.

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Assembly Code Example(1)

USART_Receive:
; Wait for data to be received
in r16, UCSRnA
sbrs r16, UDREn
rjmp USART_Receive
; Get and return received data from buffer
in r16, UDRn
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) )
;
/* Get and return received data from buffer */
return UDRn;
}

Note: 1. See ”About Code Examples” on page 17.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must
be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with
“SBRS”, “SBRC”, “SBR”, and “CBR”.

The function simply waits for data to be present in the receive buffer by checking the RXCn Flag, before reading
the buffer and returning the value.

20.7.2 Receiving Frames with 9 Data Bits

If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8n bit in UCSRnB before
reading the low bits from the UDRn. This rule applies to the FEn, DORn and UPEn Status Flags as well. Read
status from UCSRnA, then data from UDRn. Reading the UDRn I/O location will change the state of the receive
buffer FIFO and consequently the TXB8n, FEn, DORn and UPEn bits, which all are stored in the FIFO, will
change.
The following code example shows a simple USART receive function that handles both nine bit characters and
the status bits.

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Assembly Code Example(1)

USART_Receive:
; Wait for data to be received
in r16, UCSRnA
sbrs r16, RXCn
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in r18, UCSRnA
in r17, UCSRnB
in r16, UDRn
; If error, return -1
andi r18,(1<<FEn)|(1<<DORn)|(1<<UPEn)
breq USART_ReceiveNoError
ldi r17, HIGH(-1)
ldi r16, LOW(-1)
USART_ReceiveNoError:
; Filter the 9th bit, then return
lsr r17
andi r17, 0x01
ret
C Code Example(1)
unsigned int USART_Receive( void )
{
unsigned char status, resh, resl;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSRnA;
resh = UCSRnB;
resl = UDRn;
/* If error, return -1 */
if ( status & (1<<FEn)|(1<<DORn)|(1<<UPEn) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
Note: 1. See ”About Code Examples” on page 17.
For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must
be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with
“SBRS”, “SBRC”, “SBR”, and “CBR”.
The receive function example reads all the I/O Registers into the Register File before any computation is done.
This gives an optimal receive buffer utilization since the buffer location read will be free to accept new data as
early as possible.

20.7.3 Receive Compete Flag and Interrupt

The USART Receiver has one flag that indicates the Receiver state.
The Receive Complete (RXCn) Flag indicates if there are unread data present in the receive buffer. This flag is
one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e., does not

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contain any unread data). If the Receiver is disabled (RXENn = 0), the receive buffer will be flushed and
consequently the RXCn bit will become zero.
When the Receive Complete Interrupt Enable (RXCIEn) in UCSRnB is set, the USART Receive Complete
interrupt will be executed as long as the RXCn Flag is set (provided that global interrupts are enabled). When
interrupt-driven data reception is used, the receive complete routine must read the received data from UDRn in
order to clear the RXCn Flag, otherwise a new interrupt will occur once the interrupt routine terminates.

20.7.4 Receiver Error Flags

The USART Receiver has three Error Flags: Frame Error (FEn), Data OverRun (DORn) and Parity Error
(UPEn). All can be accessed by reading UCSRnA. Common for the Error Flags is that they are located in the
receive buffer together with the frame for which they indicate the error status. Due to the buffering of the Error
Flags, the UCSRnA must be read before the receive buffer (UDRn), since reading the UDRn I/O location
changes the buffer read location. Another equality for the Error Flags is that they can not be altered by software
doing a write to the flag location. However, all flags must be set to zero when the UCSRnA is written for upward
compatibility of future USART implementations. None of the Error Flags can generate interrupts.
The Frame Error (FEn) Flag indicates the state of the first stop bit of the next readable frame stored in the
receive buffer. The FEn Flag is zero when the stop bit was correctly read (as one), and the FEn Flag will be one
when the stop bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions, detecting
break conditions and protocol handling. The FEn Flag is not affected by the setting of the USBSn bit in UCSRnC
since the Receiver ignores all, except for the first, stop bits. For compatibility with future devices, always set this
bit to zero when writing to UCSRnA.
The Data OverRun (DORn) Flag indicates data loss due to a receiver buffer full condition. A Data OverRun
occurs when the receive buffer is full (two characters), it is a new character waiting in the Receive Shift Register,
and a new start bit is detected. If the DORn Flag is set there was one or more serial frame lost between the
frame last read from UDRn, and the next frame read from UDRn. For compatibility with future devices, always
write this bit to zero when writing to UCSRnA. The DORn Flag is cleared when the frame received was
successfully moved from the Shift Register to the receive buffer.
The Parity Error (UPEn) Flag indicates that the next frame in the receive buffer had a Parity Error when
received. If Parity Check is not enabled the UPEn bit will always be read zero. For compatibility with future
devices, always set this bit to zero when writing to UCSRnA. For more details see ”Parity Bit Calculation” on
page 184 and ”Parity Checker” on page 191.

20.7.5 Parity Checker

The Parity Checker is active when the high USART Parity mode (UPMn1) bit is set. Type of Parity Check to be
performed (odd or even) is selected by the UPMn0 bit. When enabled, the Parity Checker calculates the parity
of the data bits in incoming frames and compares the result with the parity bit from the serial frame. The result of
the check is stored in the receive buffer together with the received data and stop bits. The Parity Error (UPEn)
Flag can then be read by software to check if the frame had a Parity Error.
The UPEn bit is set if the next character that can be read from the receive buffer had a Parity Error when
received and the Parity Checking was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer
(UDRn) is read.

20.7.6 Disabling the Receiver

In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing receptions will
therefore be lost. When disabled (i.e., the RXENn is set to zero) the Receiver will no longer override the normal
function of the RxDn port pin. The Receiver buffer FIFO will be flushed when the Receiver is disabled.
Remaining data in the buffer will be lost

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20.7.7 Flushing the Receive Buffer

The receiver buffer FIFO will be flushed when the Receiver is disabled, i.e., the buffer will be emptied of its
contents. Unread data will be lost. If the buffer has to be flushed during normal operation, due to for instance an
error condition, read the UDRn I/O location until the RXCn Flag is cleared. The following code example shows
how to flush the receive buffer.
Assembly Code Example(1)
USART_Flush:
in r16, UCSRnA
sbrs r16, RXCn
ret
in r16, UDRn
rjmp USART_Flush
C Code Example(1)
void USART_Flush( void )
{
unsigned char dummy;
while ( UCSRnA & (1<<RXCn) ) dummy = UDRn;
}

Note: 1. See ”About Code Examples” on page 17.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must
be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with
“SBRS”, “SBRC”, “SBR”, and “CBR”.

20.8 Asynchronous Data Reception

The USART includes a clock recovery and a data recovery unit for handling asynchronous data reception. The
clock recovery logic is used for synchronizing the internally generated baud rate clock to the incoming
asynchronous serial frames at the RxDn pin. The data recovery logic samples and low pass filters each
incoming bit, thereby improving the noise immunity of the Receiver. The asynchronous reception operational
range depends on the accuracy of the internal baud rate clock, the rate of the incoming frames, and the frame
size in number of bits.

20.8.1 Asynchronous Clock Recovery

The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 20-5 illustrates the
sampling process of the start bit of an incoming frame. The sample rate is 16 times the baud rate for Normal
mode, and eight times the baud rate for Double Speed mode. The horizontal arrows illustrate the
synchronization variation due to the sampling process. Note the larger time variation when using the Double
Speed mode (U2Xn = 1) of operation. Samples denoted zero are samples done when the RxDn line is idle (i.e.,
no communication activity).

Figure 20-5. Start Bit Sampling

RxD IDLE START BIT 0

Sample
(U2X = 0) 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3

Sample
(U2X = 1) 0 1 2 3 4 5 6 7 8 1 2

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When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn line, the start bit
detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in the figure. The clock
recovery logic then uses samples 8, 9, and 10 for Normal mode, and samples 4, 5, and 6 for Double Speed
mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is received. If two
or more of these three samples have logical high levels (the majority wins), the start bit is rejected as a noise
spike and the Receiver starts looking for the next high to low-transition. If however, a valid start bit is detected,
the clock recovery logic is synchronized and the data recovery can begin. The synchronization process is
repeated for each start bit.

20.8.2 Asynchronous Data Recovery

When the receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery unit
uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in Double
Speed mode. Figure 20-6 shows the sampling of the data bits and the parity bit. Each of the samples is given a
number that is equal to the state of the recovery unit.

Figure 20-6. Sampling of Data and Parity Bit

RxD BIT n

Sample
(U2X = 0) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1

Sample
(U2X = 1) 1 2 3 4 5 6 7 8 1

The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to the three
samples in the center of the received bit. The center samples are emphasized on the figure by having the
sample number inside boxes. The majority voting process is done as follows: If two or all three samples have
high levels, the received bit is registered to be a logic 1. If two or all three samples have low levels, the received
bit is registered to be a logic 0. This majority voting process acts as a low pass filter for the incoming signal on
the RxDn pin. The recovery process is then repeated until a complete frame is received. Including the first stop
bit. Note that the Receiver only uses the first stop bit of a frame.
Figure 20-7 on page 193 shows the sampling of the stop bit and the earliest possible beginning of the start bit of
the next frame.

Figure 20-7. Stop Bit Sampling and Next Start Bit Sampling

RxD STOP 1 (A) (B) (C)

Sample
(U2X = 0) 1 2 3 4 5 6 7 8 9 10 0/1 0/1 0/1

Sample
(U2X = 1) 1 2 3 4 5 6 0/1

The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is
registered to have a logic 0 value, the Frame Error (FEn) Flag will be set.
A new high to low transition indicating the start bit of a new frame can come right after the last of the bits used
for majority voting. For Normal Speed mode, the first low level sample can be at point marked (A) in Figure 20-
7. For Double Speed mode the first low level must be delayed to (B). (C) marks a stop bit of full length. The early
start bit detection influences the operational range of the Receiver.

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20.8.3 Asynchronous Operational Range

The operational range of the Receiver is dependent on the mismatch between the received bit rate and the
internally generated baud rate. If the Transmitter is sending frames at too fast or too slow bit rates, or the
internally generated baud rate of the Receiver does not have a similar (see Table 20-2 on page 194) base
frequency, the Receiver will not be able to synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal receiver baud
rate.

Equation 1

 D + 1 S  D + 2 S
R slow = ------------------------------------------- R fast = -----------------------------------
S – 1 + D  S + SF  D + 1 S + S M

D Sum of character size and parity size (D = 5 to 10 bit)

S Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode.
SF First sample number used for majority voting. SF = 8 for normal speed and
SF = 4 for Double Speed mode.
SM Middle sample number used for majority voting. SM = 9 for normal speed and
SM = 5 for Double Speed mode.
Rslow is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate.
Rfast is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate.

Table 20-2 on page 194 and Table 20-3 on page 194 list the maximum receiver baud rate error that can be
tolerated. Note that Normal Speed mode has higher toleration of baud rate variations.

Table 20-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0)
D Recommended Max
# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total Error (%) Receiver Error (%)
5 93.20 106.67 +6.67/-6.8 ± 3.0
6 94.12 105.79 +5.79/-5.88 ± 2.5
7 94.81 105.11 +5.11/-5.19 ± 2.0
8 95.36 104.58 +4.58/-4.54 ± 2.0
9 95.81 104.14 +4.14/-4.19 ± 1.5
10 96.17 103.78 +3.78/-3.83 ± 1.5

Table 20-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D Recommended Max
# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total Error (%) Receiver Error (%)
5 94.12 105.66 +5.66/-5.88 ± 2.5
6 94.92 104.92 +4.92/-5.08 ± 2.0
7 95.52 104,35 +4.35/-4.48 ± 1.5

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Table 20-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D Recommended Max
# (Data+Parity Bit) Rslow (%) Rfast (%) Max Total Error (%) Receiver Error (%)
8 96.00 103.90 +3.90/-4.00 ± 1.5
9 96.39 103.53 +3.53/-3.61 ± 1.5
10 96.70 103.23 +3.23/-3.30 ± 1.0
The recommendations of the maximum receiver baud rate error was made under the assumption that the
Receiver and Transmitter equally divides the maximum total error.
There are two possible sources for the receivers baud rate error. The Receiver’s system clock (XTAL) will
always have some minor instability over the supply voltage range and the temperature range. When using a
crystal to generate the system clock, this is rarely a problem, but for a resonator the system clock may differ
more than 2% depending of the resonators tolerance. The second source for the error is more controllable. The
baud rate generator can not always do an exact division of the system frequency to get the baud rate wanted. In
this case an UBRRn value that gives an acceptable low error can be used if possible.

20.9 Multi-processor Communication Mode

Setting the Multi-processor Communication mode (MPCMn) bit in UCSRnA enables a filtering function of
incoming frames received by the USART Receiver. Frames that do not contain address information will be
ignored and not put into the receive buffer. This effectively reduces the number of incoming frames that has to
be handled by the CPU, in a system with multiple MCUs that communicate via the same serial bus. The
Transmitter is unaffected by the MPCMn setting, but has to be used differently when it is a part of a system
utilizing the Multi-processor Communication mode.
If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if the frame
contains data or address information. If the Receiver is set up for frames with nine data bits, then the ninth bit
(RXB8n) is used for identifying address and data frames. When the frame type bit (the first stop or the ninth bit)
is one, the frame contains an address. When the frame type bit is zero the frame is a data frame.
The Multi-processor Communication mode enables several slave MCUs to receive data from a master MCU.
This is done by first decoding an address frame to find out which MCU has been addressed. If a particular slave
MCU has been addressed, it will receive the following data frames as normal, while the other slave MCUs will
ignore the received frames until another address frame is received.

20.9.1 Using MPCMn

For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZn = 7). The ninth bit
(TXB8n) must be set when an address frame (TXB8n = 1) or cleared when a data frame (TXB = 0) is being
transmitted. The slave MCUs must in this case be set to use a 9-bit character frame format.
The following procedure should be used to exchange data in Multi-processor Communication mode:
1. All Slave MCUs are in Multi-processor Communication mode (MPCMn in UCSRnA is set).
2. The Master MCU sends an address frame, and all slaves receive and read this frame. In the Slave MCUs,
the RXCn Flag in UCSRnA will be set as normal.
3. Each Slave MCU reads the UDRn Register and determines if it has been selected. If so, it clears the
MPCMn bit in UCSRnA, otherwise it waits for the next address byte and keeps the MPCMn setting.
4. The addressed MCU will receive all data frames until a new address frame is received. The other Slave
MCUs, which still have the MPCMn bit set, will ignore the data frames.
5. When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCMn bit
and waits for a new address frame from master. The process then repeats from 2.

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Using any of the 5- to 8-bit character frame formats is possible, but impractical since the Receiver must change
between using n and n+1 character frame formats. This makes full-duplex operation difficult since the
Transmitter and Receiver uses the same character size setting. If 5- to 8-bit character frames are used, the
Transmitter must be set to use two stop bit (USBSn = 1) since the first stop bit is used for indicating the frame
type.
Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCMn bit. The MPCMn bit shares
the same I/O location as the TXCn Flag and this might accidentally be cleared when using SBI or CBI
instructions.

20.10 Examples of Baud Rate Setting

For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous
operation can be generated by using the UBRRn settings in Table 20-4. UBRRn values which yield an actual
baud rate differing less than 0.5% from the target baud rate, are bold in the table. Higher error ratings are
acceptable, but the Receiver will have less noise resistance when the error ratings are high, especially for large
serial frames (see ”Asynchronous Operational Range” on page 194). The error values are calculated using the
following equation:

BaudRate Closest Match

Error[%] =  -------------------------------------------------- – 1  100%
 BaudRate 

Table 20-4. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
fosc = 1.0000MHz fosc = 1.8432MHz fosc = 2.0000MHz
Baud
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
Rate
(bps) UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%
14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%
19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%
28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%
38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%
76.8k – – 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5%
115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%
230.4k – – – – – – 0 0.0% – – – –
250k – – – – – – – – – – 0 0.0%
Max.(1) 62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps
Note: 1. UBRRn = 0, Error = 0.0%

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Table 20-5. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 3.6864MHz fosc = 4.0000MHz fosc = 7.3728MHz
Baud
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
Rate
(bps) UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0%
4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0%
9600 23 0.0% 47 0.0% 25 0.2% 51 0.2% 47 0.0% 95 0.0%
14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0%
19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0%
28.8k 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0%
38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0%
57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0%
76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0%
115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0%
230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0%
250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8%
0.5M – – 0 -7.8% – – 0 0.0% 0 -7.8% 1 -7.8%
1M – – – – – – – – – – 0 -7.8%
(1)
Max. 230.4kbps 460.8kbps 250kbps 0.5Mbps 460.8kbps 921.6kbps
1.UBRRn = 0, Error = 0.0%

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Table 20-6. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 8.0000MHz fosc = 11.0592MHz fosc = 14.7456MHz
Baud
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
Rate
(bps) UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0%
4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0%
9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0%
14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0%
19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0%
28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0%
38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0%
57.6k 8 -3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0%
76.8k 6 -7.0% 12 0.2% 8 0.0% 17 0.0% 11 0.0% 23 0.0%
115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0%
230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0%
250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3%
0.5M 0 0.0% 1 0.0% – – 2 -7.8% 1 -7.8% 3 -7.8%
1M – – 0 0.0% – – – – 0 -7.8% 1 -7.8%
(1)
Max. 0.5Mbps 1Mbps 691.2kbps 1.3824Mbps 921.6kbps 1.8432Mbps
1. UBRRn = 0, Error = 0.0%

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Table 20-7. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 16.0000MHz fosc = 18.4320MHz fosc = 20.0000MHz
Baud
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
Rate
(bps) UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error
2400 416 -0.1% 832 0.0% 479 0.0% 959 0.0% 520 0.0% 1041 0.0%
4800 207 0.2% 416 -0.1% 239 0.0% 479 0.0% 259 0.2% 520 0.0%
9600 103 0.2% 207 0.2% 119 0.0% 239 0.0% 129 0.2% 259 0.2%
14.4k 68 0.6% 138 -0.1% 79 0.0% 159 0.0% 86 -0.2% 173 -0.2%
19.2k 51 0.2% 103 0.2% 59 0.0% 119 0.0% 64 0.2% 129 0.2%
28.8k 34 -0.8% 68 0.6% 39 0.0% 79 0.0% 42 0.9% 86 -0.2%
38.4k 25 0.2% 51 0.2% 29 0.0% 59 0.0% 32 -1.4% 64 0.2%
57.6k 16 2.1% 34 -0.8% 19 0.0% 39 0.0% 21 -1.4% 42 0.9%
76.8k 12 0.2% 25 0.2% 14 0.0% 29 0.0% 15 1.7% 32 -1.4%
115.2k 8 -3.5% 16 2.1% 9 0.0% 19 0.0% 10 -1.4% 21 -1.4%
230.4k 3 8.5% 8 -3.5% 4 0.0% 9 0.0% 4 8.5% 10 -1.4%
250k 3 0.0% 7 0.0% 4 -7.8% 8 2.4% 4 0.0% 9 0.0%
0.5M 1 0.0% 3 0.0% – – 4 -7.8% – – 4 0.0%
1M 0 0.0% 1 0.0% – – – – – – – –
(1)
Max. 1Mbps 2Mbps 1.152Mbps 2.304Mbps 1.25Mbps 2.5Mbps
1. UBRRn = 0, Error = 0.0%

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20.11 Register Description

20.11.1 UDRn – USART I/O Data Register n

Bit 7 6 5 4 3 2 1 0
RXB[7:0] UDRn (Read)
TXB[7:0] UDRn (Write)
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0

The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the same I/O
address referred to as USART Data Register or UDRn. The Transmit Data Buffer Register (TXB) will be the
destination for data written to the UDRn Register location. Reading the UDRn Register location will return the
contents of the Receive Data Buffer Register (RXB).
For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by the
Receiver.
The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set. Data written to
UDRn when the UDREn Flag is not set, will be ignored by the USART Transmitter. When data is written to the
transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the Transmit Shift Register
when the Shift Register is empty. Then the data will be serially transmitted on the TxDn pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive buffer is
accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions (SBI and CBI)
on this location. Be careful when using bit test instructions (SBIC and SBIS), since these also will change the
state of the FIFO.

20.11.2 UCSRnA – USART Control and Status Register n A

Bit 7 6 5 4 3 2 1 0
RXCn TXCn UDREn FEn DORn UPEn U2Xn MPCMn UCSRnA
Read/Write R R/W R R R R R/W R/W
Initial Value 0 0 1 0 0 0 0 0

• Bit 7 – RXCn: USART Receive Complete

This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty
(i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and
consequently the RXCn bit will become zero. The RXCn Flag can be used to generate a Receive Complete
interrupt (see description of the RXCIEn bit).

• Bit 6 – TXCn: USART Transmit Complete

This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are no
new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is automatically cleared when a
transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag
can generate a Transmit Complete interrupt (see description of the TXCIEn bit).
• Bit 5 – UDREn: USART Data Register Empty
The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the
buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty
interrupt (see description of the UDRIEn bit). UDREn is set after a reset to indicate that the Transmitter is ready.

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• Bit 4 – FEn: Frame Error

This bit is set if the next character in the receive buffer had a Frame Error when received. I.e., when the first stop
bit of the next character in the receive buffer is zero. This bit is valid until the receive buffer (UDRn) is read. The
FEn bit is zero when the stop bit of received data is one. Always set this bit to zero when writing to UCSRnA.

• Bit 3 – DORn: Data OverRun

This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive buffer is full
(two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is detected. This
bit is valid until the receive buffer (UDRn) is read. Always set this bit to zero when writing to UCSRnA.

• Bit 2 – UPEn: USART Parity Error

This bit is set if the next character in the receive buffer had a Parity Error when received and the Parity Checking
was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer (UDRn) is read. Always set this
bit to zero when writing to UCSRnA.

• Bit 1 – U2Xn: Double the USART Transmission Speed

This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous operation.
Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling the transfer
rate for asynchronous communication.

• Bit 0 – MPCMn: Multi-processor Communication Mode

This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to one, all the
incoming frames received by the USART Receiver that do not contain address information will be ignored. The
Transmitter is unaffected by the MPCMn setting. For more detailed information see ”Multi-processor
Communication Mode” on page 195.

20.11.3 UCSRnB – USART Control and Status Register n B

Bit 7 6 5 4 3 2 1 0
RXCIEn TXCIEn UDRIEn RXENn TXENn UCSZn2 RXB8n TXB8n UCSRnB
Read/Write R/W R/W R/W R/W R/W R/W R R/W
Initial Value 0 0 0 0 0 0 0 0

• Bit 7 – RXCIEn: RX Complete Interrupt Enable n

Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt will be
generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the
RXCn bit in UCSRnA is set.

• Bit 6 – TXCIEn: TX Complete Interrupt Enable n

Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt will be
generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the
TXCn bit in UCSRnA is set.

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• Bit 5 – UDRIEn: USART Data Register Empty Interrupt Enable n

Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will be generated
only if the UDRIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the UDREn bit in
UCSRnA is set.

• Bit 4 – RXENn: Receiver Enable n

Writing this bit to one enables the USART Receiver. The Receiver will override normal port operation for the
RxDn pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FEn, DORn, and
UPEn Flags.

• Bit 3 – TXENn: Transmitter Enable n

Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for
the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to zero) will not become effective
until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit
Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the
TxDn port.

• Bit 2 – UCSZn2: Character Size n

The UCSZn2 bits combined with the UCSZn1:0 bit in UCSRnC sets the number of data bits (Character SiZe) in
a frame the Receiver and Transmitter use.

• Bit 1 – RXB8n: Receive Data Bit 8 n

RXB8n is the ninth data bit of the received character when operating with serial frames with nine data bits. Must
be read before reading the low bits from UDRn.

• Bit 0 – TXB8n: Transmit Data Bit 8 n

TXB8n is the ninth data bit in the character to be transmitted when operating with serial frames with nine data
bits. Must be written before writing the low bits to UDRn.

20.11.4 UCSRnC – USART Control and Status Register n C

Bit 7 6 5 4 3 2 1 0
UMSELn1 UMSELn0 UPMn1 UPMn0 USBSn UCSZn1 UCSZn0 UCPOLn UCSRnC
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 1 1 0

• Bits 7:6 – UMSELn1:0 USART Mode Select

These bits select the mode of operation of the USARTn as shown in Table 20-8.

Table 20-8. UMSELn Bits Settings

UMSELn1 UMSELn0 Mode
0 0 Asynchronous USART
0 1 Synchronous USART
1 0 (Reserved)
1 1 Master SPI (MSPIM)(1)

Note: 1. See ”USART in SPI Mode” on page 205 for full description of the Master SPI Mode (MSPIM) operation

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• Bits 5:4 – UPMn1:0: Parity Mode

These bits enable and set type of parity generation and check. If enabled, the Transmitter will automatically
generate and send the parity of the transmitted data bits within each frame. The Receiver will generate a parity
value for the incoming data and compare it to the UPMn setting. If a mismatch is detected, the UPEn Flag in
UCSRnA will be set.

Table 20-9. UPMn Bits Settings

UPMn1 UPMn0 Parity Mode
0 0 Disabled
0 1 Reserved
1 0 Enabled, Even Parity
1 1 Enabled, Odd Parity

• Bit 3 – USBSn: Stop Bit Select

This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores this setting.

Table 20-10. USBS Bit Settings

USBSn Stop Bit(s)
0 1-bit
1 2-bit

• Bit 2:1 – UCSZn1:0: Character Size

The UCSZn1:0 bits combined with the UCSZn2 bit in UCSRnB sets the number of data bits (Character SiZe) in
a frame the Receiver and Transmitter use.

Table 20-11. UCSZn Bits Settings

UCSZn2 UCSZn1 UCSZn0 Character Size
0 0 0 5-bit
0 0 1 6-bit
0 1 0 7-bit
0 1 1 8-bit
1 0 0 Reserved
1 0 1 Reserved
1 1 0 Reserved
1 1 1 9-bit

• Bit 0 – UCPOLn: Clock Polarity

This bit is used for synchronous mode only. Write this bit to zero when asynchronous mode is used. The
UCPOLn bit sets the relationship between data output change and data input sample, and the synchronous
clock (XCKn).

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Table 20-12. UCPOLn Bit Settings

Transmitted Data Changed (Output of Received Data Sampled (Input on RxDn
UCPOLn TxDn Pin) Pin)
0 Rising XCKn Edge Falling XCKn Edge
1 Falling XCKn Edge Rising XCKn Edge

20.11.5 UBRRnL and UBRRnH – USART Baud Rate Registers

Bit 15 14 13 12 11 10 9 8
– – – – UBRRn[11:8] UBRRnH
UBRRn[7:0] UBRRnL
7 6 5 4 3 2 1 0
Read/Write R R R R R/W R/W R/W R/W
R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0

• Bit 15:12 – Reserved

These bits are reserved for future use. For compatibility with future devices, these bit must be written to zero
when UBRRnH is written.

• Bit 11:0 – UBRR[11:0]: USART Baud Rate Register

This is a 12-bit register which contains the USART baud rate. The UBRRnH contains the four most significant
bits, and the UBRRnL contains the eight least significant bits of the USART baud rate. Ongoing transmissions
by the Transmitter and Receiver will be corrupted if the baud rate is changed. Writing UBRRnL will trigger an
immediate update of the baud rate prescaler.

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