Computer Labs: The i8254 Timer/Counter
2o MIEIC
Pedro F. Souto (pfs@fe.up.pt)
September 27, 2018
Lab 2: The PC’s Timer/Counter - Part I
I Write a set of functions:
int timer_test_read_config(uint8_t timer,
enum timer_status_field field)
int timer_test_time_base(uint8_t timer, uint32_t freq)
that require programming the PC’s Timer/Counter
I These functions are at a high level for pedagogical reasons
I The idea is that you design the lower level functions (with the final
project in mind)
I In this lab we have also defined the lower level functions
I What’s new?
I Program an I/O controller: the PC’s timer counter (i8254)
I Use interrupts (Part II)
The i8254
I It is a programmable timer/counter
I Each PC has a functionally equivalent circuit, nowadays it is
integrated in the so-called south-bridge
I Allows to measure time in a precise way, independently of
the processor speed
I It has 3 16-bit counters, each of
which
I May count either in binary or
BCD
I Has 6 counting modes
i8254 Counting Modes (4 of 6)
Mode 0 Interrupt on terminal count – for counting events
I OUT goes high and remains high when count reaches 0
Mode 1 Hardware retriggerable one-shot
I OUT goes low and remains low until count reaches 0, the
counter is reloaded on a rising edge of the ENABLE input
Mode 2 Rate Generator (divide-by-N counter)
I OUT goes low for one clock cycle when count reaches 0,
the counter is reloaded with its initial count afterwards,
and ...
Mode 3 Square Wave Generator – for Lab 2
I Similar to mode 2, except for the duty-cycle: OUT will be
high for half of the cycle and low for the remaining half of
the cycle
i8254 Block Diagram
I Three independent 16-bit
counters
I Ports 40h, 41h and 42h
I MSB and LSB addressable
separately
I independent counting modes
I An 8 bit-control register
I Port 43h
I Programming of each counter
independently
i8254 Control Word
I Used to program the timers
I Written to the Control Register (0x43)
Example
Bit Value Function
7,6 Counter selection I Timer 2 in mode 3
00 0
01
10
1
2
I Couting value: 1234 = 0x04D2
5,4 Counter Initialization
01 LSB
10 MSB Control Register: 10110110
11 LSB followed by MSB
3,2,1 Counting Mode I "NOTE: Don’t care bits (X)
000 0
001 1 should be 0 to insure
x10 2
x11 3 compatibility with future Intel
100 4
101 5 products."
0 BCD
0
1
Binary (16 bits)
BCD (4 digits)
Timer2 LSB 0xD2
Timer2 MSB 0x04
How to assemble the control word?
How to assemble the control word?
Use bitwise operations
Use the macros defined in i8254.h
i8254: Read-Back Command
The command Read-Back Command Format
Bit Value Function
I Allows to retrieve 7,6 Read-Back Command
11
I the programmed configuration 5 COUNT
0 Read counter value
I and/or the current counting 4 STATUS
value 0 Read programmed mode
3 Select Timer 2
of one or more timers 1 Yes
2 Select Timer 1
I Written to the Control Register 1
1 Yes
Select Timer 0
(0x43) 0
1 Yes
Reserved
Reading of the status/count
Read-Back Status Format
I The configuration (status) is read Bit Value Function
7 Output
from the timer’s data register 6 Null Count
5,4 Counter Initialization
I The 6 LSBs match those of the 3,2,1 Programmed Mode
0 BCD
Control Word
I The counting value is also read from the timer’s data register
I If both status and count are requested, the status is the first
value returned
How to parse the the status word?
Use bitwise operations
Use the macros defined in i8254.h
i8254: Use in the PC (1/2)
I Timer 0 is used to provide a time base.
I Timer 1 is used for DRAM refresh
I Via DMA channel 0
(Not sure this is still true.)
I Timer 2 is used for tone generation
i8254: Use in the PC (2/2)
I The i8254 is mapped in the I/0 address space:
Timer 0: 0x40
Timer 1: 0x41
Timer 2: 0x42
Control Register: 0x43
I Need to use IN/OUT assembly instructions
I Minix 3 provides the SYS_DEVIO kernel call for doing I/O
#include <minix/syslib.h>
int sys_inb(port_t port, u8_t *byte);
int sys_outb(port_t port, u8_t byte);
I Need to write to the control register before accessing any
of the timers
I Both to program (control word) a timer, or to read its
configuration (read-back command)
Minix 3 and Timer 0
I At start up, Minix 3 programs Timer 0 to generate a square
wave with a fixed frequency
I Timer 0 will generate an interrupt at a fixed rate:
I Its output is connected to IRQ0
I Minix 3 uses these interrupts to measure time
I The interrupt handler increments a global variable on every
interrupt
I The value of this variable increments at a fixed, known, rate
I Minix 3 uses this variable mainly for:
I Keeping track of the date/time
I Implementing SW timers
Lab 2: Part 1 - Reading Timer Configuration (1/2)
What to do? Read timer configuration in Minix
int timer_test_read_config(uint8_t timer,
enum timer_status_field field)
1. Write read-back command to read input timer
configuration:
I Make sure 2 MSBs are both 1
I Select only the status (not the counting value)
2. Read the timer port
3. Parse the configuration read
4. Call the function timer_print_config() that we
provide you
How to design it? Try to develop an API that can be used in the
project.
int timer_get_conf(uint8_t timer, uint8_t *st);
int timer_display_conf(uint8_t timer, uint8_t st,
enum timer_status_field status);
Lab 2: Part 1 - Reading Timer Configuration (2/2)
Stuff we provide you
int timer_print_config(uint8_t timer,
union timer_status_field_val val,
enum timer_status_field field);
enum timer_status_field {
all, // configuration in hexadecimal
inital, // timer initialization mode
mode, // timer counting mode
base // timer counting base
};
enum timer_init {
INVAL_val,
LSB_only,
MSB_only,
MSB_after_LSB
};
union timer_status_field_val {
uint8_t byte; // status, in hexadecimal
enum timer_init in_mode; // initialization mode
uint8_t count_mode; // counting mode: 0, 1, ..., 5
bool bcd; // true, if counting in BCD
C Enumerated Types
I This is a user-defined type that can take one of a finite
number of values
enum timer_status_field {
all, // configuration in hexadecimal
inital, // timer initialization mode
mode, // timer counting mode
base // timer counting base
};
enum timer_status_field info = base;
I The C compiler represents each possible value of an
enumerated type by an integer value. By default:
I The first value is represented with 0
I Any other value, is one more than the previous value
I However, it is possible to assign to an enumerated value an
integer value different from the default (e.g. all = 255;)
I The use of enumerated types makes the code more readable
C Unions
I Syntatically, a union data type appears like a struct:
union timer_status_field_val {
enum timer_init in_mode; // initialization mode
uint8_t count_mode; // counting mode: 0, 1, ..., 5
bool bcd; // true, if counting in BCD
};
I Access to the members of a union is via the dot operator
I However, semantically, there is a big difference:
Union contains space to store any of its members, but not all
of its members simultaneously
I The name union stems from the fact that a variable of this
type can take values of any of the types of its members
Struct contains space to store all of its members
simultaneously
In timer_print_config() we are using it to reduce the
number of arguments passed
I And an enumerated type to specify the kind of information
Lab 2: Part 1 - Setting the Time-Base
What to do? Change the rate at which Timer 0 generates
interrupts.
int timer_test_time_base(uint8_t timer, uint32_t freq)
1. Write control word to configure Timer 0:
I Do not change 4 least-significant bits
I Mode (3)
I BCD/Binary counting
You need to read the Timer 0 configuration first.
I Preferably, LSB followed by MSB
2. Load Timer 0 with the value of the divisor to generate
the frequency corresponding to the desired rate
I Depends on the previous step
How to design it? Try to develop an API that can be used in the
project.
int timer_set_frequency(uint8_t timer, uint32_t freq);
How do we know it works? Use the date command.
I Or, even better, use the test code provided.
Lab 2: Grading Criteria
SVN (5%) Whether or not your code is in the right place (under
lab2/, of the repository’s root)
I Also, evidence of incremental development approach
Execution (80%) Including automatic code grading.
Code (15%)
return values of function/kernel calls must be checked
global variables only if you cannot do what you want, or if
they can be considered fields/members of an object (if
using object oriented design)
symbolic constants i.e. use #define
modularity both at the level of functions and at the level of
files
Self-evaluation Must submit it by filling a Google Form (check
the handout)
IMPORTANT Please follow exactly the instructions, otherwise
you may be penalized
Further Reading
I Lab 2 Handout
I i8254 Data-sheet
