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PLC with PIC16F648A microcontroller part 2

Article  in  ELECTRONICS WORLD · December 2008

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 Feature PLC/MCU

PLC WITH PIC16F648A

MICROCONTROLLER
PART 2 ASSOCIATE PROFESSOR DR MURAT UZAM FROM NIGDE
UNIVERSITY IN TURKEY PRESENTS A SERIES OF ARTICLES

T
he basic software of this ON A PROJECT THAT FOCUSES ON A MICROCONTROLLER-
programmable logic BASED PLC. THIS IS THE SECOND ARTICLE OF THE SERIES
controller (PLC) called
UZAM_PLC makes use of general AND IT COVERS THE BASIC SOFTWARE STRUCTURE OF
purpose 8-bit registers of RAM THE UZAM_PLC
data memory of PIC16F648A. For
the sake of simplicity, we restrict
ourselves to use only BANK 0, i.e. all the PLC’s scan cycle includes the “get_inputs” and “send_outputs”. The
macros, including the basic definitions following: obtain the inputs, run the user endless PLC scan cycles are obtained by
explained here, to be defined by means program, update the outputs. This cycle means of the label “scan” and the
of 8-bit registers of BANK 0. repeats as long as the PLC runs. instruction “goto scan”.
The file “definitions.inc” (all the files Before getting into the endless PLC UZAM_PLC is fixed to run at 4MHz
explained here including “definitions.inc” scan cycles, the initial conditions of the with PIC16F648A’s internal oscillator. The
can be downloaded from PLC are set up in the initialisation stage. watchdog timer is used to prevent user-
http://host.nigde.edu.tr/muzam/). It These main steps can be seen from program lock-ups. As will be explained
contains all of the basic macros and Figure 1, where “initialise” is a macro for later, the hardware timer TMR0 is utilised
definitions necessary for UZAM_PLC. In setting up the initial conditions; to obtain free-running reference timing
this article we will explain the contents “get_inputs” is a macro for getting the signals.
of this file. First of all, let’s have a look at inputs; and “send_outputs” is a macro
the file called “UZAM_plc_8i8o_1.asm” for updating the outputs. The “user PLC FILE DEFINITIONS
shown in Figure 1. It is well-known that program” must be placed between Next, let’s consider the inside of the file
“definitions.inc”. The definition of 8-bit
variables to be used for the basic
software and their allocation in BANK 0
of RAM data memory are shown in
Figure 2a and 2b respectively. Although
we can define as many inputs and
outputs as we want, for the sake of
simplicity we define four 8-bit input
registers and four 8-bit output registers
(Q0, Q1, Q2, Q3).
It is well-known that inputs taken from

It is also necessary to define and

initialise all variables used within a
PLC. Necessary functions are all
described as PIC Assembly macros to
be used in UZAM_PLC. The macros
described in this article could be
summarised as follows: “HC165”
(for handling the inputs), “HC595”
(for sending the outputs), “dbncr”
(for debouncing the inputs),
“initialise”, “get_inputs”,
“send_outputs”.

Figure 1: The view of the file “UZAM_plc_8i8o_1.asm”

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 PLC/MCU Feature

Figure 3: BANK macros

Figure 2: (a) above – The definition of 8-bit variables to be contacts always suffer from “contact bouncing”. To circumvent
used for the basic software, this problem we will define a “debouncing” mechanism for the
(b) below – Their allocation in BANK 0 of RAM data memory inputs and this will be explained later. In the “get_inputs” stage
of the PLC scan cycle, the input signals are serially taken from the
related 74HC/LS165 registers and stored in the registers. As a
result, bI0, bI1, bI2 and bI3 will hold these bouncing input
signals. After applying the “debouncing” mechanism to the
registers bI0, bI1, bI2 and bI3, we obtain “debounced” input
signals and they are stored in registers I0, I1, I2 and I3
respectively.
We don’t have to use all of these registers. For example, if we
use just the “main board”, then we have 8 inputs and therefore
we can just use bI0 and I0. Similarly, if we use the “main board”
with an “I/O extension board”, then we have 16 inputs and,
therefore, we must use bI0, bI1 and I0, I1. Currently, for the sake
of simplicity we restrict ourselves to the main board and,
therefore, we need to use bI0 and I0.
In the “send_outputs” stage of the PLC scan cycle, the output
information stored in the 8-bit registers Q0, Q1, Q2, Q3 are
serially sent out to and stored in the related TPIC6B595 registers.
This means that Q0, Q1, Q2, Q3 registers will hold output
information and they will be copied into the TPIC6B595 registers
at the end of each PLC scan cycle. We don’t have use all of these
output registers. For example, if we use just the “main board”,
then we have 8 outputs and, therefore, we can use just Q0.
Similarly, if we use the “main board” with an “I/O extension
board”, then we have 16 outputs and, so, we must use both Q0
and Q1. Currently, for the sake of simplicity we restrict ourselves
to the main board and therefore we need to use only Q0. Four 8-
bit registers, namely M0, M1, M2 and M3, are defined for
obtaining 32 memory bits (internal relays, in PLC jargon).
To be used for the debouncer macro we define eight 8-bit
registers (DBNCR, DBNCR+1, ..., DBNCR+7). In addition, the
register DBNCRRED is also defined to be used for the debouncer
macro. Temp_1 is a general temporary register declared to be
used in the macros. Temp_2 is declared to be used especially for
obtaining special memory bits as will be explained later. Timer_2
is defined for storing high byte of the free-running timing signals.

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 Feature PLC/MCU

Figure 6: The macro “HC165”

OTHER VARIABLES 16777.216ms.

The variable “LOGIC0” is defined to hold Timing diagram of the free-running
logical “0” value throughout the PLC reference timing signals is depicted in
operation. At the initialisation stage it is Figure 5. Note that the evaluation of
deposited with this value. Similarly, the TMR0 (Timer_1) is independent from PLC
variable “LOGIC1” is defined to hold scan cycles, but Timer_2 is incremented
logical “1” value throughout the PLC within the “get_inputs” stage of PLC scan
operation. At the initialisation stage it is cycle on Timer_1 overflow. This is justified
deposited with this value. as long as the PLC scan cycle takes less
The special memory bit “FRSTSCN” is than 65.536ms.
arranged to hold the value of “1” at the
first PLC scan cycle only. In the other PLC THE MACROS
Figure 4: Definitions of one-bit variables scan cycles following the first one it is The macro “HC165” is shown in Figure 6.
reset. The other special memory bit The input signals are serially taken from
“SCNOSC” is arranged to work as a “scan the related 74HC/LS165 registers and
oscillator”. This means that in one PLC stored in the registers such as bI0, bI1, bI2
scan cycle this special bit will hold the and bI3 by means of this macro. The
value of “0”, in the next one the value of “num” defines the number of
“1”, in the next one the value of “0” etc. 74HC/LS165 registers to be considered.
This will keep on going for every PLC scan This means that with this macro we can
Figure 5: Timing diagram of the free- cycle. Let us now consider the 16 obtain inputs from as many 74HC/LS165
running reference timing signals reference timing signals. registers as we wish.
(T = 0.512, 1.024... 16777.216ms) As seen later, TMR0 of PIC16F648A is However, as explained before, we
set up to count the 1/4 of 4MHz internal restrict this number to be 4 at most for the
oscillator signal, i.e. 1MHz with a prescaler sake of simplicity; “var0” is the beginning
The low byte of free-running timing arranged to divide the signal to 256. As a of the registers to which the state of
signals is stored in TMR0 (recalled as result by means of TMR0 bits (also called inputs taken from 74HC/LS165 registers
Timer_1). Timer_1) we obtain eight free-running will be stored. This implies that there
For accessing the RAM data memory reference timing signals with the “T” should be enough RAM locations reserved
easily, BANK macros are defined as shown timing periods starting from 0.512ms to after “var0” and, also, there should be
in Figure 3. The definitions of one bit 65.536ms. We’ll see later that the register enough 74HC/LS165 registers to get the
variables are depicted in Figure 4. The “Timer_2” is incremented on Timer_1 inputs from. There are some explanations
following definitions are self explanatory: overflow. This also gives us (by means of within the macro to describe how it works.
74HC165, TPIC6B595, 8 INPUTS, 8 Timer_2 bits) eight more free-running As can be seen, this macro makes use of
OUTPUTS, 32 Memory Bits. Let us now reference timing signals with the “T” previously defined “data_in”, “clock_in”
have a look at the others. timing periods starting from 131.072ms to and “sfht_ld” bits to obtain the input

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 PLC/MCU Feature

to be used. This means that with this

macro we can send output data serially to
as many TPIC6B595 registers as we wish.
However, as explained before, we
restrict this number to be 4 at most for the
sake of simplicity; “var0” is the beginning
of the 8-bit registers such as Q0 in RAM
from which the state of outputs are taken
and serially sent to TPIC6B595 registers.
This implies that there should be enough
RAM locations reserved after “var0” and,
also, there should be enough TPIC6B595
registers to hold the outputs.
There are some explanations within the
macro to describe how it works. As can be
seen, this macro makes use of previously
defined “data_out”, “clock_out” and
“latch_out” bits to send the output signals
serially to TPIC6B595 registers.
Figure 7: The macro “HC595” The macro “dbncr” is shown in Figure
8. It can be used for debouncing eight
independent buttons, switches, relay or
contactor contacts etc. The timing diagram
of one channel of this debouncer is
provided in Figure 9. It can be seen that
the output changes its state only after the
input becomes stable and waits in the
stable state for the predefined debouncing
time “dt1” or “dt2”. The debouncing is
applied to both rising and falling edges of
the input signal.
In this macro, each channel is intended
for a “normally open contact” connected
to the PIC by means of a pull-down
resistor, as this is the case with
UZAM_PLC. It can also be used without
any problem for a “normally closed
contact” connected to the PIC by means
of a pull-up resistor.
The “debouncing times” such as 20ms,
50ms or 100ms can be selected as
required depending on the application. It
is possible to pick up different debouncing
times for each channel. It is also possible
to choose different debouncing times for
rising and falling edges of the same input
signal, if necessary. This gives a good deal
of flexibility. This is simply done by
chancing the related time constant
“tcnst_01” or “tcnst_10” defining the
debouncing time delay for each channel
Figure 8: The macro “dbncr” and for both edges within the assembly
program.
Note that if the state-change-of-the-
signals from 74HC/LS165 registers. RAM locations, and serially sent out to and contact is shorter than the predefined
The macro “HC595” is shown in Figure stored in the related TPIC6B595 registers “debouncing time”, this will also be
7. The output signals are stored in the 8- by means of this macro. The “num” regarded as bouncing and it will not be
bit registers such as Q0, Q1, Q2 and Q3 in defines the number of TPIC6B595 registers taken into account. Therefore, no state-

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 Feature PLC/MCU

no state-change is issued. If the output

signal (rego,bito) = 0 and the input signal
(regi,biti) = 1, then with each “rising
edge” of the reference timing signal
“t_reg,t_bit” the related counter
“DBNCR+num” is incremented by one. In
this case, when the count value of
“DBNCR+num” is equal to the number
“tcnst_01”, this means that the input
signal is debounced properly and then
state-change from 0 to 1 is issued for the
Figure 9: Timing diagram of one channel of the debouncer output signal (rego,bito).
Similarly, if the output signal (rego,bito)
= 1 and the input signal (regi,biti) = 0,
change will be issued in this case. Each of within this macro. Then, we can set up then with each “rising edge” of the
the eight input channels of the debouncer both debouncing times dt1 and dt2 by reference timing signal “t_reg,t_bit” the
may be used independently from other means of time constants “tcnst_01” and related counter “DBNCR+num” is
channels. The activity of one channel does “tcnst_10” as dt1 = (t_reg,t_bit) x incremented by one. In this case, when
not affect the other channels. tcnst_01 and dt2 = (t_reg,t_bit) x the count value of “DBNCR+num” is
tcnst_10 respectively. equal to the number “tcnst_10”, this
THE MACRO “DBNCR” If the input signal (regi,biti) = 0 and the means that the input signal is debounced
Let us now briefly consider how the macro output signal (rego,bito) = 0 or the input properly and then state-change from 1 to
“dbncr” works. First of all, one of the signal (regi,biti) = 1 and the output signal 0 is issued for the output signal
previously defined reference timing signals (rego,bito) = 1, then the related counter (rego,bito). For this macro it is necessary
is chosen as “t_reg,t_bit” to be used “DBNCR+num” is loaded with “00h” and to define the following 8-bit variables in
RAM: Temp_1, and DBNCRRED. In
addition, it is also necessary to define
eight 8-bit variables in successive RAM
locations, the first of which is to be
defined as “DBNCR”. It is not necessary
to name the other seven variables. Each
bit of the variable DBNCRRED is used to
detect the “rising edge” of the reference
timing signal “t_reg,t_bit” for the related
channel.

THE MACRO “INITIALISE”

The macro “initialise” is shown in Figure
10. There are mainly two tasks carried out
within this macro. In the former, first
TMR0 is set up as a free-running hardware
timer with the 1/4 of 4MHz internal
oscillator signal, i.e. 1MHz, and with a
prescaler arranged to divide the signal to
256. In addition, PORTB is initialised to
make RB0 (data in) as input and the
following as outputs: RB3 (clock out), RB4
(data out), RB5 (latch out), RB6 (clock in),
RB7 (shift/load). In the latter, all utilised
RAM registers are loaded with initial “safe
values”. In other words, all utilised RAM
registers are cleared (loaded with 00h)
except for Temp_2, which is loaded with
06h. As explained before, Temp_2 holds
some special memory bits, therefore, the
initial values of these special memory bits
Figure 10: The macro “initialise” are put into Temp_2 within this macro. As
a result, these special memory bits are

www.electronicsworld.co.uk Electronics World - December 08 ❙ 33

 PLC/MCU Feature

loaded with the following initial values:

LOGIC0 (Temp_2,0) = 0, LOGIC1
(Temp_2,1) = 1, FRSTSCN (Temp_2,2) = 1,
SCNOSC (Temp_2,3) = 0.

THE MACRO “GET_INPUTS”

The macro “get_inputs” is shown in
Figure 11. There are mainly three tasks
carried out within this macro. In the first
one, the macro “HC165” is called with
the parameters “.1” and “bI0”. This
means that we will use only the “main
board” and, therefore, the macro
“HC165” is called with the parameter
“.1”. As explained before, the input
information taken from the macro is rated
as “bouncing” information and,
therefore, this information is stored in
“bI0” register. For example if we decide
to use the “main board” connected to
Figure 11: The macro “get_inputs” three of the “I/O extension board” then
we must call the macro “HC165” as
follows: “HC165 .4,bI0”. This will take
four 8-bit bouncing input information
from the 74HC/LS165 ICs and will put
them to the four successive registers
starting with the register bI0.
In the second task within this macro,
each bit of bI0, i (i = 0, 1… 7) is
debounced by the macro “dbncr” and
each debounced input signal is stored in
the related bit I0, i (i = 0, 1… 7). In
general, 20ms time delay is enough for
debouncing both rising and falling edges
of an input signal. Therefore, to achieve
these time delays, the reference timing
signal, obtained from Timer_1, is chosen
Figure 12: The macro “send_outputs” as T00 (0.512ms period) and both
“tcnst_01” and “tcnst_10” are chosen to
be “40”. Then we obtain the following:
dt1 = T00 x tcnst_01 = (0.512ms) x 40 =
20.48ms, dt2 = T00 x tcnst_10 =
(0.512ms) x 40 = 20.48ms.
Note that in this series of articles for
the sake of simplicity we use just 8 inputs
and 8 outputs. If we want to add more
inputs and to debounce these inputs,
Figure 13: A fraction of the first example program “UZAM_plc_8i8o_ex1.asm” then we must organise the macro
“dbncr” and locate necessary number of
registers within the RAM.
The last task is about incrementing the
Timer_2 on overflow of Timer_1. In this
task, “Timer_2” is incremented by one
when the falling edge of the bit
“Timer_1,7” is detected. In order to
detect the falling edge of the bit
Figure 14: A fraction of the second example program
“Timer_1,7”, “Temp_2,4” bit is utilised.
“UZAM_plc_8i8o_ex2.asm”

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PLC/MCU Feature

THE MACRO “GET_OUTPUTS” EXAMPLES respected outputs. That is to say that if

The macro “send_outputs” is shown in Up to now we have seen the hardware I0.0 = 0 then Q0.0 = 0 and, similarly, if
Figure 12. There are mainly four tasks and basic software necessary for I0.0 = 1 then Q0.0 = 1. This applies to all
carried out within this macro. In the first UZAM_PLC. It is now time to consider a 8 inputs and 8 outputs.
one, the macro “HC595” is called with the few examples. Before you can run the two For the second example, please include
parameters “.1” and “Q0”. This means simple examples considered here, you are the user program shown in Figure 14
that we will use only the “main board” expected to construct your own within the “UZAM_PLC_8i8o_1.asm” and
and, therefore, the macro “HC595” is UZAM_PLC hardware by using the then save it as
called with the parameter “.1”. necessary PCB files and producing PCBs “UZAM_PLC_8i8o_ex2.asm”. Next open it
As explained before, the output with its components. by MPLAB IDE and compile it. After that
information is taken from the register Q0 For the first example, include the user by using the PIC programmer software
and this macro sends the bits of Q0 serially program shown in Figure 13 within the take the compiled file
to TPIC6B595 register. For example, if we “UZAM_PLC_8i8o_1.asm” and then save “UZAM_PLC_8i8o_ex2.hex” and by your
decide to use the “main board” connected it as “UZAM_PLC_8i8o_ex1.asm”. The PIC programmer hardware send it to the
to three of the “I/O extension board” then next thing to do is to open it by MPLAB program memory of PIC16F648A
we must call the macro “HC595” as IDE and to compile it. After that, by using microcontroller within the UZAM_PLC.
follows: “HC595 .4,Q0”. Then, the macro the PIC programmer software take the After loading the
“HC595” will take four 8-bit output compiled file “UZAM_PLC_8i8o_ex1.hex” “UZAM_PLC_8i8o_ex2.hex”, switch the
information stored in Q3, Q2, Q1 and Q0 and by your PIC programmer hardware, 4PDT in “RUN” and the power switch in
and send them serially to the four send it to the program memory of “ON” position. Now, you are ready to test
TPIC6B595 register ICs respectively. PIC16F648A microcontroller within the the second example program. In this case
In the second task within this macro, the UZAM_PLC. the contents of “Timer_2” register are
watchdog timer is cleared. In the third task, After loading the transferred to 8 outputs.
the “FRSTSCN” special memory bit is reset. “UZAM_PLC_8i8o_ex1.hex”, switch the Of course, in these two examples the
As the final task, within this macro the 4PDT in “RUN” and the power switch in user programs include only PIC Assembly
“SCNOSC” special memory bit is toggled “ON” position. Now, you are ready to test instructions, but we will start using our
after a program scan, i.e. when it is “1” it the first example program. You can see own macro-based PLC instructions in the
is reset, when it is “0” it is set. that all the inputs are transferred to the next article. ■

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