14.5 Sample Programs

;A pushbutton switch is in the 16F84 RESET line

;

;Communications parameters:

;Timer channel TMR0 is used for synchronizing data

;transmission. The timer runs at the maximum rate of

;256 cycles per iteration. In a 4Mhz system the

;timer rate is 1Mhz, thus the bit rate is

;which is approximately 3,906 microseconds per bit.

;===========================

;switches

;===========================

; Switches used in __config directive:

;_PWRTE_OFF

;|

;|_____ * indicates setup values presently selected

;========================= ; setup and configuration ;=========================

;============================================================

; M A C R O S

;============================================================ ; Macros to select the register banks

ENDM

;=====================================================

; constant definitions for pin wiring

;=====================================================

;=====================================================

; PIC register flag equates

;=====================================================

;======================================================

; variables in PIC RAM

;=====================================================

;=========================================================

; program

;=========================================================

main:

; Port-A, bit 2 is input. Rest is output

;Prepare to set prescaler clrf TMR0 clrwdt

;Setup OPTION register for full timer speed

;|__________________________ RBPU pullups

;wait for READY switch

;to be pressed ;=========================== ready2send:

btfsc PORTA,readySW goto ready2send

;===========================

;send serial data ;===========================

;At this point program proceeds to send data through

;the serial port line

;Turn off LED

bcf PORTA,readyLED

; Read switches and store in local variable movf PORTB,w

movwf dataReg ;===========================

;call serial output

;procedure ;===========================

;===========================

endloop:

goto endloop

;============================================================ ; procedure to send serial data ;============================================================

;ON ENTRY:

;local variable dataReg holds 8-bit value to be

;transmitted through port labeled serialLN

;OPERATION:

;1. The timer at register TMR0 is set to run at

;maximum clock speed, that is, 256 clock beats.

;The timer overflow flag in the INTCON register

;is set when the timer cycles from 0xff to 0x00.

;2. Each bit (start, data, and stop bits) is sent

;at a rate of 256 timer beats. That is, each bit is

;held high or low for one full timer cycle (256

;clock beats).

;3. The procedure tests the timer overflow flag

;(tmrOVF) to determine when the timer cycle has

;ended, that is when 256 clock beats have passed.

;

sendData:

movwf bitCount

;=======================

;send START bit ;=======================

;Set line low then hold for 256 timer clock beats.

;send 8 DATA bits ;========================

;Eight data bits are sent through the serial line

;starting with the high-order bit. The data byte is

;stored in the register named dataReg. The bits are

;rotated left to the carry flag. Code assumes the bit

;is zero and sets the serial line low. Then the carry

;flag is tested. If the carry is set the serial line

;is changed to high. The line is kept low or high for

;256 timer beats.

send8:

;Code can assume the bit is a zero and set the line

;low since, if low is the wrong state, it will only

;remain for two timer beats. The receiver will not

;check the line for data until 128 timer beats have

;elapsed, so the error will be harmless. In any case,

;there is no assurance that the previous line state is

;the correct one, so leaving the line in its previous

;state could also be wrong.

;At this point timer has cycled.

;Test for end of byte, if not, send next bit

;=========================

;hold MARKING state ;=========================

;All 8 data bits have been sent. The serial line must

;now be held high (MARKING) for one clock cycle

;========================================================= ; end of program ;=========================================================

end

14.5.2 SerialRcv Program

;File name: SerialRcv.asm

;Date: May 6, 2006

;Author: Julio Sanchez

;Processor: 16F84A

;Description:

;Two programs to exercise serial communications between

;two PIC 16F84A both running at 4Mhs. One program sends

;data through a single line and the other one receives

;it. This program is the receiver.

;

;Circuit:

;Port A0 is the serial transmission line

;Port A2 is an active-low pushbutton switch that

;Port A3 is a LED that is ON when the program is

;Port-B0-B7 are 8 LEDs that display the data bits

;A pushbutton switch is in the 16F84 RESET line

;

;Communications parameters:

;Timer channel TMR0 is used for synchronizing data

;transmission. The timer runs at the maximum rate of

;256 cycles per iteration. In a 4Mhz system the

;timer rate is 1Mhz, thus the bit rate is

;which is approximately 3,906 microseconds per bit.

;Upon receiving the START bit, the program waits for

;one half a clock cycle (128 timer beats) to

;synchronize with the sender. ;===========================

;switches

;===========================

; Switches used in __config directive:

;_PWRTE_OFF

;|

;|_____ * indicates setup values presently selected

;========================= ; setup and configuration ;=========================

;============================================================

; M A C R O S

;============================================================ ; Macros to select the register banks

;=====================================================

; constant definitions for pin wiring

;=====================================================

;=====================================================

; PIC register and flag equates

;=====================================================

;======================================================

; variables in PIC RAM

;=====================================================

;=========================================================

; program

;=========================================================

main:

Bank1

; Port-A bits 0 and 2 are input. All others are output

;Prepare to set prescaler clrf TMR0 clrwdt

;Setup OPTION register for full timer speed

;|__________________________ RBPU pullups

;=========================

;wait for READY switch

;to be pressed ;=========================

;data received ;===========================

;Turn ready to receive LED off

;============================================================

;============================================================

;ON ENTRY:

;local variable dataReg is used to store 8-bit value

;received through port (labeled serialLN)

;OPERATION:

;1. The timer at register TMR0 is set to run at

;maximum clock speed, that is, 256 clock beats.

;The timer overflow flag in the INTCON register

;is set when the timer cycles from 0xff to 0x00.

;2. When the START signal is received, the code

;waits for 128 timer beats so as to read data in

;the middle of the send period.

;3. Each bit (start, data, and stop bits) is read

;at intervals of 256 timer beats.

;4. The procedure tests the timer overflow flag

;(tmrOVF) to determine when the timer cycle has

;ended, that is when 256 clock beats have passed.

;=============================================================

rcvData:

;=========================

; wait for START bit

;=========================

;=========================

;offset 128 clock beats ;=========================

;At this point the receiver has found the falling

;edge of the start bit. It must now wait 128 timer

;beats to synchronize in the middle of the sender’s

;data rate, as follows:

;======================== return

;========================================================= ; end of program ;=========================================================

end

14.5.3 Serial6465 Program

;File name: Serial6465.asm

;Last update: May 7, 2006

;Author: Julio Sanchez

;Processor: 16F84A

;Description:

;Program to exercise serial communications using a

;PIC 16F84A and two shift registers: a 74HC164, and a

;74HC165. The 74HC165 inputs 8 lines from a DIP switch

;and transmits settings to PIC through a serial line.

;PIC sends data serially to an 74HC164 which is wired

;to 8 LEDs that display the received data. A total of

;6 PIC lines are used in interfacing 8 input switches

;to 8 output LEDs.

;Circuit:

;* Port A0 is the serial transmission line which

;comes from the 74HC165.

;* Port A1 is wired to the 74HC164 CLOCK pin

;* Port A2 is wired to the 74HC164 CLEAR pin

;* 74HC164 output pins 0 to 7 are wired to LEDs.

;* Port B0 is wired to the 74HC165 Hout line

;* Port B1 is wired to the 74HC165 CLK line

;* Port B2 is wired to the 74HC165 load line

;* A pushbutton switch is in the 16F84 RESET line

;and serves to restart the program

;Communications protocol:

;Communication between PIC and the 74HC164 and

;74HC165 is synchronous since the shift registers

;clock lines serve to shift in and out the data

;bits.

;

;===========================

;switches ;===========================

;Switches used in __config directive:

;=========================================================

; program

;=========================================================

main:

; Port-A is all output Bank1

movlw b’00000000’ movwf TRISA

;Port-B line 0 is input, all others are output movlw b’00000001’

movwf TRISB Bank0

;Make sure Port-A line 2 (clear line) is high movlw b’00000100’

movwf PORTA

;dataReg contains input ;===========================

;call serial output

;procedure ;===========================

call out164 ; Call serial output procedure

;===========================

;wait forever ;=========================== endloop:

goto endloop ;============================================================

;74HC164 procedure to send serial data ;============================================================

;ON ENTRY:

;local variable dataReg holds 8-bit value to be

;transmitted through port labeled serialLN

;OPERATION:

;1. A local counter (bitCount) is initialized to

;8 bits

;2. Code assumes that the first bit is zero by

;setting the data line low. Then the high-order

;bit in the data register (dataReg) is tested.

;If set, the data line is changed to high.

;3. Bits are shifted in by pulsing the 74HC164

;clock line (CLK).

;4. Data bits are then shifted left and the bit

;counter is tested. If all 8 bits have been sent

;the procedure returns.

;Using this assumption is possible because the bit is not

;shifted in until the clock line is pulsed.

;=========================

;pulse clock line ;=========================

;Bits are shifted in by pulsing the 74HC164 CLK line

;============================================================

;74HC165 procedure to read parallel data and send

;============================================================

;OPERATION:

;1. Eight DIP switches are connected to the input

;ports of an 74HC165 IC. Its output line Hout,

;and its control lines CLK and load are connected

;to the PIC’s Port-B lines 0, 1, and 2

;respectively

;2. Procedure sets a counter (bitCount) for 8

;iterations and clears a data holding register

;(dataReg).

;3. Port-B bits are read into w. Only the lsb of

;Port-B is relevant. Value is stored in a working

;register and the meaningful bit is rotated into

;the carry flag, then the carry flag bit is

;then shifted into the data register.

;4. The iteration counter is decremented. If this

;is the last iteration the routine ends. Otherwise

;the bitwise read-and-write operation is repeated.

;======================================================== end

14.5.4 TTYUsart Program

;File name: TTYUsart.asm

;Last update: May, 2006

; Author: Julio Sanchez

;Processor: 16F84A

;Description:

;Program to emulate USART operation in PIC code. Uses

;PIC-to-LCD interface. Display has 2 lines, each with

;16 characters.

;Program operation:

;Characters received from the RS232 line are displayed on

;the LCD. LCD lines scroll automatically. A pushbutton

;activates the send operation by transmitting the text

;string: Readywhich is also displayed on the LCD.

;

;Program communications and LCD parameters are stored in

;#define statements. These statements can be edited to

;accommodate a different set-up. Program uses delay loops

;for interface timing.

;

;WARNING:

;Code assumes 4Mhz clock. Delay routines must be

;edited for faster clock

;

;BAUD RATE CALCULATIONS:

;A 4Mhz clock oscillator has a clock frequency of 1 Mhz:

;Since the baud rate is the number of clock cycles per

;second, for a 4Mhz clock it is:

;bit time = ——— sec. = 208.33 microseconds

;4,800

;Calculating one half the baud rate allows resetting the

;clock from the edge to the center of a time pulse:

;

;|<======== falling edge of start bit

;| |

;208/2 = 104

;The PIC clock counts up from 0 to 255. So to implement

;a 104 microsecond delay we must start counting at

;clock beat:

;plus one microsecond for movlw instruction used to

;initialize the clock:

; For one full baud rate delay:

;The following two constants are stored in #define

;statements:

;Setting the prescaler to TMR0 reduces the baud rate

;to one-half. Other prescaler values will reduce the

;baud rate accordingly.

;

;Wiring diagram:

;RB4-RB7 ===> LCD data lines 4 to 7 (output)

;RB0 =======> MAX202 T2in line (output)

;RA0 =======> MAX202 R2out line (input)

;RA1 =======> LCD E line (output)

;RA2 =======> LCD RS line (output)

;RA3 =======> LCD R/W line (output - not used)

;RA4 =======> Pushbutton switch 1

; (input - active low)

;

;===========================

;switches ;===========================

;Switches used in __config directive:

;_PWRTE_OFF

;|

;|_____ * indicates setup values presently selected

;========================= ; setup and configuration ;=========================

processor 16f84A include <p16f84A.inc>

__config _XT_OSC & _WDT_OFF & _PWRTE_ON & _CP_OFF

;============================================================ ; M A C R O S ;============================================================

#define LCDlimit .16; Number of characters per line

;4800 baud clock countdown values

;Code reduces rate to 2400 baud by entering a minimal

;presclaer to TRM0

#define halfBaud .152 ; For one-half bit time

#define fullBaud .48 ; For one full bit time

;

;Note: The constants that define the LCD display line

;addresses have the high-order bit set in

;order to facilitate the controller command

;

;=====================================================

;buffer and variables in PIC RAM ;=====================================================

;Create a 16-byte storage area

408 Chapter 14

; Variables for serial communications

tempData ; Temporary storage for bit manipulations

;RB4-RB7 ===> LCD data lines 4 to 7 (output)

;RB0 =======> MAX202 T2in line (output) movwf TRISB

Bank0

;Clear bits in Port-A output lines

call delay_5 call initLCD

initialization

call delay_5 ; Wait again

; Set Port-B, line 0 high so start bit is detected bsf PORTB,0

;============================

;wait for start command ;============================

;Program waits until pushbutton number 1 is pressed

;to continue execution. Pushbutton 1 is active low

;and wired to RA4

pb1Wait:

;display and send “Ready-” ;============================

;Set LCD base address

call line1

; Initialize system for UART emulation at 2400 baud call initTTY

;Display on LCD and test serial transmission by sending

;the string “Ready-”

;Store character in local line buffer using indirect

;addressing

;16-byte buffer named lineBuf starts at address 0x0c

;============================================================ ; initialize LCD for 4-bit mode ;============================================================ initLCD:

;Initialization for Densitron LCD module as follows:

;4-bit interface

;2 display lines of 16 characters each

;cursor on

;left-to-right increment

;cursor shift right

;no display shift

; Set 4-bit mode command must be repeated movlw 0x28

;=======================

;Procedure to delay

;42 microseconds ;======================= delay_125:

;=======================

;Procedure to delay

;5 milliseconds ;======================= delay_5:

;=============================

;long delay sub-routine

;(for debugging) ;============================= long_delay

;========================

;send 2 nibbles in

;4-bit mode ;========================

;Procedure to send two 4-bit values to Port-B lines

;7, 6, 5, and 4. High-order nibble is sent first

;ON ENTRY:

;w register holds 8-bit value to send

send8:

;=================

;merge bits ;=================

;Routine to merge the 4 high-order bits of the

;value to send with the contents of Port-B

;so as to preserve the 4 low-bits in Port-B

;Logic:

;AND value with 1111 0000 mask

;AND Port-B with 0000 1111 mask

;Now low nibble in value and high nibble in

;PortB are all 0 bits:

;PortB = 0000 bbbb

;OR value and Port-B resulting in:

;ON ENTRY:

;w contains value bits

;ON EXIT:

;w contains merged bits

merge4:

;Set address register

;to LCD line 1 ;========================

;ON ENTRY:

;Address of LCD line 1 in constant LCD_1

line1:

;========================

;Set address register

;to LCD line 2 ;========================

;ON ENTRY:

;Address of LCD line 2 in constant LCD_2

line2:

return

;==========================

;scroll LCD line 2 ;==========================

;Procedure to count the number of characters displayed on

;each LCD line. If the number reaches the value in the

;constant LCDlimit, then display is scrolled to the second

;LCD line. If at the end of the second line, then the

;second line is scrolled to the first line and display

;continues at the start of the second line

;reset to the first line.

;At this point the end of the LCD line was reached

;Test if this is also the end of the second line

;End of second LCD line line2End:

;Scroll second line to first line. Characters to be

;scrolled are stored in buffer starting at address 0x0c.

;16 characters are to be moved

;First clear LCD

;============================

;clear line buffer ;============================

;Use indirect addressing to store 16 blanks in the

;buffer located at 0x0c

;============================================================ ; initialize for TTY ;============================================================

;Procedure to initialize RS232 reception

;Assumes:

;First initialize receiver to RS-232-C line parameters

;Disable global and peripheral interrupts

;|______________________ Global interrupts