Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

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