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

The sender program, named SerialSnd, performs the following initialization operations:

1.Line RA2 is initialized for input since the pushbutton switch is located on this line. Lines RB0 to RB7 are also input, since they are connected to the DIP switch array.

2.The prescaler is assigned to the Watchdog Timer so that channel TMR0 runs at full processor speed.

3.Interrupts are disabled.

Initialization code is as follows:

; Port-A, bit 2 is input. All others are output

;Prepare to set prescaler clrf tmr0 clrwdt

;Setup OPTION register for full timer speed

;|__________________________ RBPU pullups

Once initialized, the program performs the following functions:

1.The SEND READY LED is turned on.

2.Code monitors the SEND pushbutton switch.

3.Once the switch is pressed, the program turns off the SEND READY LED.

4.The state of the DIP switches is obtained by reading RB0 to RB7.

5.The byte from Port-B is sent through the serial line.

The following code fragment shows the 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 transmission

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

return