;Title 1khurts.asm ;a 1.00kHz square wave generator. ;program runs a xtal oscillator PIC16f872 to exactly 1000 hertz. ;comments: email@example.com include "P16f872.inc" ; use definition file for 16F872 ; configure the chip for a crystal oscillator, with code protection and watchdog timer off. __CONFIG _CP_OFF & _XT_OSC &_WDT_OFF ; define RAM =, give RAM registers names. CBLOCK 0x20 ; RAM starts at address 20h d1 ENDC ; org 0x0000 ; start address = 0000h ; INITIALISE PORTS ; binary used to see individual pin level movlw b'00000000' ; all port pins = low movwf PORTA movlw b'00000000' movwf PORTB movlw b'00000000' movwf PORTC bsf STATUS,RP0 ; set RAM Page 1 for TRIS registers ; initialize ports ; set all pins to outputs, and analog pins to digital. movlw b'00000000' ; all IO pins = outputs movwf TRISA movlw b'00000000' movwf TRISB movlw b'00000000' movwf TRISC movlw b'00000110' ; all analog pins = digital movwf ADCON1 bcf STATUS,RP0 ; back to RAM page 0 ; LED FLASH LOOP Loop bsf PORTC,4 ; RC4 = high nop ; these two nop's make each half of cycle the same delay. nop call Delay bcf PORTC,4 ; RC4 = low call Delay goto Loop ; trial 1kHz delay subroutine ; delay is period/2 ; 1kHz = 1000/s => period = .001 = 1ms ; 1ms/2 = 0.5 ms = .0005s ; 0.0005x1Mips = 500 instruction cycles (clock is 4MHz). ; 500 - 5 (overhead of calling delay, and looping) = 495 Delay movlw 0xA4 ;A4 = 164, see below, Delay is 2 instruction cycles so 495 - 2 = 493. movwf d1 Count decfsz d1 ;493 required by Count, Count is 3 instructions each loop + 1 for skip, 492, w/o skip 492/3=164=A4 goto Count return END
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<A HREF="http://www.piclist.com/techref/microchip/1khurts.htm"> PIC 16F872 oscillator at exactly 1000 hertz</A>
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