Code:
+LIST P=16F506 ; DIRECTIVE TO DEFINE PROCESSOR ;-------------------------------------------------------------------------------------------------------------------------------------- ; FILE DESCRIPTION ;-------------------------------------------------------------------------------------------------------------------------------------- ; ; EVERY TIMER PERIOD THE MAIN CODE EXECUTES FIRST. THEN THE PROGRAM WILL WAIT FOR THE TMR0 VALUE TO REACH 255 SO THAT IT WILL KNOW ; WHEN THE TMR0 ROLLOVER IS ABOUT TO HAPPEN AND SYNCHRONIZES ON THAT ROLLOVER TO REACH INSTRUCTION CYCLE PRECISION. ; ; THE FOLLOWING CODE DEMONSTRATES HOW TIMED EVENTS CAN BE CREATED USING TIMER0, THE PERIOD IS FIXED AND IS THE PERIOD OF THE TIMER0 ; ROLLOVER (DIFFERENT PRESCALER VALUES MAY BE USED) ; TIMER0 MAY BE PRELOADED FROM FOR DIFFERENT TIMING (PLEASE SEE NOTE *ADVANCED * BELOW) ; ; THE MAIN LOOP WITH USER CODE DOES NOT HAVE TO HAVE ISOCHRONOUS LOOPS ! ANYTHING UP TO 65276 INSTRUCTION CYCLES WILL DO ! ; ;-------------------------------------------------------------------------------------------------------------------------------------- ; VARIABLE DEFINITION ;-------------------------------------------------------------------------------------------------------------------------------------- DELAYCOUNT EQU H'0D' ; USED IN A FIXED DELAY ;-------------------------------------------------------------------------------------------------------------------------------------- ; PROGRAM CODE ;-------------------------------------------------------------------------------------------------------------------------------------- ORG H'0000' ; RESET VECTOR INITIALIZE MOVLW B'11000111' ; CONFIGURE TIMER0 TO RUN FROM INSTRUCTION CLOCK WITH 1:256 PRESCALER OPTION ; WE WILL HAVE ROLLOVERS EVERY 65.5 MS (15.3 HZ) @ FOSC = 4 MHZ MOVLW D'254' ; PRELOAD TIMER0 FOR FAST STARTUP MOVWF TMR0 ; ;------- ----------------------------------------------------------------------------------------------- SYNCHRONIZE COMF TMR0,W ; ROUGH SYNCHRONISATION: WAIT FOR TIMER0 TO REACH 255 SKPZ ; IS TMR0 NOW 255 ? GOTO SYNCHRONIZE ; NO, GO LOOK AGAIN ; TIMER0 NOW HAS VALUE 255, THE PRESCALER VALUE IS NOW 3,4,5 OF 6 ;------- ----------------------------------------------------------------------------------------------- ; HERE THERE MUST BE A DELAY OF EXACTLY 248 INSTRUCTION CYCLES WHEN USING A 1:256 PRESCALER ; (CAN BE ANY USEFUL CODE) MOVLW D'82' ; 247 INSTRUCTION CYCLES MOVWF DELAYCOUNT ; DECFSZ DELAYCOUNT,F ; GOTO $-1 ; NOP ; 1 INSTRUCTION CYCLE ;------- ----------------------------------------------------------------------------------------------- ; FINE SYNCHRONISATION: WE WILL ARRIVE HERE JUST BEFORE THE PRESCALER ROLLS OVER (AND ; INCREMENTS TIMER0), HERE WE WILL COUNT HOW MANY INSTRUCTION CYCLES WE ARRIVED BEFORE THE ; ACTUAL ROLLOVER - WHICH GIVES US THE TIME WE NEED TO DELAY BCF STATUS,5 ; SELECT PAGE 0, WE CAN ONLY JUMP TO A DESTINATION IN THE LOWER 256 INSTRUCTIONS OF A MEMORY ; PAGE, A MEMORY PAGE IS 512 INSTRUCTIONS IN SIZE MOVLW D'3' ; ASSUME WE WILL HAVE TO SKIP THE DELAY (JUMP THREE INSTRUCTIONS) ADDWF TMR0,W ; W = UNCHANGED OR W = W - 1 (ADD TO THE DELAY DEPENDING ON HOW MUCH TO SOON WE ARRIVED) ADDWF TMR0,W ; W = UNCHANGED OR W = W - 1 (ADD TO THE DELAY DEPENDING ON HOW MUCH TO SOON WE ARRIVED) ADDWF TMR0,W ; W = UNCHANGED OR W = W - 1 (ADD TO THE DELAY DEPENDING ON HOW MUCH TO SOON WE ARRIVED) ; W IS NOW 0,1,2 OF 3 (0 = THREE CYCLES DELAY, 3 = NO DELAY) ADDWF PCL,F ; DELAY AS MUCH AS NEEDED /JUMP AHEAD AS MUCH AS NEEDED NOP ; PCL + 0 NOP ; PCL + 1 NOP ; PCL + 2 ;------- ----------------------------------------------------------------------------------------------- PSEUDO_INT ; PCL + 3 ; THE TIMER0 VALUE IS NOW 0, THE PRESCALER VALUE WILL ALWAYS BE 4 ; WE WILL GET BACK HERE EVERY 65536 INSTRUCTIONS (1:256 PRESCALER) ; WE MAY USE UP TO 65276 INSTRUCTION CYCLES HERE (1:256 PRESCALER) ;MOVLW D'32' ; * ADVANCED * ;MOVWF TMR0 ; NOTE:TO CHANGE THE PERIOD, WE CAN PRELOAD TIMER0 HERE, OR, WE CAN DELAY A BIT FIRST TO CHANGE ; THE PRESCALER VALUE, AND THEN PRELOAD TIMER0) ; * ADVANCED * NOP ; DO SOMETHING NOP ; DO SOMETHING MORE ; ETC... ; ETC... ; ETC... ;------- ----------------------------------------------------------------------------------------------- GOTO SYNCHRONIZE ; ;--------------------------------------------------------------------------------------------------------------------------------------
file: /Techref/member/AT-planet-T9/High precision timed events without interrupts for 12-bit PIC (instruction cycle accurate) using a non-isochronous main loop.htm, 6KB, , updated: 2010/10/2 12:57, local time: 2024/11/24 14:10,
owner: AT-planet-T9,
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