Recently I was struggling to achieve very accurately synchronised camera and light triggers for a real-time computer vision project that I was working on, my original PIC embedded system for triggering everything using a PIC micro-controller had a fairly reliable accuracy of about 250us which was sufficient for a few years, but for various reasons this needed to be reduced by a factor of 10 to about 25us. The problem was that as I reduced the timer’s overflow period it started to become quite inaccurate and erratic.<\/p>\n
It is possible to achieve quite accurate timing on your embedded software system using a PIC, however it can take a bit of work to tune things. Most articles that introduce the use of timers on the PIC use the Timer0 module as an example, maybe because it has a ‘0’ in it?<\/p>\n
Now, when using Timer0 to produce a repeated time interval, its High & Low overflow registers must be manually reset in the interrupt handler to setup the next time period, all while the timer is still counting away.<\/p>\n
This it makes it difficult to know what values to put into the registers to get an accurate overflow period – you end up trying to account for the interrupt latency and counting assembly instructions in the interrupt handler to estimate times etc.<\/p>\n
The upshot is that it’s hard to get a reliable overflow period with a desired accuracy when using Timer0.<\/p>\n
Example, initial Timer0 setup code:<\/p>\n
\r\nvoid init_timers() {\r\n\r\n \/\/ See http:\/\/www.enmcu.com\/software\/timer0calculatorandcodegeneration\r\n \/\/ for values.\r\n\r\n INTCON.GIE=1; \/\/ global interrupt enable\r\n INTCON.PEIE=1; \/\/ peripheral interrupt enable\r\n INTCON.TMR0IF = 0x0; \/\/ Clear timer0 overflow interrupt flag\r\n INTCON.TMR0IE = 1; \/\/ enable the timer0 by setting TRM0IE flag\r\n\r\n T0CON.T08BIT = 0; \/\/ 16 Bit timer\r\n T0CON.T0CS = 0; \/\/ Internal clock\r\n T0CON.PSA = 1; \/\/ Set scaler to 1:4\r\n\r\n \/\/ Set timer counts\r\n TMR0H = TIMER_HIGH;\r\n TMR0L = TIMER_LOW;\r\n\r\n T0CON.TMR0ON = 1; \/\/ Turn Timer0 on.\r\n}\r\n\r\n\/\/ interrupt handler for the timer overflow\r\nvoid interrupt(void) {\r\n\r\n \/\/ Is this a Timer 0 interrupt?\r\n if (INTCON.TMR0IF) {\r\n \/\/ yes!\r\n \r\n \/\/ Reset the timer values\r\n \/\/ These are actually now incorrect\r\n \/\/ as time has elapsed since the timer\r\n \/\/ overflow and now!\r\n TMR0H = TIMER_HIGH;\r\n TMR0L = TIMER_LOW;\r\n \r\n \/\/ Reset interrupt flag\r\n INTCON.TMR0IF = 0;\r\n\r\n \/\/ Increase some counter\r\n ++counter;\r\n }\r\n}\r\n<\/pre>\nThe best thing to do is to ditch Timer0 and switch to Timer2 instead. Timer2 automatically handles this kind of pattern (and it’s not much harder to use even though it’s got a ‘2’ in its name!).<\/p>\n