Timers - what you should know

[newguy]

Reposted from the other forum.... I figured it would be useful to have it here too.

Hopefully this helps to clear up some of the confusion regarding timers. I will use an example that is driven by the internal (system) clock.

Whatever the master crystal frequency is, the internal (system) clock is always the crystal frequency divided by 4.

Example:

Crystal: 4 MHz
System clock: 1 MHz (1 microsecond period)

The system clock is what drives the timer modules (if set up to use the internal clock.) However, there is a prescaler which can also be set. A prescaler is a programmable "divide by N" block where N is the number of input clock cycles it takes to get one output cycle out of the prescaler.

Example:

Let's assume that you want to use timer 0. Timer 0's prescaler can be set to 1 (no divide at all), 2, 4, 8, ....., all the way up to 256. If it was set to 256, then it would take 256 system clock cycles to get one cycle out of the prescaler. Therefore timer 0 would then count up at a rate of one count every 1 microsecond (system clock) x 256 (prescaler setting) = 256 microseconds/count.

Note that some timers can also have postscalers.

A timer will generate an interrupt every time it overflows, and if its' interrupt is enabled. Timer 0 can be set to either 8 bit mode or 16. In 8 bit mode, it overflows every 256 counts. It would take 65,536 counts in 16 bit mode.

To completely set up timer 0 in 8 bit mode with the system clock as its source and with a prescaler of 256, you would code:

Code:

[nucloid]

Thank you for the informative post -- a very nice tutorial for us newbies.

I very much appreciate it.


cheers


(now I am looking for "interrupts and interrupt driven routines for the first timer")

[asmboy]

before starting to code with CCS -c i did a lot in pure ASM - one aspect of dealing with timers in a NON interrupt fashion is to set a prescaler value early in the program for a 16 bit timer to get a useful 'tick' increment - then set the timer count to a value that will give your timeout on overflow and then poll said timer's overflow bit to see if the time limit has been exceded - since READING the bit is the only reset - no matter how many times the timer may have rolled over beyond the MINIMUM timeout you set - you still know - that by - checking for a '1' in the overflow bit tells you are sure your time is Up. for timer1 it is the TMR1IF bit in the PIR1 register- for example on the 16f887 - all the other major timers have the same kind of flag- BEWARE that some VERY low end pics DO NOT - but talking really low end like 16c5xx and so on. hope you can use this tidbit

[smiles]

[newguy]

It's all in the original post, but I'll restate it slightly differently.

A timer never stops unless your program stops it. It's always ticking away, regardless of what your code is having the processor do.

An interrupt is just that - it causes the processor to "interrupt" whatever it is doing at the moment the interrupt occurs. Sort of like your doorbell. When your doorbell rings, are you able to instantaneously answer the door? No. You may be sitting down reading a book or standing in the kitchen cooking. Whatever you're doing, you must first set whatever you're doing aside in a 'safe' manner so that you can return to that task at some point in the future and pick up where you left off. If you're reading a book, you'll use a bookmark to save your place, and if you're cooking, you may turn down the stove so that whatever you're cooking won't burn.

So, with this in mind, let's mentally tackle a timer interrupt. A timer interrupt 'fires', and the processor begins the rather long task of setting everything aside so that it can attend to the interrupt. While it is doing this - saving work (data/registers), then determining which interrupt(s) require attention, and finally jumping into the ISR which needs to be performed - the timer is still ticking away.

Exactly what the 'count' of the timer actually is when your interrupt gets around to servicing it is unknown. There is no way of knowing what it will be if you have a program with interrupts other than the timer itself. So, if an accurate timer-derived clock is necessary, you must somehow work around this uncertainty. That's where the 6 in the equation in the original post came about.

In that example, 250 'ticks' = 1ms. However, that timer only fires when it rolls over from 255 to 0 (which is 256 'ticks'). To get the timer to skip 6 'ticks', your ISR must read the current value of the timer's count, add 6 to it, and then replace this higher value in the timer's count register.

Clear?

[smiles]

Yes, I understand why we must skip 6
So
void main() {
int i;
setup_timer_0(RTCC_INTERNAL|RTTC_8_BIT|RTCC_DIV_4);
enable_interrupts(INT_RTCC);
enable_interrupts(GLOBAL);
...
i=0;
...
while(TRUE); // loop forever
}
When it receives command enable_interrupts(GLOBAL), it must finish declaring i=0, then go to RTCC_isr(), add the value of timer resistor to 6, then the timer starts and it back to void main() ?
is that right ?
if right, then when the timer value transit between 255 and 0, one_millisecond will be TRUE ?
it also mean
void RTCC_isr(void) {
timer0low = timer0low + 6; // the timer active here
one_millisecond = TRUE; //ignore this command and turn back to main program until timer value transit between 255 and 0 ?
}
Hope you answer me, thanks so much !

[newguy]

[smiles]

Thanks so much

[hayee]

Thanks everyone for this timer0 tutorial. I have learnt lot of things from that. I have a simple question regarding this. Maybe this is a stupid question. I want to know that what this line means or what is the function of this line?

[newguy]

That address, 0xfd6, is the address of the low byte of timer 0 in that PIC's memory. You can find this information in the PIC's data sheet in the memory organization - special function registers - section and in the timer 0 section itself. That line of code assigns a variable, timer0low, to fall at that address. So a read/write of that variable directly reads/writes the low byte of timer 0 itself.

[vinceykw]

may i know where is the 4us come from...

[PCM programmer]

Read the first part of his explanation:

[asmallri]

[harshanahnd]

Hi,

How to setup timer0 in 16 bit mode? What is the keyword that we should use in setup_timer_0 function?

Thanks

[rhaguiuda]