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I have learned that the maximum 'delay' possible in pic16f877a running with a source of 11.0592MHz (11059200 / 4 actually) is 23 ms.. without using software delays (delay_ms( ))..

the calculation was max_delay = ((FF-00)xPrescalarxTimeperiod) = (255x256x3.62xe-7) = ~23ms..

My question is how to 'increase this delay to 1 second' without using software delay?

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Are you setting up the prescaler for the WDT or for a timer? – Thomas O Oct 26 '10 at 9:25
for timer only.. please help me.. – Vicky Rao Oct 26 '10 at 9:53
1  
Unfortunately it's not possible without some kind of software delay, so here's one way to do it: every 23 ms run a software interrupt which increments a counter, after 43 counts you will have reached 0.989 seconds; for more precise results use a faster timer setting and increase counts appropriately. – Thomas O Oct 26 '10 at 10:27
Sir, I agree with what you said.. I will work on it.. Thank you very much.. – Vicky Rao Oct 26 '10 at 10:43
I would like to specify, it is possibly to do it without using a software delay similar to delay_ms which is very inefficient. a counter in an interrupt is maybe 30 clock cycles every 20 mS. – Kortuk♦ Oct 26 '10 at 23:13

1 Answer

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Why is it hard?

This is not the easiest thing to do. you have picked a crystal oscillator that is specifically chosen to give easy division to common baud rates.

For example:

11.0592MHz/9600 is 1152

11.0592MHz/115200 is 96

That means you can hit some standard baud rates perfectly with a certain divider. Gives very consistent UART. Now you want to count time.

Counting 1 Second

If you take a 11.0592MHZ then divide by 256 and 64 you get a value of 675. This means if you want an accurate count of seconds you need to have that interrupt every 256 * 64 clock cycles and count with a variable in your interrupt you can hit it.

How to make it easier

You have two options, accepting error. First, your equation you divided by 255 since you are counting from 0 to 255, but since the interrupt is on overflow it is actually as you count to 256, so you should be dividing by 256.

My clock has error?

Decide how much precision you need. Often you do not need spot on precision, if this is the case, accept that it is almost a second and use what Thomas said in comment. Count to as close as you can get and cope. Timer 1 will allow you to divide by 65536 and spend more time doing things other than handling interrupts.

I need precision in my clock!

You need to pick an oscillator value that is a multiple of 32768 Hz. What you want is something that can be reduced to 1Hz from division by a power of 2.

If you need the nice baud rate and a nice counter, the wiki page has a mark for RTC(good for real time clock). 4.608 MHz is a great thing, it divides to 1Hz pretty easy and is good for 115200 baud also.

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As a side note, RTCs still suffer from error and need to be calibrated. – Kortuk♦ Oct 27 '10 at 1:31
With the guidance of Mr.Kortuk, Kellen and Thomas I have increased the delay.. The code goes like this: – Vicky Rao Oct 27 '10 at 4:13
#int_timer0 timer0ISR() { x=x+1; if(x==43) {portd= ~portd; x=0;}}.. followed by 'main'.. I have connected LEDs to portd that blinks at an interval of 1 second, without any software delay.. thank you all.. – Vicky Rao Oct 27 '10 at 4:29

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