topic Re: Timer in MC9S08GT60 in 8-bit Microcontrollers

Timer in MC9S08GT60

ppanicker — Mon, 17 Aug 2009 15:58:40 GMT

Hi,

My application code requires me to send data through the SPI interface every one minute. I want to write an interrupt routine using the timer. I am using MC9S08GT60, running at 8MHz.

I am not sure how to initialize the timer module. And set the register values to get an interrupt every one minute.

Thanks,

Poornima

Re: Timer in MC9S08GT60

PeterHouse — Mon, 17 Aug 2009 21:25:33 GMT

Poornima,

Find a divisor setup which will make 65536 counts last longer than one second. Get as short as possible without going less than one second.

Calculate the exact number of counts for one second and set this as the modulo. If it is not exact and you need EXACT for the long term you may need to adjust the modulo count at each interrupt to make sure the long term count is exact. If you need EXACT over the short term, then you will possibly need to change crystals - I do not think this will be the case for an 8 MHz crystal as I believe there is an exact divisor.

Enable the Overflow interrupt and put some code in it.

Good Luck,

Peter House

Re: Timer in MC9S08GT60

bigmac — Tue, 18 Aug 2009 01:56:10 GMT

Hello Poomima,

I don't think that it is possible to directly achieve a 1 minute TPM overflow period with a bus frequency of 8MHz. However, if a much shorter TPM overflow period, perhaps 10 milliseconds, is generated, a one minute timeout period could then be achieved by counting every 6000 overflows.

With a bus frequency of 8 MHz, a TPMMOD setting of 39999 and a prescale value of 2 should give 10 ms overflow period.

Using an 16-bit global variable tcount, the TPM overflow ISR then might incorporate the following code snippet:

tcount--;

if (tcount == 0) {

tcount = 6000;

SPI_process();

}

Regards,

Mac

Message Edited by bigmac on 2009-08-18 05:07 AM