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PIC24/dsPIC timer period is 1 clock too long

Posted by Dan Henry on August 23, 2010
From the "Timers" Reference Manual sections for these devices (Example 11-1 and Figure 11-4 in all manuals) we can see that prvSetupTimerInterrupt() in port.c should be changed from:

const unsigned long ulCompareMatch = ( configCPU_CLOCK_HZ / portTIMER_PRESCALE ) / configTICK_RATE_HZ;


to:

const unsigned long ulCompareMatch = ( configCPU_CLOCK_HZ / portTIMER_PRESCALE ) / configTICK_RATE_HZ - 1;

RE: PIC24/dsPIC timer period is 1 clock too long

Posted by Richard on August 23, 2010
Thanks for the feedback. Its not the first time I have made that mistake, and I'm sure it won't be the last.

However, I'm not sure your solution is completely accurate because of the operator precedence. Should it not be:

const unsigned long ulCompareMatch = ( ( configCPU_CLOCK_HZ / portTIMER_PRESCALE ) / configTICK_RATE_HZ ) - 1;

If you agree with this I will change it in SVN straight away so it does not get lost in the crowds.

Regards.

RE: PIC24/dsPIC timer period is 1 clock too long

Posted by Dan Henry on August 23, 2010
Multiplicative operators, of which '/' is one, have higher precedence than additive operators, so the added parentheses are unnecessary. However, explicitness is always good in my book, so I say leave them in the way you have it.

RE: PIC24/dsPIC timer period is 1 clock too long

Posted by Richard on August 23, 2010
... the SVN copy has been updated.

Regards.


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