Tick time incorrect in CodeWarrior DEMO

shirleychen wrote on Thursday, March 11, 2010:

I found the tick time is incorrect in both CodeWarrior DEMO project.
Compare value need to be added to TC0 before enable TFLG1 in file port.c,
in function void interrupt vPortTickInterrupt( void ).
The value that is added on TC0 is based on the system bus frequency.
In my case is 2500 @ 20MHz bus clock.
Anybody has the same experience?

davedoors wrote on Friday, March 12, 2010:

I just looked at the code but don’t understand your comments because I cannot see anything being added to TC0. Can you explain further?

shirleychen wrote on Friday, March 12, 2010:

From the code generated by the Processor Expert TickTimer.c, interrupt service routine as below

  TC0 += CmpVal;                       /* Add value corresponding with periode */
  TFLG1 = 1;                           /* Reset interrupt request flag */
  TickTimer_OnInterrupt();             /* Invoke user event */

CmpVal(in this case = 2500) is added on to TC0 to generate the 1kHz interrupt.
When the vector for TC0 is redirected to vPortTickInterrupt( void ) in port.c file as below,
void interrupt vPortTickInterrupt( void )
#if configUSE_PREEMPTION == 1
/* A context switch might happen so save the context. */

/* Increment the tick … */

/* … then see if the new tick value has necessitated a
context switch. */
               TC0 += 2500;   //  I added the code here
TFLG1 = 1;   

/* Restore the context of a task - which may be a different task
to that interrupted. */
TC0 += 2500;  // I added code here
TFLG1 = 1;
Without the the code I added, the TC0 interrupt will be enabled with TC0 = 0, that is the next ISR will come in when the TC0 counts 0xFFFF cycle, which exceed 1ms period, that was why the tick time appeared longer than it suppose to be.