Large range delay at medium frequency for MC

Question

I have a program that varies the step time and duty cycle for a 6 step commutation sequence. It takes an 8-bit ADC result and stores it as the TMR0 value, when TMR0 overflows, the ISR selects the next sequence.

I have 2 problems:

1. This only works at low INTOSC frequencies, where UART will not work, and
2. I don't get a large enough step range.

Any general ideas on how to create a larger ranged delay at a higher frequency? My first idea is to use a nested loop, still using a TMR0 value, then putting that inside of a loop the steps the inverse of TMR0 times...

Answer 1

One way to approach the problem would be to introduce some additional variables that keep track of the timer interval and how many times it's been called so that a fixed number of interrupts may be ignored. For example the following C pseudo-code to slow the execution time down by a factor of 10.

uint_8 ticks_left, timer_interval;

void my_isr()
{
    ticks_left--;
    if (ticks_left == 0)
    {
         disable_timer();
         // Move to next ADC etc
    } else
         setup_timer(timer_interval);
    }
}

void main()
{
    timer_interval = your_calculated_value;
    ticks_left = 10;
    setup_timer(timer_interval);
}

It won't be perfectly accurate because of the (probably less than 10 cycle) instruction delay between the interrupt and the new timer value being set, although if that's a concern you could check the assembler output to find the number of cycles it will take and reset the timer to a slightly lower value in the ISR if that's likely to be a problem.