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Timer 2: determine best prescaler, postscaler, period

 
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lindsay.wilson.88



Joined: 11 Sep 2024
Posts: 39

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Timer 2: determine best prescaler, postscaler, period
PostPosted: Wed Sep 25, 2024 4:35 pm     Reply with quote

Don't know if this is useful, but I needed to figure it out so maybe someone else does as well ;-)

Suppose you want to have timer 2 interrupts running at, or as close as possible to, a frequency specified by the user (e.g. over the UART). There are three parameters which need to be chosen:

prescaler (1,4,16)
postscaler (1-16)
period (0-255)

I wrote a program which goes through each combination of prescaler and postscaler, works out the period, and then figures out which combination results in the closest frequency match. I used floats for most of the calculations because I'm lazy, so it's not particularly fast (30-40ms on a 39MHz 18F4520), but for my application it's fine as the user only configures the timer frequency occasionally.

The resulting frequency is pretty close - the error is no worse than maybe 0.2% over the entire frequency range (in my case, 150Hz to 65kHz). For greater precision, timer 0 is better since it's 16-bit, but there were various reasons I prefer timer 2, mainly the interrupt-on-match behaviour rather than interrupt-on-overflow. Long story ;-)

Header:

Code:
#include <18F4520.h>
#device ADC=10

#FUSES NOWDT                    //No Watch Dog Timer

#use delay(clock=39.3216MHz,crystal=9.8304MHz)
#use rs232(baud=9600,parity=N,UART1,bits=8)
#use FAST_IO(ALL)


Main code:

Code:
#include <main.h>
#include <stdlib.h>
#include <math.h>

// Text received over UART
char uart_text[10];

// Main oscillator frequency
int32 fosc;

// Desired frequency we want timer to run at
int16 tmr2_desired_freq;

// Test values for prescaler, postscaler and period. The program runs
// through lots of combinations of each to find the best.
int8 tmr2_test_prescaler;
int8 tmr2_test_postscaler;
float tmr2_test_period;

// Actual frequency resulting from using the above test values
float tmr2_test_freq;

// Frequency error - difference between desired and actual
float tmr2_freq_error;

// Minimum frequency error so far - used to check whether the current
// test values result in a better frequency match or not
float tmr2_min_freq_error;

// The best values of prescaler, postscaler and period. These result
// in the smallest possible error
int8 tmr2_best_prescaler;
int8 tmr2_best_postscaler;
int8 tmr2_best_period;

// Timer interrupt routine - give an output pulse just to see what's happening
#int_timer2
void timerinterrupt()
{
   output_high(PIN_C0);
   output_low(PIN_C0);
}

void main()
{
   // General setup
   setup_adc_ports(NO_ANALOGS,VSS_VDD);
   set_tris_c(0b10000000); // Remember to leave the UART TX as output!
   setup_timer_2(T2_DIV_BY_1,100,16); // Set timer to something to start with
   enable_interrupts(int_TIMER2);
   enable_interrupts(GLOBAL);
   fosc=39321600;
   
   while(TRUE)
   {
      // Get desired frequency from the user over UART
      printf("Enter desired frequency\r\n");
      gets(uart_text);
      tmr2_desired_freq=atol(uart_text);
      printf("Desired frequency: %lu\r\n",tmr2_desired_freq);
     
      // Set the minimum error to something very large and initialise
      // the other test parameters to those that give the lowest
      // frequency(i.e. "safest")in case best values aren't
      // found for some reason
      tmr2_min_freq_error=100000;
      tmr2_best_prescaler=16;
      tmr2_best_postscaler=16;
      tmr2_best_period=255;
     
      // Now go through every possible combination of prescaler and postscaler
      // values. Work out the period which gives the desired frequency. Find
      // the closest integer value to this. Then work out the frequency resulting
      // from this integer value. Work out the error between this frequency and the
      // desired frequency. if this error is less than the previous minimum error,
      // update the best values.
      for(tmr2_test_prescaler=1; tmr2_test_prescaler<17; tmr2_test_prescaler*=4) // 1,4,16     
      {
         for(tmr2_test_postscaler=1; tmr2_test_postscaler<17; tmr2_test_postscaler++) // 1-16
         {
            // Work out the test period. Do as float to handle negative and fractional values.
            tmr2_test_period=(fosc/((float)4*tmr2_test_prescaler*tmr2_test_postscaler*tmr2_desired_freq))-1;
            if(tmr2_test_period<0){tmr2_test_period=0;} // If less than zero, set to zero
            if(tmr2_test_period>255){tmr2_test_period=255;} // If greater than 255, set to 255
            tmr2_test_period=floor(tmr2_test_period+0.5); // Round to nearest integer
           
            // Work out the frequency obtained by using this period. Again, do as float to handle fractional values.
            tmr2_test_freq=fosc/((float)4*tmr2_test_prescaler*(tmr2_test_period+1)*tmr2_test_postscaler);
           
            // Work out the absolute error between the test and desired frequencies
            tmr2_freq_error=abs(tmr2_test_freq-(float)tmr2_desired_freq);
           
            // If the current error is less than the minimum error, we've found a possible candidate
            // for the best choice of parameters. Update the minimum error and the best prescaler,
            // postscaler and period values
            if(tmr2_freq_error<tmr2_min_freq_error)
            {
               tmr2_min_freq_error=tmr2_freq_error;
               tmr2_best_prescaler=tmr2_test_prescaler;
               tmr2_best_postscaler=tmr2_test_postscaler;
               tmr2_best_period=tmr2_test_period;
            }
         }
      }
     
      // By this point, we should have best choices for the prescaler, postscaler and period
      // Update the actual timer with these values. Need to use switch because you can't just
      // pass the prescaler value directly to the setup_timer_2 function.
      switch(tmr2_best_prescaler)
      {
         case 1: setup_timer_2(T2_DIV_BY_1,tmr2_best_period,tmr2_best_postscaler); break;
         case 4: setup_timer_2(T2_DIV_BY_4,tmr2_best_period,tmr2_best_postscaler); break;
         case 16: setup_timer_2(T2_DIV_BY_16,tmr2_best_period,tmr2_best_postscaler); break;
      }
     
      // Print results
      printf("Best results:\r\n");
      printf("Prescaler %u\r\n",tmr2_best_prescaler);
      printf("Postscaler %u\r\n",tmr2_best_postscaler);
      printf("Period %u\r\n",tmr2_best_period);
   }
}
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