//this code initializes a 16 bit timer for use as a pwm and implements a pid controller//I use this code for temperature correction, with the plant value being an adc thermistor reading, //and the pwm hooked up to a heater (through a mosfet switch)//this code is good for small plant values within a signed 16 bit range that require a direct pwm output#ifndef __pid_h__#define __pid_h__#include <avr/io.h>#include <inttypes.h>//the desired plant valueconst uint16_t SET_POINT = 512;//stores the integral sumint32_t integral;//the previous errorint16_t prev_error;void pid_setup(void) { //pwm data direction DDRB |= _BV(PB1); //set the prescaler and wgm modes for ICR1 top fast pwm with no prescaling TCCR1A |= _BV(COM1A1) | _BV(WGM11); TCCR1B |= _BV(WGM13) | _BV(WGM12) | _BV(CS10); //the higher the compare value, the longer the pulse stays high //0 is completely off, 0xFFFF is completely on OCR1A = 0; ICR1 = 0xFFFF;}//apply a pid correction//input the current plant value//the gains are implemented as constants, but could easily be made global variablesvoid pid_correct(int16_t plant_value) { //the plant error //make sure error is positive when you need to apply a larger pwm value (wider positive width) int16_t error = plant_value - SET_POINT; //int16_t error = ((int16_t) SET_POINT) - plant_value; //proportional * pGain int32_t proportional = error * 200; //integral * iGain int32_t temp_sum = integral + ( error * 10 ); //check that the integral doesn't get out of the desired range if ( temp_sum > 0xFFFF ) integral = 0xFFFF; else if (temp_sum < (-0xFFFF) ) integral = -0xFFFF; else integral = temp_sum; //differential * dGain //if you used the other error calculation, use the other differential as well int32_t differential = ( (error - prev_error) * 20 ); //int32_t differential = ( (prev_error - error) * 20 ); //the final sum, with an overflow check int32_t temp_pid_sum = proportional + integral + differential; uint16_t pid_total; //set max pwm value at ~95% of 2^16 (change this to whatever you like) if ( temp_pid_sum > 62258 ) pid_total = 62258; else if ( temp_pid_sum < 0 ) pid_total = 0; else pid_total = (uint16_t) temp_pid_sum; //set pwm compare value to change the duty cycle OCR1A = pid_total; //store the previous error prev_error = error;}#endif