Hanshin Cho

Having problem using PWM to control a servo motor.

Discussion created by Hanshin Cho on May 11, 2010
Latest reply on May 14, 2010 by Hanshin Cho

I am using a MC9S12DT256 with an E-clock of 8MHz, 8bit PWM.

the servo motor requires a period of 20ms with a minimum pulse of 0.7ms and a max pulse of 2.5ms.

The problem is that when I hook it up to the motor it just rotate left a little. (same rotation at 0.7ms and 2.5ms)

I can't seem to see where my code or calculations went wrong.

I divided the 8Mhz clock by 10 to get 0.8MHz(Scaled A), then divided that by 64 to get 12500HZ.

I want a 20ms period for the motor so my period register will be:
0.020=(1/12500)*PWMPERx     which gives PWMPERx=250

Then for the duty register I want 0.8ms out of 20 ms period which is a duty of  4%
so 0.04=(250-PWMDTY0)/250

Any help is much appreciated.

Here is my code:


#include <hidef.h>

#include "derivative.h"

#include <mc9s12dt256.h>


void main(void) { 


 PWMCTL=0x00;       // 8bit pwm
 PWMSCLA=0x05;     //  A/(2*5) =SA
 PWMSCLB=0x05;     //  B/(2*5) =SB
 PWMCLK=0xff;           // select scaled clock
 PWMPOL=0x00;        // start low
 PWMPRCLK=0x06;   //  SA/64 = clock
 PWMCAE=0x00;         // left aligned
 PWME=0xff;                 // PWM enabled
 PWMPER0=0xfa;        // 20ms=250/12500