Driving Hobby Servos

Hobby RC Servo modules like this can be controlled from KFLOP.

Most Servo Modules require a 1-2ms pulse at 50Hz to command their position.  

This is somewhat difficult to generate with normal PWM generators as high Time resolution is required at a slow rate.  KFLOP Hardware PWM generators have a minimum frequency of 254Hz.

A KFLOP User C Program can be used to enable and disable a PWM generator in a way that one pulse is output only every N PWM cycles.  PWM gernerator #1 is sacrificed to generate a fixed long PWM pulse that the C Program can monitor to count cycles as well as determine when the next PWM cycle for all the PWM generators will begin.

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#include "KMotionDef.h"   void ServiceHobbyPWM(void);   int ncycles, v1, v2;   main() {     int LowClocks, scaler=150;     float PWMPeriod, BaseFreq=16.6667e6, PulseGap=20e-3, SafetyTime=200e-6;     // Set base PWM to be high for entire time less 200us           LowClocks = SafetyTime * BaseFreq / scaler;     PWMPeriod = 256.0*scaler/BaseFreq;     ncycles = PulseGap/PWMPeriod;     printf("PWM Period = %fus\n", PWMPeriod*1e6);     printf("Low Clocks = %d\n", LowClocks);     printf("Skip Cycles = %d\n", ncycles);           SetBitDirection(26,1);  // define bit as an output     ClearBit(26);     SetBitDirection(27,1);  // define bit as an output     ClearBit(27);     SetBitDirection(28,1);  // define bit as an output     ClearBit(28);           FPGA(IO_PWMS_PRESCALE) = scaler;    // divide clock by 256 (250Hz)     FPGA(IO_PWMS) = 255-LowClocks;      // Mostly high pulse     FPGA(IO_PWMS+1) = 1;            // Enable     ClearBit(26);           v1 =  800e-6 * BaseFreq / scaler;  // set Pulse time     v2 = 1700e-6 * BaseFreq / scaler;  // set Pulse time       printf("PWM settings v1 = %d v2 = %d\n", v1,v2);       for (;;)     {         ServiceHobbyPWM();     } }     void ServiceHobbyPWM(void) {     static int i=0;     static WaitLevel=0;           if (!WaitLevel) // waiting for low?     {         if (!ReadBit(26)) // yes is it low?         {             WaitLevel=1;  // yes, change state             if (++i == ncycles)  // count mostly high pulses, special cycle?             {                 FPGA(IO_PWMS+2) = v1;   // Pulse size                 FPGA(IO_PWMS+3) = 1;    // Enable                 FPGA(IO_PWMS+4) = v2;   // Pulse size                 FPGA(IO_PWMS+5) = 1;    // Enable                 i=0;             }             else             {                 FPGA(IO_PWMS+3) = 0;   // Disable                 FPGA(IO_PWMS+5) = 0;   // Disable             }         }     }     else     {         if (ReadBit(26)) // no, waiting for high, is it high?         {             WaitLevel=0; // yes, change state         }     } }

The code above prints this:

PWM Period = 2303.995425us
Low Clocks = 22
Skip Cycles = 8
PWM settings v1 = 88 v2 = 188

It generates the timing shown below.  Note the yellow trace is the reference PWM used by the C Program to count the cycles.  Two PWM pulses are emitted every 8 cycles.  The cyan trace shows a pulse width of 780us (specified in the program to be 800us) with a period of 18.5ms.  The magenta trace shows a pulse width of 1.7ms (specified in the program to be 1700us) with a period of 18.5ms.