Servo (PWM)+ ADC MSP430G2553

Hi Werner,

A handful of thoughts:

• What is the voltage level for VCC on the servo?  If you are using the 3.3V Vcc on the launchpad, this could potentially cause issues.  Most servos (that I have experience with) like to have a Vcc in the 4V to 6V range, and will sometimes have erratic behavior at 3.3V Vcc.  When controlling a servo with a launchpad, I typically use a 5V source to power the servo, then let the 3.3V PWM control the position (3.3V is typically sufficient for the PWM, but there is the off chance that it may require a level shifter).
• Why are you using 1.1MHz as your constant for MCLK?  You are using the calibrated DCO values for 1MHz, so just use 1000000.
• Why are you using 46Hz for your PWM frequency constant?  Why not just use 50Hz (as expected by the servo)?
• Why are you using Timer0_A0 interrupt?  This, combined with the 1 second delay is causing your ADC sample to not run during the "__delay _cycles()" instruction (since the 430 does not do nested interrupts by default).  You shouldn't need the timer ISR at all, the hardware will automatically create the PWM for you with no additional input once it is set up correctly.
• Since you are using SMCLK/8 (125KHz) as the clock source for the timer, but your calculations for PWM period use 1.1MHz, you are not going to end up with the correct frequency.
• Take a look at some of the example code for how to create a PWM using output mode 7 (Rest/Set), this will help you understand how to create the PWM.
• When calculating the PWM duty cycle (from the ADC, you need to add some sort of scaling to make the ADC input range match to the usable PWM duty cycle range for a servo (typically 5 to 10% duty cycle).

My suggestion is to redo the constants at the top of code to give better values (MCLK, PWM_PERIOD/DUTY, etc).  Then correctly set up the PWM from these values.  Use the ADC ISR to scale the possible ADC values to the usable range (5 to 10% duty cycle).  The G2553 has a second Timer_A, so you could use this to add delay between ADC sample intervals.  The best way would be to use the ADC10 with setting the Sample and Hold Source Select to use the timer as the trigger for the ADC conversion (see the code examples for an example on how to do this).  The final consideration would be that when you change the PWM duty cycle, you need to check for the case where you are setting CCR1 below the current TAR, which would cause the counter to loop through to CCR0 before changing the output.  To check for this, when you go to change the CCR1 value, first halt the timer, compare the new CCR1 to TAR, and if TAR is greater, reset the counter when you restart the timer module after updating CCR1.  This should help prevent weird glitches when changing the PWM duty cycle.

Mike

Werner,

this one here is an absolute NO-GO:

// CCR0 interrupt service routine
#pragma vector = TIMER0_A0_VECTOR
__interrupt void Timer0_A0( void )
{
  TACCR1 = servo1pos;          // Set time for servo
  __delay_cycles(1000000);     // 1 second delay     <----- NO WAY! You are in an interrupt!
  ADC10CTL0 |= ENC + ADC10SC;  // Start sampling
}

Try to get rid of all those delay_cycles(). There is absolutely no need for them. At least not during the main program flow. In the init before the main program starts it might be OK. I did almost the same as you want to do. Maybe my sorce code for that helps. I removed some parts of it because I had an additional multiplexed seven segment display connected. This was most of the code and it would have only been confusing. So sorry, if I maybe deleted a thing I thought it was belonging to the display but was necessary for the servo control.

/*
 * main.c
 *
 *  Created on: 13.11.2014
 *      Author: Dennis Eichmann
 */



// ------------------------------------------------- MSP430G2553 Microcontroller
//
//                           MSP430G2553
//                          -------------
//                   VCC  =| 1 o      20 |=  GND
//      BUTTON LEFT P1.0  =| 2        19 |=  P2.6 PWM OUTPUT
//     BUTTON RIGHT P1.1  =| 3        18 |=  
//    BUTTON CENTER P1.2  =| 4        17 |=  TEST/SBWTCK
//       BUTTON SET P1.3  =| 5        16 |=  RST/NMI/SBWTDIO
//         VREF OUT P1.4  =| 6        15 |=  
//                        =| 7        14 |=  P1.6 ANALOG IN
//                        =| 8        13 |=  
//                        =| 9        12 |=  
//                        =| 10       11 |=  
//                          -------------
//
// -----------------------------------------------------------------------------



#include "msp430g2553.h"
#include <stdint.h>



#define ADC_VAL_CCR_FACTOR      97752
#define ADC_VAL_CCR_DIVIDER     100000

#define BUTTON_LEFT_PRESSED     !( P1IN & 0x01 )
#define BUTTON_RIGHT_PRESSED    !( P1IN & 0x02 )
#define BUTTON_CENTER_PRESSED   !( P1IN & 0x04 )
#define BUTTON_SET_PRESSED      !( P1IN & 0x08 )



volatile uint32_t adc10_value = 511;           // Preset ADC result with 1.5ms
         uint16_t timer_value = 1500;          // Preset timer value with 1.5ms



void main( void )
{
  WDTCTL     = ( WDTPW | WDTHOLD );            // Stop watchdog timer

  BCSCTL1    =  CALBC1_1MHZ;                   // Set DCO range
  DCOCTL     =  CALDCO_1MHZ;                   // Set DCO step and modulation

  P1REN      =  ( 0x01 | 0x02 | 0x04 | 0x08 ); // P1.0/1/2/3 input buttons resistor enable
  P1OUT      =  ( 0x01 | 0x02 | 0x04 | 0x08 ); // P1.0/1/2/3 input buttons pull-up resistor

  P2SEL      =  0x40;                          // P2.6 signal output PWM TA0.1
  P2DIR      =  0x40;                          // P2.6 set to output

  TA0CCR0    =  20000;                         // Period
  TA0CCR1    =  1500;                          // Duty-cycle for P1.6
  TA0CCTL1   =  OUTMOD_7;                      // Reset/set for P1.6
  TA0CTL     =  ( TASSEL_2 | MC_1 );           // SMCLK, up-mode

  // Vref+/Vss, 64 clocks S&H, ref output on, 2,5V ref, ref on, ADC10 on, interrupt enable
  ADC10CTL0  =  ( SREF_1 | ADC10SHT_3 | REFOUT | REF2_5V | REFON | ADC10ON | ADC10IE );
  ADC10CTL1  =  ( INCH_6 | ADC10DIV_7 );        // Input channel A6 on P1.6, clock divided by 8
  ADC10AE0   =  0x40;                           // Analog input A6 enabled

  __bis_SR_register( GIE ); // Enable global interrupts

  ADC10CTL0 |= ( ENC | ADC10SC ); // Enable and start conversion

  while( 1 ) // Endless-loop (main-program)
  {
    if( BUTTON_SET_PRESSED )
    {
      timer_value = (1000 + ((adc10_value * ADC_VAL_CCR_FACTOR) / ADC_VAL_CCR_DIVIDER)); // Set duty-cycle to pot-adjusted value
    }
    else if( BUTTON_LEFT_PRESSED )
    {
      timer_value = 1000; // Set duty-cycle to 1.0ms
    }
    else if( BUTTON_RIGHT_PRESSED )
    {
      timer_value = 2000; // Set duty-cycle to 2.0ms
    }
    else if( BUTTON_CENTER_PRESSED )
    {
      timer_value = 1500; // Set duty-cycle to 1.5ms
    }

    if( timer_value < 1000 )
    {
      timer_value = 1000;
    }
    else if( timer_value > 2000 )
    {
      timer_value = 2000;
    }

    TA0CCR1 = timer_value;
  }
}



#pragma vector = ADC10_VECTOR
__interrupt void ADC10_ISR ( void )
{
  adc10_value = ADC10MEM;
  ADC10CTL0 |= ( ENC | ADC10SC ); // Enable and start conversion
}

I had four buttons, servo to the left, servo to the right, servo center and reading the potentiometer position. All buttons between pin of the micro and GND. The pot was connected to the refence voltage of the micro. No timer ISR, this is done in hardware.

Maybe that helps a little bit.

Dennis