MSP430FR2311: Set Onboard OA as Inverting OA w/ Gain

Hi Diana,

First, thank you for your detailed post.

Regarding your question, you're referencing the correct figure in the User's Guide for an inverting op-amp mode/configuration. In Table 19-2, you can see the different modes and PGA gain settings. The columns "MSEL", "PSEL", "NSEL", and "GAIN" are all register fields (sections or groups of bits in certain control registers) that would be configured for your requirements.

For example, you need to operate in Inverting PGA mode. Thus, MSEL would equal 00b. As you can see in Figure 19-8 and Table 19-7, the MSEL field is defined by Bits 1 to 0 in the SAC PGA Control Register, SACxPGA. The complete list of SAC registers can be found in Table 19-5.

Now, the Driver Library allows users to configure modules like the SAC at a much higher, abstracted level (using an API) than what I've described above, which is the register level. After looking in the MSP430 DriverLib for MSP430FR2xx_4xx Devices User's Guide (v2.80.00.01), I didn't find many functions for the SAC module. Like you, I wasn't able to find any references to SACxPGA or SACxSMEL after searching in the 'sac.c' and 'sac.h' files found under the project folder -> driverlib -> MSP430FR2xx_4xx in CCS. Perhaps, additional functions for the SAC will be added in the future.

In the meantime, I'd recommend taking a look at our register-level code examples as a foundation for getting started. Then, you can add your changes to the SACxPGA register to this example moving forward.

msp430fr231x_SAC_L1_03.c

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//******************************************************************************
//  MSP430FR231x Demo - TRI0+SAC0, Two-Stage amplifier,
//                      Use ADC to sample SAC0 ouput
//
//  Description: Configure SAC0 and TRI0 as two-stage amplifier.
//  As the code example practice, TRI0- and TRI0O are connected directly. 
//  And OA0- and OA0O are connected as below to implement x3 amplifier. 
//  The input signal which is in the range 0~1/2Vcc is input through TRI0+,
//  TRI0 output is connected to SAC0 positive input terminal internally.
//  Use ADC CH A3 to sample SACO output. If higher than 1/2 VCC, turn on LED.
//  Both SAC0 and TRI0 are configured to low speed and low power mode.
//  Need to add external circuit to silicon, make sure SAC0 and TRI0
//  work in amplifier mode.
//  ACLK = n/a, MCLK = SMCLK = default DCODIV ~1MHz.
//  Note: Please select SBW interface instead of JTAG interface while debugging 
//        the TRI because the TRI pins are alternatively used with JTAG pins.
//
//                MSP430FR231x
//             -------------------
//         /|\|          P1.2/OA0-|----||---------R2=100Kohm---GND
//          | |                   |  R1=200Kohm
//          | |          P1.3/OA0O|----||              
//          --|RST       P1.4/OA0+|
//            |         P1.5/TRI0O|----||
//            |         P1.6/TRI0-|----||
//            |         P1.7/TRI0+|<------0~1/2*Vcc
//            |                   |
//            |               P1.0|----> LED
//
//   Darren Lu
//   Texas Instruments Inc.
//   July 2015
//   Built with IAR Embedded Workbench v6.30 & Code Composer Studio v6.1
//******************************************************************************
#include <msp430.h>

unsigned int adcResult;

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

  P1DIR |= BIT0;                            // Select P1.0 as output
  P1OUT &= ~BIT0;                           // Set P1.0 output low
  PM5CTL0 &= ~LOCKLPM5;                     // Disable the GPIO power-on default high-impedance mode
                                            // to activate previously configured port settings

  //Select P1.2 P1.3 P1.4 as SAC function
  //Select P1.5 P1.6 P1.7 as TRI fucntion
  P1SEL0 |= BIT2 + BIT3 + BIT4 + BIT5 + BIT6 + BIT7;
  P1SEL1 |= BIT2 + BIT3 + BIT4 + BIT5 + BIT6 + BIT7;

  TRI0CTL = TRIPM + TRIEN;                  // Enable TRI, Select low speed and low power mode

  SAC0OA |= PSEL1;                          // Select TRI output as SAC positive input source
  SAC0OA |= NMUXEN + PMUXEN;                // Enable negative and positive input
  SAC0OA |= OAPM;                           // Select low speed and low power mode
  SAC0OA |= SACEN + OAEN;                   // Enable SAC and OA

  // Configure ADC10
  ADCCTL0 &= ~ADCENC;                       // Disable ADC
  ADCCTL0 = ADCSHT_2 | ADCON;               // ADCON, S&H=16 ADC clks
  ADCCTL1 = ADCSHP;                         // ADCCLK = MODOSC; sampling timer
  ADCCTL2 = ADCRES;                         // 10-bit conversion results
  ADCIE = ADCIE0;                           // Enable ADC conv complete interrupt
  ADCMCTL0 = ADCINCH_3 | ADCSREF_0;         // A3 ADC input select = OA output
                                            // Vref = DVCC

  while(1)
  {
      ADCCTL0 |= ADCENC | ADCSC;          // Sampling and conversion start
      __bis_SR_register(LPM0_bits | GIE); // Enter LPM0, ADC_ISR will force exit
      __no_operation();                   // For debug only

      if (adcResult > 512)                // OA output > 1/2 VCC
          P1OUT |= BIT0;                  // Set P1.0 LED on
      else
          P1OUT &= ~BIT0;                 // Clear P1.0 LED off
  }
}

// ADC interrupt service routine
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=ADC_VECTOR
__interrupt void ADC_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(ADC_VECTOR))) ADC_ISR (void)
#else
#error Compiler not supported!
#endif
{
  switch(__even_in_range(ADCIV, ADCIV_ADCIFG))
  {
      case ADCIV_NONE:
          break;
      case ADCIV_ADCOVIFG:
          break;
      case ADCIV_ADCTOVIFG:
          break;
      case ADCIV_ADCHIIFG:
          break;
      case ADCIV_ADCLOIFG:
          break;
      case ADCIV_ADCINIFG:
          break;
      case ADCIV_ADCIFG:
          adcResult = ADCMEM0;            // Read ADC memory
          __bic_SR_register_on_exit(LPM0_bits);// Exit from LPM
          break;
      default:
          break;
  }
}

Regards,

James

MSP Customer Applications

Hi James. Thanks for the quick reply! I quickly realized that Driver Library was probably a no go for setting MSEL and the gain. The next issue I've been having is that I cannot find the SACxPGA register in the MSP430FR2311.h file, which isn't a part of Driver Library. I've been able to understand the example you shared pretty well, but as far as I can see, the registers used there are all defined in MSP430FR2311.h. I'm sure the SACxPGA register is easily accessible, however I haven't been able to find its declaration in the header file so I have no idea how to call it.