MSP430FR2355: Pulse oximeter with MSP430FR2355

Hi, Mourya, 

MSP430FR2355 has built-in DAC to generate the bias offset for built-in OPA (DAC and OPA are all a part of the SAC in MSP430FR2355). So there is no need to connect the pin 5 of transimpedance offset pin to MSP430FR2355. 

There is no need to connect the OA0 output to OA2 inverting input externally. You can do the connection internally by setting SAC2PGA.MSEL=11b and SAC2OA.NSEL=01b. 

In MSP430FR2353/2355, the SAC0 is internally connected with SAC2, and the SAC1 is internally connected with SAC3. And the SAC outputs are also internally connected to the built-in comparator and bult-in ADC12bit.  

Please refer to the figure 11 in the application report of "How to Use the Smart Analog Combo in MSP430™ MCUs" (slaa833) for detailed information/setting of cascading SACs. You can also refer to figure 6-1 (eCOMP0, SAC0, SAC2 Interconnection) in the MSP430FR2355/2353 datasheet. Both documents have the detailed information for SACs internal connection with other peripherals. 

Regarding the schematic for SLAA897, we didn't build the board. But we do have some internal schematic to internal test purpose. If you are interested, please contact TI sales and check with MSP430 BU team. 

Thanks, 

Lixin 

Hi Lixen,

Thank you very much for your reply and helping me clear my doubts. One  thing I noticed is that in the SLAA897 document (Section 2.2)  it is written that to configure SAC2 as Inverting PGA Mode the settings should be (PSEL=01, NSEL=01, MSEL=00). But I think it is when we are externally connecting the OA0 Output to OA2 Negative I/P. But as you said for internal connection , I have to use PSEL=01, NSEL=01, MSEL=11 for SAC2.

For Pulse oximeter design I Need to sample the output of both the Amplifier stages(OA0O & OA2O), but only SAC0 & SAC1 Outputs are connected internally to the ADC Inputs. For SAC2 I have to configure the output pin for SAC2 and connect it externally to ADC input pin. 

Based on the above understanding I have configured SAC,ADC & TIMER as shown below.

 // Configure reference module
	         PMMCTL0_H = PMMPW_H;                    // Unlock the PMM registers
	         PMMCTL2 = INTREFEN | REFVSEL_2;         // Enable internal 2.5V reference
	         while(!(PMMCTL2 & REFGENRDY));          // Poll till internal reference settles




	        /*******************SAC0 configurations as TIA******************/
	         P1SEL0 |=   BIT2  ;             // Select  P1.2  OA0- PIN function
	         P1SEL1 |=   BIT2  ;             // Select  P1.2  OA0- PIN function

	         SAC0DAC = DACSREF_1;                        // Select 2.5V int Vref as DAC reference
	           SAC0DAT = 0x0FFF;                         // Set SAC DAC data to 2.5V
	           SAC0DAC |= DACEN;                         // Enable DAC
	         SAC0OA = NMUXEN + PMUXEN + PSEL_1 + NSEL_0; //Select positive INPUT AS DAC and negative pin input AS PHOTODIODE
	         SAC0PGA |= MSEL_0;                          // INVERTING PGA MODE
	         SAC0OA &= ~OAPM;                            // Select HIGH speed and HIGH power mode
	         SAC0OA |= SACEN + OAEN;                     // Enable SAC and OA

	         /****************SAC2 configurations AS INVERTING PGA************/
	                                 P3SEL0 |= BIT1    ;             // Select P3.1   OA2 OUTPUT function
	                                 P3SEL1 |= BIT1    ;             // Select P3.1   OA2 OUTPUT function

	                                 SAC2DAC = DACSREF_1;                       // Select 2.5V int Vref as DAC reference
	                                   SAC2DAT = 0x0FFF;                         // Set SAC DAC data to 2.5V
	                                   SAC2DAC |= DACEN;                         // Enable DAC
	                                 SAC2OA = NMUXEN + PMUXEN + PSEL_1 + NSEL_1;//Select positive INPUT AS DAC and negative pin input AS O/P of OA0
	                                 SAC2PGA |= GAIN0 + GAIN2 + MSEL_3;    // Set inverting PGA mode with Gain=16
	                                 SAC2OA &= ~OAPM;                           // Select HIGH speed and HIGH power mode
	                                 SAC2OA |= SACEN | OAEN; // Enable SAC and OA

 /******************SAC3 configurations AS DAC OUTPUT********************/
	                      P3SEL0 |= BIT5   ;           // Select P3.5  OA3O function
	                      P3SEL1 |= BIT5  ;         //  OA is used as buffer for DAC

	                      SAC3DAC = DACSREF_1;                       // Select 2.5V int Vref as DAC reference
	                      SAC3DAT = 3340;                         // Set SAC DAC data to INITIAL VALUE
	                      SAC3DAC |= DACEN;                         // Enable DAC
	                      SAC3OA = NMUXEN + PMUXEN + PSEL_1 + NSEL_0;//Select DAC MODE
	                      SAC3PGA |= MSEL_1;                       // Set OA AS BUFFER MODE
	                      SAC3OA &= ~OAPM;                           // Select HIGH speed and HIGH power mode
	                      SAC3OA |= SACEN | OAEN; // Enable SAC and OA

	                      /*******************SAC1 configurationsAS DAC OUTPUT**********************/
	                      P1SEL0 |= BIT5  ;            // Select P1.5  OA1O function
	                      P1SEL1 |= BIT5 ;          //  OA is used as buffer for DAC

	                     SAC1DAC = DACSREF_1;                       // Select 2.5V int Vref as DAC reference
	                     SAC1DAT = 3340;                         // Set SAC DAC data to INTIAL VALUE 
	                     SAC1DAC |= DACEN;                         // Enable DAC
	                     SAC1OA = NMUXEN + PMUXEN + PSEL_1 + NSEL_0;//Select DAC MODE
	                     SAC1PGA |= MSEL_1;                       // Set OA AS BUFFER MODE
	                     SAC1OA &= ~OAPM;                           // Select HIGH speed and HIGH power mode
	                    SAC1OA |= SACEN | OAEN; // Enable SAC and OA

	                    /***********ADC CONFIGURATION***********/

	         ADCCTL0 &= ~ADCENC;                       // Disable ADC
	       ADCCTL0 |= ADCSHT_4 | ADCMSC | ADCON;  //0000010010010010; 64 ADCLK CYCLES, MSC BIT HIGH, ADC ON

	       ADCCTL1 |= ADCSHP | ADCSHS_1 | ADCCONSEQ_1;   //0000010010010010;
	       ADCCTL2 &= ~ADCRES; //CLEAR RESOLUTION
	       ADCCTL2 |= ADCRES_2; //12 BIT RESOLUTION
	       ADCMCTL0 |= ADCSREF_1|ADCINCH_1|ADCINCH_3; // OA0 OUTPUT AS A1 ADC INPUT ,VR+= VREF+, ALSO OA2 OUPUT EXTERNALLY CONNECTED TO A3 ADC INPUT
	       ADCIE |= ADCIE0; // ENABLE CONVERSION COMPLETE INTERRUPT
	       ADCCTL0 |= ADCENC|ADCSC; // ENABLE CONVERSION & START  CONVERSION

	       /******************** Configure Timer ******************/

	       TB0CTL = TBSSEL__ACLK + TBCLR; //ACLK & Reset timer
	       TB0CCTL1 = OUTMOD_2; // TOGGLE & RESET
	       TB0CCTL0  = CCIE; //INTERRUPT ENABLE
           TB0CCR0 = 31; //SAMPLING RATE 512 SPS
           TB0CCR1 = 10; //Allow plenty of time for the signal to become stable before sampling
           TB0CTL |= MC_1; // UP MODE


// Timer B0 interrupt service routine
#pragma vector=TIMER0_B0_VECTOR
__interrupt void Timer0_B0_ISR(void)
{
int i;

 if((SAC3OA & SACEN)) //  CHECK IF VSLED IS ON

 {
     SAC3DAC |= DACEN;
     SAC3DAT = vs_LED_level;
     SAC0DAT = vs_dc_offset;
     is_IR = 0;                          // IR LED OFF

     ir_sample = ADCMEM0;              // Read the IR LED results
     i = ADCMEM0;
                                                 // Enable the next conversion sequence.
                                                 // The sequence is started by TA1
             ADCCTL0 &= ~ADCENC;
             ADCCTL0 |= ADCENC;

.................

, As per the reference code of Slaa458,In the Timer ISR It first checks whether the DAC Output is connected to IR LED or not. If it is connected it turns IR OFF and turn VS LED ON, vice versa in the else statement. 

In the new design both LED's have their own DAC Output so , Should I need to check ON/OFF status OF the SAC3 & SAC1 alternately to switch both LED'S one at a time? I mean I can Write code like below line

if((SAC3OA & SACEN)) //  CHECK IF IR LED IS ON instead of if ((DAC12_0CTL & DAC12OPS)) as used in slaa458?

I hope my code is in the right direction. 

Hi, Lixen,

One more thing I wanted to ask is that the SAC0 +ve input is connected to its DAC & not the SPO2 sensor DB9 Connector.

What value should I keep for SAC0DAT ? Since DAC value of other SAC's are changing according to the SPO2 Signal But for SAC0 the DAC value needs to be fixed?

Hi, Sumit, 

If you are following the slaa897 for the connection: SAC0 connected to photodiode and SAC2 as second amplifier, and the photodiode sensing signal dynamic range is stable,  the SAC0_DAC can be fixed value. The SAC2_DAC can be dynamic to track the DC component of the photodiode signal. 

Thanks, 

Lixin