Getting Precise with MSP430 Sigma-Delta ADC Peripherals Vincent Chan MSP430 Business Development Manager TI Asia

Transcription

1 Getting Precise with MSP43 Sigma-Delta ADC Peripherals Vincent Chan MSP43 Business Development Manager TI Asia 25 Texas Instruments Inc, Slide 1

2 Agenda Sigma-Delta basics & benefits Understanding the SD16_A Choosing an MSP43 integrated ADC Lab exercise: Hands-on with the SD16_A 25 Texas Instruments Inc, Slide 2

3 Sigma-Delta Basics AMPLITUDE TIME Simple analog: 1-bit ADC 1-bit DAC Integration (Σ) & difference ( ) stages Complex digital: filter stage Over sampled input Decimation filtered output 25 Texas Instruments Inc, Slide 3

4 Σ- vs. Successive Approximation Sigma-Delta typically higher resolution 1 s ksps Σ- vs. 1 s ksps SAR Tolerance of analog step changes sample-to-sample Inherent for SAR Step changes at the input must cycle through the Σ- filter stage Σ- architecture ~9% digital: easier to integrate 25 Texas Instruments Inc, Slide 4

5 SD16 Overview SD16 Control Block F42x & FE42x Multiple channels Single external input per channel Up to 256 OSR V REF Temperature sensor Reference 1.2V f M Divider Group/Start Conversion Logic MCLK SMCLK ACLK TACLK Channel x 1MHz f M Ax Ax.1 Ax Ax Ax Ax Ax Ax PGA 2 nd Order Σ Mod SD16PREx SD16MEMx 25 Texas Instruments Inc, Slide 5

6 SD16_A Overview F42x & F2x3 Single channel Multiple input pairs V REF 1.2V Reference f M Divider Divider MCLK SMCLK ACLK TACLK Input buffer AV CC measure 3kHz to 1.1MHz f M divider A A1 A2 + - A3 + - A4 + - A5 + - A6 + - A7 + - BUF PGA 2 nd Order Σ Mod Start Conversion Logic SD16MEM Up to 124 OSR Temp. sensor A5 25 Texas Instruments Inc, Slide 6

7 Conversion Modes Single conversion = Result written to SD16MEMx Conversion SD16SC Set in SW Auto-clear Continuous conversion Conversion Conversion Conversion Conv SD16SC Set in SW Cleared in SW Clearing SD16SC immediately stops conversion: The value in SD16MEMx can change and should be read prior to halting conversion 25 Texas Instruments Inc, Slide 7

8 SD16_A Input Design Four external input pairs Fully differential Internal channels: Temperature AV CC / 11 Offset shunt Selectable current vs. speed input buffer PGA: 1, 2, 4, 8, 16 & 32x A A1 A2 + - A3 + - A4 + - A5 + - A6 + - A7 + - Input Channels * BUF PGA Temp. sensor 2 nd Order Σ Mod A5 SD16AEx bits for internal A IN - connection to AV SS * Buffer not in F2x3 devices to SD16 Ax- SD16AEx Port Pin Control AV SS 1 from Ax- pin to GPIO disable 25 Texas Instruments Inc, Slide 8

9 Analog Input Range What is V REF? What is the PGA setting? GAIN V +.5V Applies to all inputs & modes V FSR = V ref GAIN / 2 PGA V +.15V -.15V -.5V -.6V 25 Texas Instruments Inc, Slide 9

10 Input Select vs. Channel Select SD16_A: 1 channel, 4 external inputs per channel MSP43F42x & MSP43F2x3 SD16: 3 channels, 1 external input per channel MSP43FE42x & MSP43F42x Channels are independent & can operate in parallel Inputs are multiplexed into each channel & must be selected/sampled sequentially 25 Texas Instruments Inc, Slide 1

11 Using Preload: Multi-Channel Only Offsets decimation filter for # of f M clocks Staggers conversion results channel-to-channel Used to introduce phase delay in the conversion flow for signal compensation Example: electricity meter I & V phase compensation SD16OSRx = 32 f M cycles: SD16PRE = 8 Delayed Conversion Conversion Conversion 32 1 st Sample Ch (erroneous) 32 1 st Sample Ch 32 SD16PRE1 = Conversion Conversion Conversion Conversion Start of Conversion 1 st Sample Ch1 Time 25 Texas Instruments Inc, Slide 11

12 Input Step Response Key for mux switching Decimation filter must cycle out the delta SD16INTDLYx sets automatic settling time to 1 st conversion interrupt f M = 1.48MHz; OSR = 256 f SAMPLE = 496 ksps -> t SETTLE(MAX) ~ 732usec Asynchronous Step Conversion 25 Texas Instruments Inc, Slide 12

13 Internal Reference SD16REFON SD16REFON V REF 1.2V AV CC V REF 1.2V AV CC 1 1 Internal External AV SS Internal Only AV SS SD16VMIDON SD16VMIDON Reference Reference Internal 1.2V reference 2ppm temperature coefficient V REF Options: External ref: SD16REFON =, SD16VMIDON = Internal ref: SD16REFON = 1, SD16VMIDON = Internal ref w/ buffered output: SD16REFON = 1, SD16VMIDON = 1 For temperature (A6): use internal reference 25 Texas Instruments Inc, Slide 13

14 Internal Reference Settling Time SD16VMIDON = SD16VMIDON = 1 C VREF = 47nF Ref buffer = +1x faster reference settling Disable once settled 25 Texas Instruments Inc, Slide 14

15 Modulator Clock Selection f M comparison using D427: (continuous conversion mode, reference buffer enabled) ACLK = kHz, 256 OSR, SD16INTDLY_ ~7.8ms/conversion ~95uA: CPU in LPM3 (~19uA: CPU in LPM) SMCLK = 1.48MHz, 256 OSR, SD16INTDLY_ ~244us/conversion ~195uA: CPU in LPM ~32x Faster ~15% Greater Current 25 Texas Instruments Inc, Slide 15

16 Interrupt Vector Generator 15 x x x x Source SD16IV Contents No interrupt pending SD16MEMx overflow 2h SD16CCTL IFG 4h Reserved 6h Reserved 8h Reserved Ah Reserved Ch Reserved Eh Reserved 1h One shared interrupt vector: SD16_A IFG & overflow Fast decoding reduces code size/cpu load 25 Texas Instruments Inc, Slide 16

17 SD16IV Handling With Assembler SD16_ISR add add &SD16IV,PC ; Offset to to Jump Jump table table reti reti ; SD16IV =,, no no int. int. jmp jmp Over_ISR ; Overflow handler CH1_ISR ; Handle channel 1 interrupt Over_ISR ; Handle overflow interrupt 25 Texas Instruments Inc, Slide 17

18 SD16IV Handling With C // // SD16_ISR #pragma vector=sd16_vector interrupt void void SD16_ISR(void) { switch ( even_in_range(sd16iv, 16)) 16)) { case case 2: 2: Handle overflow; break; case case 4: 4: Channel 1 IFG; IFG; break; } } 25 Texas Instruments Inc, Slide 18

19 Using even_in_range() versus 25 Texas Instruments Inc, Slide 19

20 Selecting an MSP43 ADC f SAM PLE (ksps) reference channels res SINAD A min max (typ) IN Ref IN Ref OUT Ref I_OUT triggering gain features ADC Vss to Vref /2.5V +/-1mA SW/Timer/Cont N/A DTC ADC Vss to Vref /2.5V +/-1mA SW/Timer/Cont N/A Conv Mem SD16 3 ind ~ /-6mV V +/-1mA SW/Cont to 32x Preload SD16_A 4 mux'd ~.3 ~ /-6mV V +/-1mA SW/Cont to 32x Buffered input Voltage range to be measured? Max frequency for A IN? How much resolution? Differential inputs? Reference range? Multiple channels? Bits Sigma-Delta SAR Slope 1 1 1k 1k 1k 1M Samples per Second 25 Texas Instruments Inc, Slide 2

21 Lab Exercise 1: Measuring Voltage Configure SD16_A Measure variable voltage Convert to mv s Update LCD with result each second 25 Texas Instruments Inc, Slide 21

22 Lab 1 Software Flow SD16_A Configuration: 1x Gain Slow input buffer Channel A1+/- Internal V REF f M = ACLK, 256 OSR Single conversion 2 s complement WDT ISR (1sec) Exit LPM3 on wake Init Configure Peripherals Measure Offset Single sample A7+/- Reconfigure for A1+/- Main Enable Ref, Start Conv Enter LPM3 Calculate Voltage Update Display SD16 ISR Handle converison result/offset Disable Ref 25 Texas Instruments Inc, Slide 22

23 Lab 1 SD16_A Setup // // Add Add SD16_A SD16_A Configuration Here Here SD16CTL SD16CTL = ; SD16CCTL = = ; // // Get Get internal internal offset offset SD16INCTL = ; SD16CCTL = = ; while(!(sd16cctl & )); offset offset = SD16MEM; // // Configure for for external external potentiometer SD16INCTL = ; SD16CCTL = ; Add appropriate SD16_A configuration code Configure for single conversion from A7 Start conversion & poll IFG Reconfigure for single conversions from A1 25 Texas Instruments Inc, Slide 23

24 Lab 1 SD16_A Main // // Main Main while while (1) (1) { SD16CTL SD16CTL = = ; SD16CCTL = = ; ; _BIS_SR(LPM3_bits + GIE); GIE); Disp_Value(2, (float)result * MV_PER_LSB); } Enable internal reference Start a conversion 25 Texas Instruments Inc, Slide 24

25 Lab 1 SD16_A ISR // // Add Add SD16_A SD16_A ISR ISR Here Here #pragma #pragma vector=sd16_vector interrupt void void SD16_isr(void) { switch switch ( even_in_range(sd16iv, 4)) 4)) { case case 2: 2: break; break; case case 4: 4: result result = ; SD16CTL SD16CTL &= &= ~( ); break; break; } } Get conversion and subtract measured offset Turn off internal reference 25 Texas Instruments Inc, Slide 25

26 Lab Exercise 2: Measuring Pressure Configure SD16_A Measure pressure sensor Convert to mbar Update LCD with result each second air pressure 25 Texas Instruments Inc, Slide 26

27 Lab 2 Software Flow SD16_A Configuration: 32x Gain Medium input buffer Channel A+/- Init Configure Peripherals Measure Offset Single sample A7+/- Reconfigure for A+/- (continuous) External V REF f M = MCLK, 124 OSR Continuous conversion 2 s complement WDT ISR (1sec) Exit LPM on wake Main Enter LPM Calculate Pressure Update Display SD16 ISR Handle converison result/offset 25 Texas Instruments Inc, Slide 27

28 Lab 2 SD16_A Setup // // Add Add SD16_A SD16_A Configuration Here Here SD16CTL SD16CTL = ; SD16CCTL = = ; // // Get Get internal internal offset offset SD16INCTL = ; SD16CCTL = = ; while(!(sd16cctl & )); offset offset = SD16MEM; // // Configure for for external external potentiometer SD16INCTL = ; SD16CCTL = ; Add appropriate SD16_A configuration code Configure for single conversion from A7 Start conversion & poll IFG Reconfigure for continuous conversions from A 25 Texas Instruments Inc, Slide 28

29 SD16_A: 1 And 1 Uses 16-bits with flexible input architecture Take into account reference settling SD16INTDLYx: Is it long enough? Match buffer settings for optimum power/performance Understand f M, OSR & sampling rate relationship Match the ADC with the application requirements Not just a matter of resolution! 25 Texas Instruments Inc, Slide 29

30 Lab 1 SD16_A Setup Solution // // Add Add SD16_A SD16_A Configuration Here Here SD16CTL SD16CTL = SD16VMIDON+SD16REFON+SD16SSEL1; SD16CCTL = = SD16BUF_1+SD16DF+SD16SNGL; // // Get Get internal internal offset offset SD16INCTL = SD16INCH_7; SD16CCTL = = SD16SC; SD16SC; while(!(sd16cctl & SD16IFG)); offset offset = SD16MEM; // // Configure for for external external potentiometer SD16INCTL = SD16INCH_1; SD16CCTL = SD16BUF_1+SD16DF+SD16SNGL+SD16IE; 25 Texas Instruments Inc, Slide 3

31 Lab 1 SD16_A Main Solution // // Main Main while while (1) (1) { SD16CTL SD16CTL = = SD16VMIDON+SD16REFON; SD16CCTL = = SD16SC; SD16SC; _BIS_SR(LPM3_bits + GIE); GIE); Disp_Value(2, (float)result * MV_PER_LSB); } 25 Texas Instruments Inc, Slide 31

32 Lab 1 SD16_A ISR Solution // // Add Add SD16_A SD16_A ISR ISR Here Here #pragma #pragma vector=sd16_vector interrupt void void SD16_isr(void) { switch switch ( even_in_range(sd16iv, 4)) 4)) { case case 2: 2: break; break; case case 4: 4: result result = SD16MEM-offset; SD16CTL SD16CTL &= &= ~(SD16VMIDON+SD16REFON); break; break; } } 25 Texas Instruments Inc, Slide 32

33 Lab 2 SD16_A Setup Solution // // Add Add SD16_A SD16_A Configuration Here Here SD16CTL SD16CTL = ; ; // // This This is is the the default, default, delete delete line line SD16CCTL = = SD16BUF_2+SD16OSR_124+SD16DF+SD16SNGL; // // Get Get internal internal offset offset SD16INCTL = SD16GAIN_32+SD16INCH_7; SD16CCTL = = SD16SC; SD16SC; while(!(sd16cctl & SD16IFG)); offset offset = SD16MEM; // // Configure for for external external potentiometer SD16INCTL = SD16GAIN_32+SD16INCH_; SD16CCTL = SD16BUF_2+SD16OSR_124+SD16DF+SD16SC+SD16IE; Make sure external reference has settled before offset measurement is made! 25 Texas Instruments Inc, Slide 33