ECE2049: Embedded Computing in Engineering Design C Term Spring Lecture #14: Using the ADC12 Analog-to-Digital Converter

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Vivian Shelton
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1 ECE2049: Embedded Computing in Engineering Design C Term Spring 2018 Lecture #14: Using the ADC12 Analog-to-Digital Converter Reading for Today: Davies 9.2-3, 9.7, MSP430 User's Guide Ch 28 Reading for Next Class: Example code HW #4 (on web): Due Monday 2/15 (to box outside AK-011 by 9 am) Lab #2 (on web): Bonus sign-off Fri 2/9 (5 pm). Report due Tues 2/13 (in class) Exam #2 FRIDAY 2/16/2018 at 9 am in AK-116 Last Class: >> Introduction to Analog-to-Digital Conversion >> Mapping analog sensors to an ADC > FSR, Resolution, Dynamic Range 3 Key Concepts for Using Analog-to-Digital Converters 1) Full Scale Range (FSR) Max or total range of analog values that can be represented For ADC: FSR = V ref+ V ref- = volts 2) Resolution (Single bit resolution) Smallest change in value that can be measured For ADC: Resolution = FSR/number of codes = FSR/(2 k ) = volts/bit (where k = number of bits) 3) Dynamic Range Ratio of the largest to the smallest values that can be measured (in decibels) For ADC: DR = 20log 10 (2 k ) db (where k = number of bits)

2 Example. A simple digital current meter might measure current by measuring the voltage across a small sensing resistor. How could we use the ADC12 to measure across the range 0 to 1 A to 1 ma accuracy? Yes! Can we get 0.1 ma accuracy? No, not unless we change G such that 2.5 V A Elements of our solution: FSR = 2.5V, 12 bit ADC gives Resolution of ma Now we have to program MSP430F5529 to achieve this! >>> Knowing how your external sensor works and how to map it to the ADC you're using is as critical as knowing how to program the microprocessor to read the ADC!!

3 MSP430F5529 Analog-to-Digital Converter (ADC12_A) 16 channel, 12-bit sample and hold ADC (200k samples per second max) 12 External Analog Inputs A0-A7, A12-15 use same pins as Ports 6 & 7 4 internal Analog Inputs A8-A11 Configure and use by setting values in various control registers ADC12CTL0 controls the following options -- Sample and Hold time (ADC12SHT1x and ADC12SHT0x) -- Multiple sample conversion method (ADC12MSC) -- Reference Voltages (ADC12REF2_5V and ADC12REF_ON) -- ADC12ON bit -- Enable and start conversion (ADC12ENC and ADC12SC) -- Overflow/Conversion time int. enables (ADC120VIE, ADC12TVIE) ADC12CTL1 controls the following options -- Conversion start address (ADC12CSTARTADDx ) -- Sample and hold source select (ADC12SHSx) -- Sample and Hold pulse mode selectable (ADC12SHP) -- Invert signal sample and hold (ADC12ISSH) -- ADC12 clock divider (ADC12DIVx) -- ADC12 clock source select(adc12sselx) -- Conversion mode select (ADC12CONSEQx) -- ADC12 busy/conversion not complete bit (ADC12BSY) >> Results from each channel are stored in the low 12 bits of one of 16 Conversion Memory Registers (ADC12MEMx) >> Each memory register has a corresponding Conversion Memory Control Register (ADC12MCTLx) Each ADC12MCTLx controls the following options for its Memory Register -- End of Sequence (EOS) = Is this channel the end on a sequence of channels that are to be converted

4 -- Select Reference Voltages (ADC12SREFx) = -- Analog input channel selection (ADC12INCHx) = So what does the programmer need to do to use ADC12_A? 1) Select ADC Core Behavior: In ADC12CTL0 and ADC12CTL1 registers -- Clock source and divider -- Sample and hold behavior -- Reference Voltages 2) Select Conversion Mode required: ADC12CONSEQx bits in ADC12CTL1 reg -- Single channel or a sequence of channels -- Also single conversion or repeated conversions

5 3) Select input channel(s): INCHx bits in ADCMCTLx registers >> ADC12 has 12 (external) analog input signals and 4 internal inputs --> ADC12's External Analog Inputs A0 A7 and A12-A15 are multiplexed with Port 6 and 7 pins! Ex: Assume input channels A6 and A7 are to be used with ADC12_A. The Port Selection bits for those pins should be set to 1 = Function Select >> Internal input channels 8, 9 & 11 (ADC12INCHx = 1000, 1001, 1011) are connected to different chip reference voltages Could be used to do health monitoring >> Internal input channel 10 (ADC12INCH_10 = 1010) is connected to an internal Temperature Sensor. 4) Enable appropriate interrupts -- ADC12IE register -- Do not have to use interrupts, but useful for repeated measurements -- Write ISR (should handle all possible ADC interrupts with some default behavior... a switch statement) 5) Enable and Start Conversion(s) -- ADC1CTL0 register Ex. Continuing on with the digital current meter, assume that the gain G is set so 1A equals the full scale voltage of 2.5V. Assume that the the analog input voltage is applied to A0. How would we set the ADC12 registers?

6 // Some code to implement the current sensor example from last // class. Input voltage range 0 to 2.5V corresponds to 0 to 1A. unsigned int in_value; // Reset REFMSTR to hand over control of internal reference // voltages to ADC12_A control registers REFCTL0 &= ~REFMSTR; // Initialize control register ADC12CTL0 = // ADC12SHT0x = 9h (384 clk cycles), MCS = 0 = no burst mode // ADC12 REF2_5V = 1 (2.5V), ADCREFON = 1 = use internal ref volt // and ADC12ON = 1 = turn ADC on ADC12CTL0 = ADC12SHT0_9 ADC12REFON ADC12REF2_5V ADC12ON; // Initialize control register ADC12CTL1 = // ADC12CSTART_ADDx = 0000 = start conversion with ADC12MEM0, // ADC12SHSx = 00 = use SW conversion trigger, ADC12SC bits // ADC12SHP = 1 = SAMPCON signal sourced from sampling timer, // ADC12ISSH = 0 = sample input signal not inverted, // ADC12DIVx = 000= divide ADC12CLK by 1, // ADC12SSEL=00= ADC clock ADC12OSC (~5 MHz), // ADC12CONSEQx = 00 single channel, single conversion, // ADC12BUSY = 0 = no ADC operation active ADC12CTL1 = ADC12SHP; // Set conversion mem control register ADC12MCTL0 = // EOS = 0, SREF =001 -->Voltage refs = GND to (Vref+) // INCHx = 0000 = analog input from A0 ADC12MCTL0 = ADC12SREF_1 + ADC12INCH_0; P6SEL = BIT0; // Set Port 6 Pin 0 to FUNCTION mode for ADC ADC12CTL0 &= ~ADC12SC; // clear the start bit //Enable and start (single) conversion (not using ADC int) ADC12CTL0 = ADC12SC + ADC12ENC; // Poll busy bit waiting for conversion to complete while (ADC12CTL1 & ADC12BUSY) no_operation(); in_value = ADC12MEM0 & 0x0FFF; // keep only low 12 bits >> How would we use the results from the ADC in our program?

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