1 EECS 373 Design of Microprocessor-Based Systems Prabal Dutta University of Michigan Lecture 11: Sampling, ADCs, and DACs Oct 12, 2010 Slides adapted from Jonathan Hui

2 Announcements Homework, Labs, and Minute Quizzes Office Hours –Tuesday, Oct 12, 2:30 PM – 3:30 PM, in EECS 2334 –Thursday, Oct 14, 1:30 PM – 3:00 PM, in CSE 4773 –Last ones before the midterm Midterm –Thursday, Oct 21, 2010 –10:40 AM – 11:30 AM (50 minutes) –In-class Labs

3 Outline Minute quiz Announcements Sampling ADC DAC

4 We live in an analog world Everything in the physical world is an analog signal –Sound, light, temperature, pressure Need to convert into electrical signals –Transducers: converts one type of energy to another Electro-mechanical, Photonic, Electrical, … –Examples Microphone/speaker Thermocouples Accelerometers

5 Transducers convert one form of energy into another Transducers –Allow us to convert physical phenomena to a voltage potential in a well-defined way.

6 Convert light to voltage with a CdS photocell V signal = (+5V) R R /(R + R R ) Choose R=R R at median of intended range Cadmium Sulfide (CdS) Cheap, low current t RC = C l *(R+R R ) –Typically R~50-200k  –C~20pF –So, t RC ~20-80uS –f RC ~ 10-50kHz Source: Forrest Brewer

Many other common sensors (some digital) Force –strain gauges - foil, conductive ink –conductive rubber –rheostatic fluids Piezorestive (needs bridge) –piezoelectric films –capacitive force Charge source Sound –Microphones Both current and charge versions –Sonar Usually Piezoelectric Position –microswitches –shaft encoders –gyros Acceleration –MEMS –Pendulum Monitoring – Battery-level voltage – Motor current Stall/velocity – Temperature Voltage/Current Source Field –Antenna –Magnetic Hall effect Flux Gate Location –Permittivity –Dielectric Source: Forrest Brewer

8 Going from analog to digital What we want How we have to get there SoftwareSensorADC Physical Phenomena Voltage or Current ADC Counts Engineering Units Physical Phenomena Engineering Units

9 Representing an analog signal digitally How do we represent an analog signal? –As a time series of discrete values  On MCU: read the ADC data register periodically V Counts

10 Choosing the horizontal range What do the sample values represent? –Some fraction within the range of values  What range to use? Range Too Small Range Too Big Ideal Range

11 Choosing the horizontal granularity Resolution –Number of discrete values that represent a range of analog values –MSP430: 12-bit ADC 4096 values Range / 4096 = Step Larger range  less information Quantization Error –How far off discrete value is from actual –½ LSB  Range / 8192 Larger range  larger error

12 Converting between voltages, ADC counts, and engineering units Converting: ADC counts  Voltage Converting: Voltage  Engineering Units

13 A note about sampling and arithmetic Converting values in 16-bit MCUs vtemp = adccount/4095 * 1.5; tempc = (vtemp-0.986)/ ;  tempc = 0 Fixed point operations –Need to worry about underflow and overflow Floating point operations –They can be costly on the node

14 Choosing the sample rate What sample rate do we need? –Too little: we can’t reconstruct the signal we care about –Too much: waste computation, energy, resources Example: 2-bytes per sample, 4 kHz  8 kB / second What about sampling jitter? Remember Lab 1?

15 Shannon-Nyquist sampling theorem If a continuous-time signal contains no frequencies higher than, it can be completely determined by discrete samples taken at a rate: Example: –Humans can process audio signals 20 Hz – 20 KHz –Audio CDs: sampled at 44.1 KHz

16 Use anti-aliasing filters on ADC inputs to ensure that Shannon-Nyquist is satisfied Aliasing –Different frequencies are indistinguishable when they are sampled. Condition the input signal using a low-pass filter –Removes high-frequency components –(a.k.a. anti-aliasing filter)

17 Designing the anti-aliasing filter Note  is in radians  = 2  f Exercise: Find an R+C pair so that the half-power point occurs at 30 Hz

18 Can use dithering to deal with quantization Dithering –Quantization errors can result in large-scale patterns that don’t accurately describe the analog signal –Introduce random (white) noise to randomize the quantization error. Direct Samples Dithered Samples

19 Outline Minute quiz Announcements Sampling ADC DAC

20 The basics of Analog-to-Digital Conversion So, how do you convert analog signals to a discrete values? A software view: 1.Set some control registers : Specify where the input is coming from (which pin) Specify the range (min and max) Specify characteristics of the input signal (settling time) 2.Enable interrupt and set a bit to start a conversion 3.When interrupt occurs, read sample from data register 4.Wait for a sample period 5.Repeat step 1

21 Architecture of the TI MSP430 ADC subsystem

22 ADC Features Texas Instruments MSP430 Atmel ATmega 1281 Resolution12 bits10 bits Sample Rate200 ksps76.9 ksps Internally Generated Reference Voltage 1.5V, 2.5V, Vcc1.1V, 2.56V Single-Ended Inputs1216 Differential Inputs014 (4 with gain amp) Left Justified OptionNoYes Conversion ModesSingle, Sequence, Repeated Single, Repeated Sequence Single, Free Running Data Buffer16 samples1 sample

23 ADC Core Input –Analog signal Output –12-bit digital value of input relative to voltage references Linear conversion

24 SAR ADC SAR = Successive-Approximation-Register –Binary search to find closest digital value –Conversion time log(bits)

25 SAR ADC SAR = Successive-Approximation-Register –Binary search to find closest digital value 1 Sample  Multiple cycles

26 SAR ADC 1 Sample  Multiple cycles

27 Sample and Conversion Timing Timing driven by: –TimerA –TimerB –Manually using ADC12SC bit Signal selection using SHSx Polarity selection using ISSH

28 Voltage Reference Voltage Reference Generator –1.5V or 2.5V –REFON bit in ADCCTL0 –Consumes energy when on –17ms settling time External references allow arbitrary reference voltage Exercise: If you want to sample Vcc, what Vref should you use? InternalExternal Vref+ 1.5V, 2.5V, VccVeRef+ Vref- AVssVeRef-

29 Sample Timing Considerations Port 6 inputs default to high impedance When sample starts, input is enabled –But capacitance causes a low-pass filter effect  Must wait for the input signal to converge

30 Software Configuration How it looks in code: ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP;

31 Inputs and Multiplexer 12 possible inputs –8 external pins (Port 6) –1 Vref+ (external) –1 Vref- (external) –1 Thermistor –1 Voltage supply External pins may function as Digital I/O or ADC. –P6SEL register

32 Conversion Memory 16 sample buffer Each buffer configures sample parameters –Voltage reference –Input channel –End-of-sequence CSTARTADDx indicates where to write next sample

33 Conversion Modes Single-Channel Single-Conversion –Single channel sampled and converted once –Must set ENC (Enable Conversion) bit each time Sequence-of-Channels –Sequence of channels sampled and converted once –Stops when reaching ADC12MCTLx with EOS bit Repeat-Single-Channel –Single channel sampled and converted continuously –New sample occurs with each trigger (ADC12SC, TimerA, TimerB) Repeat-Sequence-of-Channels –Sequence of channels sampled and converted repeatedly –Sequence re-starts when reaching ADC12MCTLx with EOS bit

34 Software Configuration How it looks in code: Configuration ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; ADC12MCTL0 = EOS | SREF_1 | INCH_11; Reading ADC data m_reading = ADC12MEM0;

35 A Software Perspective command void Read.read() { ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; ADC12MCTL0 = EOS | SREF_1 | INCH_11; call Timer.startOneShot( 17 ); } event void Timer.fired() { ADC12CTL0 |= ENC; ADC12IE = 1; ADC12CTL0 |= ADC12SC; } task void signalReadDone() { signal Read.readDone( SUCCESS, m_reading ); } async event void HplSignalAdc12.fired() { ADC12CTL0 &= ~ENC; ADC12CTL0 = 0; ADC12IE = 0; ADC12IFG = 0; m_reading = ADC12MEM0; post signalReadDone(); }

36 A Software Perspective command void Read.read() { ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; ADC12MCTL0 = EOS | SREF_1 | INCH_11; call Timer.startOneShot( 17 ); } event void Timer.fired() { ADC12CTL0 |= ENC; ADC12IE = 1; ADC12CTL0 |= ADC12SC; } task void signalReadDone() { signal Read.readDone( SUCCESS, m_reading ); } async event void HplSignalAdc12.fired() { ADC12CTL0 &= ~ENC; ADC12CTL0 = 0; ADC12IE = 0; ADC12IFG = 0; m_reading = ADC12MEM0; post signalReadDone(); }

37 A Software Perspective command void Read.read() { ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; ADC12MCTL0 = EOS | SREF_1 | INCH_11; call Timer.startOneShot( 17 ); } event void Timer.fired() { ADC12CTL0 |= ENC; ADC12IE = 1; ADC12CTL0 |= ADC12SC; } task void signalReadDone() { signal Read.readDone( SUCCESS, m_reading ); } async event void HplSignalAdc12.fired() { ADC12CTL0 &= ~ENC; ADC12CTL0 = 0; ADC12IE = 0; ADC12IFG = 0; m_reading = ADC12MEM0; post signalReadDone(); }

38 A Software Perspective command void Read.read() { ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; ADC12MCTL0 = EOS | SREF_1 | INCH_11; call Timer.startOneShot( 17 ); } event void Timer.fired() { ADC12CTL0 |= ENC; ADC12IE = 1; ADC12CTL0 |= ADC12SC; } task void signalReadDone() { signal Read.readDone( SUCCESS, m_reading ); } async event void HplSignalAdc12.fired() { ADC12CTL0 &= ~ENC; ADC12CTL0 = 0; ADC12IE = 0; ADC12IFG = 0; m_reading = ADC12MEM0; post signalReadDone(); }

39 A Software Perspective command void Read.read() { ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; ADC12MCTL0 = EOS | SREF_1 | INCH_11; call Timer.startOneShot( 17 ); } event void Timer.fired() { ADC12CTL0 |= ENC; ADC12IE = 1; ADC12CTL0 |= ADC12SC; } task void signalReadDone() { signal Read.readDone( SUCCESS, m_reading ); } async event void HplSignalAdc12.fired() { ADC12CTL0 &= ~ENC; ADC12CTL0 = 0; ADC12IE = 0; ADC12IFG = 0; m_reading = ADC12MEM0; post signalReadDone(); }

40 MCU Kernel Driver Interrupts and Tasks ADC Application command void Read.read() { ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; ADC12MCTL0 = EOS | SREF_1 | INCH_11; call Timer.startOneShot( 17 ); } event void Timer.fired() { ADC12CTL0 |= ENC; ADC12IE = 1; ADC12CTL0 |= ADC12SC; } task void signalReadDone() { signal Read.readDone( SUCCESS, m_reading ); } async event void HplSignalAdc12.fired() { ADC12CTL0 &= ~ENC; ADC12CTL0 = 0; ADC12IE = 0; ADC12IFG = 0; m_reading = ADC12MEM0; post signalReadDone(); }

41 Outline Minute quiz Announcements Sampling ADC DAC

42 A decoder-based DAC architecture in linear and folded forms

43 A binary-scaled DAC architecture in linear and folded forms Much more efficient Monotonicity not guaranteed May experiences glitches

44 DAC output signal conditioning Often use a low-pass filter May need a unity gain op amp for drive strength

45 Questions? Comments? Discussion?