Dealing with Analog Signals A Microcontroller View

Dealing with Analog Signals A Microcontroller View Jonathan Hui University of California, Berkeley EECS194-5

An Analog World • Everything in the physical world is an analog signal • Sound, light, temperature, gravitational force • Need to convert into electrical signals • Transducers: converts one type of energy to another • Electro-mechanical, Photonic, Electrical, … • Examples • Microphone/speaker • Thermocouples • Accelerometers EECS194-5

An Analog World • Transducers • Allow us to convert physical phenomena to a voltage potential in a well-defined way. EECS194-5

Engineering Units Physical Phenomena Voltage ADC Counts Sensor ADC Software Going From Analog to Digital • What we want • How we have to get there Physical Phenomena Engineering Units EECS194-5

Sampling Basics • How do we represent an analog signal? • As a time series of discrete values  On the MCU: read the ADC data register periodically V Counts EECS194-5

Range Too Big Range Too Small Ideal Range Sampling Basics • What do the sample values represent? • Some fraction within the range of values  What range to use? EECS194-5

Sampling Basics • 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 EECS194-5

Sampling Basics • Converting: ADC counts  Voltage • Converting: Voltage  Engineering Units EECS194-5

Sampling Basics • Converting values in 16-bit MCUs vtemp = adccount/4095 * 1.5; tempc = (vtemp-0.986)/0.00355;  tempc = 0 • Fixed point operations • Need to worry about underflow and overflow • Floating point operations • They can be costly on the node EECS194-5

Sampling Basics • 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 • But the mote only has 10 kB of RAM… EECS194-5

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 EECS194-5

Sampling Basics • 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) EECS194-5

Direct Samples Dithered Samples Sampling Basics • 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. EECS194-5

Analog-to-Digital Basics • So, how do you convert analog signals to a discrete values? • A software view: • 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) • Enable interrupt and set a bit to start a conversion • When interrupt occurs, read sample from data register • Wait for a sample period • Repeat step 1 EECS194-5

Block Diagram (MSP430) EECS194-5

ADC Features EECS194-5

ADC Core • Input • Analog signal • Output • 12-bit digital value of input relative to voltage references • Linear conversion EECS194-5

SAR ADC • SAR = Successive-Approximation-Register • Binary search to find closest digital value EECS194-5

SAR ADC • SAR = Successive-Approximation-Register • Binary search to find closest digital value 1 Sample  Multiple cycles EECS194-5

SAR ADC 1 Sample  Multiple cycles EECS194-5

Timing driven by: TimerA TimerB Manually using ADC12SC bit Signal selection using SHSx Polarity selection using ISSH Sample and Conversion Timing EECS194-5

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 • Want to sample Vcc, what Vref to use? EECS194-5

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 EECS194-5

How it looks in code: ADC12CTL0 = SHT0_2 | REF1_5V | REFON | ADC12ON; ADC12CTL1 = SHP; Software Configuration EECS194-5

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 orADC. P6SEL register Is this a multiplexor as you saw in CS150? Inputs and Multiplexer EECS194-5

16 sample buffer Each buffer configures sample parameters Voltage reference Input channel End-of-sequence CSTARTADDx indicates where to write next sample Conversion Memory EECS194-5

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 Conversion Modes EECS194-5

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(); } EECS194-5

Interrupts and Tasks 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(); } Application Kernel Driver MCU ADC EECS194-5

Interrupts and Tasks • Tasks are run-to-completion • Used to signal application events • Break up computation in the application • Interrupts • Generated by the hardware • Preempt execution of tasks • Interrupts and tasks can schedule new tasks Task Task Task Handler Interrupt Hardware EECS194-5

Dealing with Analog Signals A Microcontroller View