eecs 373 design of microprocessor-based systems prabal dutta university of michigan

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EECS 373Design of Microprocessor-Based Systems

Prabal DuttaUniversity of Michigan

Lecture 11: Sampling, ADCs, and DACsOct 12, 2010

Slides adapted from Jonathan Huihttp://www.cs.berkeley.edu/~jwhui

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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

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Outline

• Minute quiz

• Announcements

• Sampling

• ADC

• DAC

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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

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Transducers convert one form of energy into another

• Transducers– Allow us to convert physical phenomena to a

voltage potential in a well-defined way.

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Convert light to voltage with a CdS photocell

Vsignal = (+5V) RR/(R + RR)

• Choose R=RR at median of intended range

• Cadmium Sulfide (CdS)• Cheap, low current

• tRC = Cl*(R+RR)– Typically R~50-200k– C~20pF – So, tRC~20-80uS– fRC ~ 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

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Going from analog to digital

• What we want

• How we have to get there

SoftwareSensor ADC

PhysicalPhenomena

Voltage orCurrent

ADC Counts Engineering Units

PhysicalPhenomena

Engineering Units

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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

)(xf sampled

)(xf

t

ST

V Counts

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Choosing the horizontal range

• What do the sample values represent?– Some fraction within the range of values

What range to use?

rV

tRange Too Small

rV

tRange Too Big

rV

rV

tIdeal Range

rV

rV

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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

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Converting between voltages, ADC counts, and engineering units

• Converting: ADC counts Voltage

• Converting: Voltage Engineering Units

ADCN

4095

4095

RRADCin

RR

RinADC

VVNV

VV

VVN

t

rV

rV

inV

00355.0

986.0TEMP

986.0)TEMP(00355.0

TEMPC

CTEMP

V

V

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A note about sampling and arithmetic

• 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

00355.0

986.0TEMP TEMP

C

V

4095TEMP

RRADC

VVNV

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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?

)(xf sampled

)(xf

t

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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

)(xf

maxf

maxsamples 2 ff

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Use anti-aliasing filters on ADC inputs toensure 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)

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Designing the anti-aliasing filter

• Note is in radians = 2f

• Exercise: Find an R+C pair so that the half-power point occurs at 30 Hz

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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

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Outline

• Minute quiz

• Announcements

• Sampling

• ADC

• DAC

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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 conversion3. When interrupt occurs, read sample from data

register4. Wait for a sample period5. Repeat step 1

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Architecture of the TI MSP430 ADC subsystem

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ADC Features

Texas Instruments MSP430

Atmel

ATmega 1281

Resolution 12 bits 10 bits

Sample Rate 200 ksps 76.9 ksps

Internally Generated Reference Voltage

1.5V, 2.5V, Vcc 1.1V, 2.56V

Single-Ended Inputs 12 16

Differential Inputs 0 14 (4 with gain amp)

Left Justified Option No Yes

Conversion Modes Single, Sequence, Repeated Single, Repeated Sequence

Single, Free Running

Data Buffer 16 samples 1 sample

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ADC Core

• Input– Analog signal

• Output– 12-bit digital value of

input relative to voltage references

• Linear conversion

4095

4095

RRADCin

RR

RinADC

VVNV

VV

VVN

RV

RVinV

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SAR ADC

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

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SAR ADC

• SAR = Successive-Approximation-Register– Binary search to find closest digital value

1 Sample Multiple cycles

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SAR ADC

1 Sample Multiple cycles

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Sample and Conversion Timing

• Timing driven by:– TimerA– TimerB– Manually using ADC12SC bit

• Signal selection using SHSx• Polarity selection using ISSH

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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?

Internal External

Vref+ 1.5V, 2.5V, Vcc VeRef+

Vref- AVss VeRef-

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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

ns800pF40011.9)kΩ2( Ssample Rt

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Software Configuration

• How it looks in code:

ADC12CTL0 = SHT0_2 | REF1_5V |

REFON | ADC12ON;

ADC12CTL1 = SHP;

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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

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Conversion Memory

• 16 sample buffer

• Each buffer configures sample parameters

– Voltage reference– Input channel– End-of-sequence

• CSTARTADDx indicates where to write next sample

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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

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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;


post signalReadDone();

}

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ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

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ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

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ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

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ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

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MCU

Kernel Driver

Interrupts and Tasks

ADC

Application



ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

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Outline

• Minute quiz

• Announcements

• Sampling

• ADC

• DAC

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A decoder-based DAC architecture in linear and folded forms

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A binary-scaled DAC architecture in linear and folded forms

• Much more efficient• Monotonicity not guaranteed• May experiences glitches

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DAC output signal conditioning

• Often use a low-pass filter• May need a unity gain op amp for drive strength

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Questions?

Comments?

Discussion?

eecs 373 design of microprocessor-based systems prabal dutta university of michigan

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