Dealing with Analog Signals A Microcontroller View

2008 EECS194-5 1

Dealing with Analog SignalsA Microcontroller View

Jonathan HuiUniversity of California, Berkeley

2008 EECS194-5 2

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

2008 EECS194-5 3

An Analog World

• Transducers– Allow us to convert physical phenomena to a voltage

potential in a well-defined way.

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Going From Analog to Digital

• What we want

• How we have to get there

SoftwareSensor ADC

PhysicalPhenomena

Voltage ADC Counts Engineering Units

PhysicalPhenomena

Engineering Units

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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

)(xf sampled

)(xf

t

ST

V Counts

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Sampling Basics

• 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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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

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Sampling Basics

• 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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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

00355.0

986.0TEMP TEMP

C

V

4095TEMP

RRADC

VVNV

2008 EECS194-5 10

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…

)(xf sampled

)(xf

t

2008 EECS194-5 11

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

)(xfmaxf

maxsamples 2 ff

2008 EECS194-5 12

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)

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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.

Direct Samples Dithered Samples

2008 EECS194-5 14

Analog-to-Digital Basics

• 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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Block Diagram (MSP430)

2008 EECS194-5 16

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

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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

• Want to sample Vcc, what Vref to use?

Internal External

Vref+ 1.5V, 2.5V, Vcc VeRef+

Vref- AVss VeRef-

2008 EECS194-5 23

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

• Is this a multiplexor as you saw in CS150?

2008 EECS194-5 26

Conversion Memory

• 16 sample buffer

• Each buffer configures sample parameters

– Voltage reference

– Input channel

– End-of-sequence

• CSTARTADDx indicates where to write next sample

2008 EECS194-5 27

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();

}

2008 EECS194-5 30




ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

2008 EECS194-5 31




ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

2008 EECS194-5 32




ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

2008 EECS194-5 33




ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

2008 EECS194-5 34

MCU

Kernel Driver

Interrupts and Tasks

ADC

Application



ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}

2008 EECS194-5 35

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

Hardware

Interrupt

Task Task Task

Handler

Dealing with Analog Signals A Microcontroller View

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