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
2008 EECS194-5 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
)(xf sampled
)(xf
t
ST
V Counts
2008 EECS194-5 6
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
2008 EECS194-5 7
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
2008 EECS194-5 8
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
2008 EECS194-5 9
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)
2008 EECS194-5 13
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
2008 EECS194-5 15
Block Diagram (MSP430)
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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
2008 EECS194-5 17
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
2008 EECS194-5 22
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
2008 EECS194-5 24
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
2008 EECS194-5 28
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;
2008 EECS194-5 29
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();
}
2008 EECS194-5 30
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();
}
2008 EECS194-5 31
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();
}
2008 EECS194-5 32
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();
}
2008 EECS194-5 33
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();
}
2008 EECS194-5 34
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();
}
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
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