Lab 3: ADC

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Lab 3 ADC
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Signal Path
Produce a proper output voltage level
Signal Conditioning (if necessary)
Sensors

• Amplification
• Filtering

Analog to Digital Conversion (ADC)

• Light
• Temperature
• Acceleration
• Humidity
• Pressure
• etc.

• Resistance
• Capacitance
• Current
• Voltage
• etc.

10011101
Convert to voltage
Converts voltage to digital number
Todays labs
3
General View
Each storage register associated with a control
register
Positive/Negative voltage reference define the
upper and lower limits of the conversion
16 conversion result storage registers
4 clock sources available for ADC12 clock
Software configurable sample-and-hold period
12 input channels available
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Conversion Formula

• This is a conversion formula of ADC
• For MSP430 ADC12 module
• It is 12-bit ADC
• If Vin gt Vref gt output 4095 (max)
• If Vin lt Vref- gt output 0 (min)

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Input Channels

• 12 input channels available
• 8 external input pins A0 A7
• 2 external voltage reference pins VeREF, VeREF-
• 1 internal temperature sensor
• 1 supply voltage monitor
• Multiplexed with P6 GPIO
• Setting P6SEL to choose the
• function you want

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Internal Voltage Reference Generator

• Voltage references are used as a base of the
  conversion
• Input voltage compares to the voltage reference
  to generate the conversion result
• A stable voltage reference is crucial to an
  accurate conversion
• Usually we use a special circuit to generate
  stable voltage reference
• MSP430 has a built-in voltage reference generator
• It has two selectable voltage levels
• 1.5V and 2.5V

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Voltage References Options

• 3 positive reference sources
• AVcc Positive supply voltage
• VREF internal voltage reference
• Can be 1.5V or 2.5V, depends on your selection
• VeREF external positive voltage reference
• 2 negative reference sources
• AVss Negative supply voltage (GND)
• VeREF- -- external negative voltage reference
• 6 combinations in total

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ADC12 Conversion Modes

• Single channel single-conversion
• A single channel is converted once
• Sequence-of-channels
• A sequence of channels is converted once
• Repeat-single-channel
• A single channel is converted repeatedly
• Repeat-sequence-of-channels
• A sequence of channels is converted repeatedly

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Conversion Memory

• 16 ADC12MEMx memory registers to store conversion
  results
• Each ADC12MEMx is configured with an associated
  ADC12MCTLx control register.
• EOS End of sequence, indicates last conversion
• SREFx Select voltage reference
• INCHx Select Input Channel
• CSTARTADDx bits define the first memory used for
  conversion
• This bit is in ADC12CTL1 control register

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ADC12MCTLx
Use in sequence of channels mode
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Example

• Depends on the setting of conversion mode,
  ADC12MCTLx and CSTARTADDx, the ADC will have
  different behavior

Sequence of Channels
CSTARTADDx 0000
CSTARTADDx 0100
Single-channel
If you need 5 input channels
If you need 7 input channels
CSTARTADDx 0000
CSTARTADDx 0100
You need to set ADC12MCTL0 to ADC12MCTL4 (choose
the input channel and voltage reference for each
ADC12MEM). You need to set the EOS bit in
ADC12MCTL4 to 1, indicates the sequence of
conversion end here. And the conversion results
will store in corresponding ADC12 memory registers
You need to set ADC12MCTL8 to ADC12MCTL14 (choose
the input channel and voltage reference for each
ADC12MEM). You need to set the EOS bit in
ADC12MCTL14 to 1, indicates the sequence of
conversion end here. And the conversion results
will store in corresponding ADC12 memory registers
You need to set ADC12MCTL0 (choose the input
channel and voltage reference). And the
conversion result will store in ADC12MEM0
You need to set ADC12MCTL8 (choose the input
channel and voltage reference). And the
conversion result will store in ADC12MEM8
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Conversion Clock

• 4 clock sources available
• ADC12OSC ADC12 internal oscillator
• MCLK
• SMCLK
• ACLK
• For simplicity, we use ADC12OSC in our lab
• Independent of the system clock

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ADC12 Core

• Two phases for a conversion
• Sample-and-hold
• A/D conversion
• Conversions in MSP430 ADC12
• Triggered by a SHI signal
• Two sample modes
• Extended Sample Mode
• Pulse Sample Mode

SHI signal triggers a conversion and controls
the length of sample-and-hold period
SHI signal triggers a conversion, the length of
the sample-and-hold period is controlled by other
register
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The SHI Signal

• Four selectable sources for SHI
• ADC12SC bit
• A single bit in ADC12 control register, set this
  bit to high will trigger a conversion
• Timer_A output unit 1 (TA1)
• Timer_B output unit 0 (TB0)
• Timer_B output unit 1 (TB1)

Lab_2 slide
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Sample And Hold Revisit

• There is a sample and hold circuit before A/D
  conversion
• Mostly integrated with the ADC chip
• When no conversion, switch S1 is open
• When a conversion start
• S1 closed
• Input signal charge C1
• S1 open, C1 holds the value of input signal
• A/D conversion
• Sample and hold time
• Time between S1 close and re open
• If it is too short
• C1 will not fully charged (error)

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Sample Timing Considerations

• For an accurate conversion, the sample-and-hold
  time must be long enough to charge up CI to Vs
• Rs will affect the sample-and-hold time
  (tsample)
• For MSP430F1611, RI 2K ohm (max) CI 40 pF
  (max)
• Minimum tsample for 12-bit conversion

gt
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Setting tsample
If we use ADC internal oscillator (ADC12OSC) as
ADC12 clock source. Frequency of ADC12OSC is
about 5MHz, if you set SHT0x to 0100, then the
sample-and-hold time is 64(1/5000000) second
12.8 µs
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Rs Source Resistance

• Many devices didnt specify their source
  resistance (source impedance/output impedance)
• Measuring source impedance is not an easy task
• For most of the case, set the SHTx bit to 0001 or
  0010 should work
• My suggestion is
• its only 16 possible setting, just try it out!

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ADC12 Interrupts

• ADC12 has 18 interrupt sources
• ADC12IFG0-ADC12IFG15 flag
• Associated with each ADC12MEMx, indicates a
  conversion of this memory is complete
• ADC12OV ADC12MEMx overflow
• A conversion result is written to any ADC12MEMx
  before its previous conversion result was read
• ADC12TOV conversion time overflow
• Another conversion is requested before the
  current conversion is completed

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ADC12IV, Interrupt Vector Generator

• All ADC12 interrupts are source to a single
  interrupt vector
• Similar to Timer_A3
• The highest priority enabled ADC12 interrupt
  generates a number in the ADC12IV register
• Highest priority interrupt generates a number
• Branch to the ADC ISR, read ADC12IV, process the
  associate sub-routine, leave the ADC ISR
• And then, second highest priority interrupt
  generates another number
• Branch to the ADC ISR again, .

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Reset of Interrupts

• Overflow interrupts (memory overflow and
  conversion time overflow) will reset
  automatically when
• Any access, read or write, of the ADC12IV
  register
• ADC12IFG0-ADC12IFG15 flag will reset when
• Accessing their associated ADC12MEMx register

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Interrupt Enable Register
Enable the interrupt of corresponding ADC12
memory (ADC12MEM). Usually, you only need to
enable one of the source.
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ADC12 Configuration
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ADC12 Configuration
Enable/disable internal voltage reference
generator, set the voltage reference to 1.5/2.5 V
Enable/disable overflow interrupt
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ADC12 Configuration
We use this bit in ADC12CTL0 register to trigger
a conversion. You can try out the other source of
trigger. The benefit of using Timer_A or Timer_B
trigger sources is you dont need CPU to handle
the trigger
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ADC12 Configuration
Use Pulse Sample Mode
We use ADC12 internal oscillator as clock source
Depends on your requirement, choose the proper
conversion sequence mode
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Example Single Channel, single conversion

• Only one input channel, one conversion at a time

Initialization
Configure the ADC ADC12CTL0 1. setting
SHTx 2. setting voltage reference
ADC12CTL1 1. setting CSTARTADDx (pointed to
one memory) 2. setting SHSx 3. SHP 1
4. setting ADC12 clock source 5. select
single channel, single conversion
mode ADC12MCTLx 1. depends on the conversion
mode you choose, and the input channel and
voltage reference you want, set proper ADC12MCTLx
register
Turn-on ADC12
Set ADC12ON to 1
Enable Conversion
Trigger a Conversion
Set ENC to 1
An ADC interrupt generated when the conversion
complete. The conversion result is available at
the memory register you choose
You can use a timer to trigger a conversion, in
the timer ISR, set the ADC12SC bit to 1
You need to enable the corresponding interrupt.
The one selected by CSTARTADDx
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Example Sequence of Channels

• Multiple input channels, a sequence of conversions

Configure the ADC ADC12CTL0 1. setting
SHTx 2. setting voltage reference 3.
set MSC bit to 1 (multiple conversion
automatically start) ADC12CTL1 1. setting
CSTARTADDx (pointed to one memory) 2.
setting SHSx 3. SHP 1 4. setting
ADC12 clock source 5. select
Sequence-of-channels mode ADC12MCTLx 1.
depends on the conversion mode you choose, and
the input channel and voltage reference you want,
set proper ADC12MCTLx register 2. depends
on the number of channels you want, set the EOS
bit to 1 in the proper ADC12MCTLx register
Initialization
Turn-on ADC12
Set ADC12ON to 1
Enable Conversion
Trigger a Conversion
Set ENC to 1
An ADC interrupt generated when the last
conversion complete. The conversion results are
available at the memory registers you choose
You can use a timer to trigger a conversion, in
the timer ISR, set the ADC12SC bit to 1
You need to enable the interrupt of the last
conversion memory.
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Todays Labs

• Lab 1 Light Sensor
• use the sample code on website
• choose a light sensor on Taroko
• configure the ADC to take samples from the light
  sensor
• when the light sensor is covered by hand, turn on
  a LED when the hand remove, turn off the LED
• set a threshold

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Lab2 Infrared Proximity Sensor

• Connect the sensor to Taroko
• configure the ADC to take samples from the sensor
• use an obstacle(hand, paper, etc.) to approach
  the sensor
• if distance lt 10 cm, Red LED on
• if distance gt 20 cm, Green LED on
• else, Yellow LED on

Vcc
GND
Signal
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Lab 3 Accelerometer

• Connect the accelerometer module to Taroko
• Configure a GPIO pin as a supply voltage source
  for the accelerometer module
• configure the ADC to take samples from multiple
  input channels
• Configure the LEDs
• when accelerate in X direction, turn on Red LED
  accelerate in -X direction, turn off Red LED
• when accelerate in Y direction, turn on Green
  LED accelerate in -Y direction, turn off Green
  LED
• when accelerate in Z direction, turn on Yellow
  LED accelerate in -Z direction, turn off Yellow
  LED