1. Lab 3: ADC

2. 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.
Convert to voltage
Converts voltage to digital number
Today’s 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

4. Conversion Formula This is a conversion formula of ADC
For MSP430 ADC12 module
It is 12-bit ADC
If Vin > Vref+ => output 4095 (max)
If Vin < Vref- => output 0 (min)

5. Input Channels 12 input channels available Multiplexed with P6 GPIO
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

6. 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

7. 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

8. 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

9. 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

10. Use in sequence of channels mode
ADC12MCTLx
Use in sequence of channels mode

11. 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

12. 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

13. ADC12 Core Two phases for a conversion Conversions in MSP430 ADC12
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

14. 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

15. 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)

16. 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
=>

17. 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/ ) second = 12.8 μs

18. Rs: Source Resistance
Many devices didn’t 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
it’s only 16 possible setting, just try it out!

19. 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

20. 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, ……….

21. 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

22. Interrupt Enable Register
Enable the interrupt of corresponding ADC12 memory (ADC12MEM). Usually, you only need to enable one of the source.

23. ADC12 Configuration

24. Enable/disable overflow interrupt
ADC12 Configuration
Enable/disable internal voltage reference generator, set the voltage reference to 1.5/2.5 V
Enable/disable overflow interrupt

25. 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 don’t need CPU to handle the trigger

26. We use ADC12 internal oscillator as clock source
ADC12 Configuration
Use Pulse Sample Mode
We use ADC12 internal oscillator as clock source
Depends on your requirement, choose the proper conversion sequence mode

27. 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

28. 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.

29. Today’s 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

30. 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 < 10 cm, Red LED on
if distance > 20 cm, Green LED on
else, Yellow LED on
Signal
GND
Vcc

31. 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