1. Analog/Digital Subsystem
Converting analog signals to digital values
CS-280
Dr. Mark L. Hornick

2. Analog Input refers to some type of input signal whose value is continuously variable
Measuring analog input is very common requirement for embedded systems
Sensors are used to convert a physical signal to a time-varying voltage
CS-280
Dr. Mark L. Hornick

3. Some examples of physical quantities that are measured in devices you use
Temperature (HVAC)
Force/pressure/sound (Auto)
Position/Velocity/Acceleration (Robotics, Cameras)
Oxygen (Auto)
Biometrics (ECG, BP)
Signal strength, battery charge (mobile phone)
CS-280
Dr. Mark L. Hornick

4. Analog to Digital Conversion is
The process that discretizes a signal from its continuous (analog) form to its corresponding digital approximation
ADC or A/D
CS-280
Dr. Mark L. Hornick

5. Discretization explanation
Consider an analog signal that will vary between two values – say VL and VH volts
Discretization refers to the “levels” the ADC is able to resolve the analog signal to:
a 2-bit converter can resolve 4 different discrete levels
CS-280
Dr. Mark L. Hornick

6. Resolution of Discretization
The resolution of an ADC is limited by the number of bits within the ADC
What is the resolution of a 2-bit converter that is trying to measure a 0-5v signal?
How can the resolution be improved?
CS-280
Dr. Mark L. Hornick

7. Atmel Atmega32 A/D Subsystem
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Dr. Mark L. Hornick

8. Atmega32 has a built-in 10-bit A/D converter
1024 discrete levels
PortA pins are used to sense the input analog signal
8 different signals can be connected simultaneously
You cannot use PortA for output
Measures the voltage on a PortA pins w.r.t. ground
E.g. ADC0 vs. ground
Can also measure relative voltage between pins
E.g. ADC0 vs. ADC1
CS-280
Dr. Mark L. Hornick

9. The absolute voltage that can be measured is limited
Measured voltage should be no higher than a reference voltage supplied to the AREF pin
VM <= VAref
AREF is tied to a potentiometer
Voltage can be varied between 0v and Vcc (5v)
The measured voltage VM
VM = 0v is converted to a value of 0
VM = VAref is converted to a value of 1023
VM > VAref is clipped to a value of 1023
Don’t try to measure voltages higher than 5v
Otherwise, you’ll fry the chip
CS-280
Dr. Mark L. Hornick

10. Several I/O Registers are used to control the A/D subsystem
ADCSRC (0x06) – A/D Control/Status Register
SFIOR (0x30) – Special Function Register
Used for other subsystems as well as A/D
ADMUX (0x07) – A/D Multiplex Register
ADCH (0x05) – High byte of converted value
ADCL (0x04) – Low byte of converted value
CS-280
Dr. Mark L. Hornick

11. Enabling the ADC via ADCSRA – A/D control/status register
Meaning of the various bits in the register:
ADPS0-ADPS2 : ADC clock frequency divider
set all to 1 to run ADC at 1/128th of Atmega32 clock
ADIE – Enable ADC Interrupt
Interrupt is triggered when AD conversion is complete
ISR Vector is 0x20
ADIF – automatically set when AD Interrupt occurs
Automatically cleared when ISR is executed
Or writing 1 to it (SBI) clears it
ADATE – set to enable auto-trigger
Trigger source is determined by SFIOR register
ADSC – Start AD conversion
In Free Running Mode, start first conversion
automatically returns to 0 when conversion is complete
ADEN – Set to enable ADC
CS-280
Dr. Mark L. Hornick

12. SFIOR – Special Function Register
Only meaningful if ADATE in ADCSRA is SET
In Free Running Mode, ADC Interrupts will not occur
ADC will just continuously convert as fast as it can
CS-280
Dr. Mark L. Hornick

13. ADMUX – another A/D control/status register
MUX2-MUX0 – Used to pick the PortA pin to sense
000 for PA0; 111 for PA7
MUX4-MUX3
Gain setting; just set to 0 for now
REFS1-REFS0
Reference voltage; just clear REFS1, set REFS0 for now
ADLAR – see next slide
CS-280
Dr. Mark L. Hornick

14. The ADCH and ADCL I/O Registers hold the 10-bit result
If ADLAR in ADMUX is set:
ADCL must be read first, because ADCL and ADCH are automatically reset after ADCH is read
If ADLAR in ADMUX is clear:
CS-280
Dr. Mark L. Hornick

15. A/D conversion takes a relatively long time
How much?
65 – 250 microseconds
i.e clock cycles at 16MHz
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Dr. Mark L. Hornick

16. ADC Results Discretization Q is a quantum Apply 1.34 volts
Q = (Vmax-Vmin)/2n
Q = (5 – 0)/210 = 5/1024 = 4.88 mV
Apply 1.34 volts
Result: 274 (0x112)
ADC “reports” volts
CS-280
Dr. Mark L. Hornick