Digital Electronics EEE3017W

1
ADC Control Register

• This register is known as ATDC
• It is used to configure various ADC parameters

2
ADC Data Format

• The highest 8-bit 2s complement number that we
  can represent is 7F and the most negative is 80

3
ADC Timing

• The PRS bits set the clock prescaler
• This needs to be set to ensure optimal clock
  frequency into the ADC
• For the ADC to convert as fast as possible it
  needs to receive a 2MHz clock
• There are many different usable processor speeds
• To ensure that the clocks speeds match, the ADC
  contains a clock divider which ensures that the
  ADC receives the correct clock frequency

4
Example

• The monitor in our board initializes the clock
  frequency to 18.874368 MHz. This must be divided
  down to 2MHz to be feed into the ADC. What PRS
  value is needed?

5
Solution
6
ADC Pin Enable Register

• As the ADC shares pins with ports
• We need a way to disable port pins and enable
  analogue inputs
• This is done by setting the bits in this register

7
ADC Status and Control Register

• There is only 1 ADC onboard the GT16
• This means that we need to specifically select
  which input will be converted
• This register is used to control ADC interrupts
  and which input is to be converted

8
Types of ADC

• There are a large number of different types of
  ADC.
• We will consider some of the more common types
• Ramp Convertors
• Successive Approximation Convertors
• Dual Slope Integration Convertors
• Flash Convertors
• Sigma Delta Convertors

9
Speed vs. Resolution
10
Ramp Convertors

• Very crude ADC
• Consists of a counter which is feed into a DAC
• The counter starts at zero and counts up until
  the output of the DAC exceeds the input voltage
• The counter code is then taken as the converted
  output
• Very slow
• Conversion time not constant
• Not very popular

11
Flash Convertors

• Speed
• Very fast (up to GHz)
• Resolution
• 6 to 12-bits
• Cost
• Expensive
• Power Consumption
• High power consumption due to large number of
  comparators
• Uses
• Building block in other ADCs
• High performance applications
• High speed oscilloscopes
• RF Test Equipment

Taken from www.n-denkei.com
12
Flash Convertors

• Uses a resistor network and comparators to
  determine output code
• n resistors for an n-bit ADC, divide Vref into
  n-1 different voltage levels
• Vin is then compared to the Vref
• The output of the comparators is put into a
  priority encoder

13
Successive Approximation Convertors

• Speed
• Fairly fast
• Resolution
• 8 to 18-bits
• Cost
• Low Cost
• Uses
• PC DAQ Cards
• Onboard ADCs in Microcontrollers
• ADCs in low cost equipment

14
Successive Approximation Convertors
Taken from Analog Devices MT-021

• Uses a binary search system to find output code
• If we have a 3-bit convertor with an input
  voltage which corresponds to an output between
  the values of 101 and 110
• The convertor makes the following approximations
• 100 (too low)
• 110 (too high)
• 101 is the final result

15
Dual Slope Integration Convertors

• Speed
• Fairly slow
• Resolution
• Up to 24-bits
• Cost
• Depends on resolution
• Uses
• Popular in DMMs

Taken from Analog Devices MT-027
16
Dual Slope Integration Convertors

• Integrate the input voltage for a known period of
  time
• Usually by charging a capacitor
• This capacitor is then discharged by integrating
  a known negative voltage
• The time taken for the capacitor to fully
  discharge is measured with a counter
• The output of the counter is then the convertors
  output

Taken from Analog Devices MT-027
17
Sigma Delta Convertors

• These are quite complicated devices
• Use a delta sigma DAC in a feedback loop
• Typically slower than Flash but offer good
  resolution at a good price
• Often used in PC Sound Cards

Taken from Analog Devices MT-023
18
ADC Example 1

• This is an example of how to use the ADC in 8-bit
  continuous mode
• The port is sampled and the result is put on Port
  As LEDs

19
ADC Example 1

• XDEF _Startup
• INCLUDE 'derivative.inc
• org ROM1Start
• _Startup
• LDHX 047F Initialize the stack
  pointer
• TXS
• LDA 53
• STA SOPT Disable the watchdog timer
• MOV FF, PTADD Make PTA an output
• MOV FF, PTAD LED's are off for a logic high
• MOV A4, ATD1C
• MOV 01, ATD1PE Set the PTB0/AD1P0 pin as an
  ADC input, rest of the port is IO
• MOV 20, ATD1SC Start the converter running in
  continuous mode, no
• interrupts
• mainLoop
• LDA ATD1RH
• COMA Invert all the bits of the result
  because LED's are active
  low

20
ADC Example 2

• This is an example of how to use the ADC in 8-bit
  interrupting, 1 shot mode
• The ADC interrupt initiates the next conversion
• The port is sampled and the result is put on Port
  As LEDs

21
ADC Example 2

• XDEF _Startup
• INCLUDE 'derivative.inc
• org ROM1Start
• _Startup
• LDHX 047F Initialize the stack
  pointer
• TXS
• LDA 53
• STA SOPT Disable the watchdog timer
• MOV FF, PTADD Make PTA an output
• MOV FF, PTAD LED's are off for a logic high
• MOV A4,ATD1C
• MOV 01,ATD1PE The PTB0/AD1P0 pin is an ADC
  input
• MOV 40,ATD1SC Start the converter in 1 shot
  mode, with interrupts
• CLI Globally enable interrupts
• mainLoop NOP
• BRA mainLoop
• ADC_IRQ
• LDA ATD1RH