ELN5622 Embedded Systems Class 7 Spring, 2003

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ELN5622Embedded SystemsClass 7Spring, 2003
Aaron Itskovichitskovich_at_rogers.com
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Overview

• Introduction
• Definitions
• Op Amps -- a quick review
• Digital to analog conversions
• Analog to digital conversions

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Introduction to analog input and output

• Physical phenomena are often analog in value --
  they can take on a continuous range of values
  rather than discrete values
• Examples include temperature, speed, position,
  pressure
• For a microprocessor to operate on continuous
  physical values, conversion between analog and
  digital values is needed
• Building blocks to perform the conversions are
• Digital to analog converters (DACs)
• Analog to digital converters (ADCs)

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Transducers

• Used to convert a process variable into an
  electrical signal or vice versa
• Sensors
• Potentiometer (position)
• Strain gauge, piezoelectric device (force)
• Thermistor, thermocouple (temperature)
• Photoconductive cell, phototransistor (light)
• Current transformer, SENSEFET (current)
• Microphone (sound)
• Actuators
• Solenoids, relays, speakers
• Darlington transistors, SCRs, thyristors

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Signal conditioning

• Most transducers output low-level signals
• Usually less than 1V, may be millivolts or
  microvolts
• Signal may also be noisy
• Need to apply signal conditioning before A/D
  conversion
• Amplification
• Filtering
• Linearization

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Operational Amplifiers

• Useful in the design of DACs and ADCs because of
  their performance characteristics
• Open loop gain of several hundred thousand
• Input current approximately 0, output impedance
  approximately zero

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Digital to Analog conversion

• Definitions
• Offset minimum analog voltage
• Span Maximum analog value - minimum analog
  value
• Common spans 0-5 V, 0-10 V, 4-20 mA, /- 5 V
• Weight analog change corresponding to a change
  in a bit position of the digital number (varies
  by bit position)
• Step size span / 2n (n bits in code),
  weight of the LSB (also known as the resolution)
• Example Analog signal in range 5 to -5 volts,
  8-bit digital number
• Span 10 volts
• Offset -5 volts
• Step size 10 / 256 39.1 mV
• Weights 5, 2.5, 1.25, .625, ... .039

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Digital to analog conversion

• Function take an n-bit digital input and output
  a corresponding analog voltage
• DAC systems normally consist of three components
• An accurate reference voltage
• The DAC itself
• Op amp for output buffering
• Ideal DAC would convert n-bit code Bn-1 ... B1 B0
  to output voltage as shown below
• Vout Span x ( Bn-1 2-1 Bn-2 2-2 ... B0 2-n
  ) Offset
• Vout will be a fractional value of the "full
  scale" voltage (span-offset)
• Maximum is the digital value of 111111...111 (all
  1s)

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Weighted resistor DACs

• Weighted resistor DACs
• Use an op amp and a current divider network to
  implement the conversion function

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Interfacing DAC

• In principle, any DAC can be interfaced to any
  microprocessor system
• In practice, some combinations of DACs and
  microprocessors are easier than others and
  require much simpler hardware and software in the
  interface
• Interfacing an 8-bit DAC to an 8-bit
  microprocessor is easy
• Write to port connected to DAC and signal DAC to
  begin
• When DAC word gt uP word, some problems can exist
• How do we interface a 12-bit DAC to an 8-bit I/O
  bus without having glitches in the analog output?
• Must use a double buffering scheme, as described
  in the text but best illustrated from Sho87

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DAC 68HC11

• No on-board D/A
• Must use an external converter
• Ex. DAC0808
• 8-bit D/A
• Connect to a parallel I/O port

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Analog to digital conversion

• The function of ADCs is to quantize the analog
  voltage and then output the corresponding digital
  code value
• As with the DAC conversion, a full-scale analog
  voltage will be divided into 2n quantization
  levels or steps for an n-bit digital coding
  scheme
• Slow approach -- counting conversion

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Successive approximation

• Successive approximation is a much faster method

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The analog to digital subsystem in the 68HC11

• The ADC system in the 68HC11 uses a variation of
  the successive approximation converter
• DAC is replaced by a series of capacitors that
  are charged to the voltages that correspond to
  the weights of each bit
• Much like a capacitive ladder network
• Capacitors are charged during a sample period
  then held during the approximation phase
• Each capacitor starting with the one that
  corresponds to the MSB is switched in turn into
  the SAR circuit for the comparison process

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Capacitor Ladder Network for A2D conversion in
HC11
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• 68HC11 A/D
• Supports 8 input ADC channels
• Channels are located on port E
• Channel 0 on PE0 -- not available on EVBU due to
  use of jumper J2!
• Channel 1 on PE1, etc.
• In performing A/D conversions, 4 conversions are
  performed as a "block," each taking 32 cycles --
  128 cycles total
• Control registers
• OPTION (1039)
• ADPU and CSEL bits
• ADCTL (1030)
• Control and status information
• ADR1 - ADR4 (1031 - 1034)
• Result registers

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A2D operations on the HC11

• To enable A/D operations on the HC11
• Enable the capacitor charging operations
• The system charge pump must be enabled at reset
  by setting the A/D power up bit (ADPU) in the
  system OPTION register
• This is used to charge the capacitors for the
  successive-approximation circuit
• Disabled by default to conserve power
• After enabling charge pump, the MCU should wait
  at least 100 usec before initiating A/D
  conversion
• Allows capacitor voltages to stabilize

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A2D operation on the HC11

• To enable A/D operations on the HC11
• Select clock for successive-approximation
  register (SAR) circuitry
• A/D can use the E clock or an internal RC circuit
• Use E clock if it is greater than 750 KHz (it is
  for the EVBU!)
• CSEL bit in OPTION register selects clock source
  (0 E clock, 1 RC circuit)
• Must also apply high and low reference voltages
  (VRH and VRL) to the chip that fixes span and
  offset -- 3 volt span is recommended minimum (VDD
  and VSS hardwired to the reference inputs on the
  EVBU)

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A2D operation on the HC11

• Single vs. continuous conversion
• Single conversion
• HC11 performs one set of conversions and stops
• Remember that one set is actually 4 conversions
• To select this, set the SCAN bit in ADCTL to 0
• Writing to ADCTL initiates conversion
• Also clears the CCF bit
• When conversion is complete, CCF bit is set
• No interrupt, so you must poll
• Read data from ADR1-ADR4
• To start another conversion, you must write to
  ADCTL again

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A2D operation on the HC11

• Continuous conversion
• Set SCAN bit to 1
• Writing to ADCTL initiates conversion
• Also clears CCF
• CCF set after first block of 4 conversions is
  complete
• ADR1 - ADR4 continue to be updated
• Round-robin fashion
• Each register will be updated every 128 cycles
  (32 cycles for each conversion)
• When you read a register, value may be up to 128
  cycles old

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A2D operation on the HC11

• Single channel vs. multiple channel
• Single channel
• Channel is sampled 4 consecutive times and the
  resulting 4 conversions are placed into ADR1-4
• Each conversion takes 32 clock cycles
• Set MULT bit to 0 in ADCTL register
• Use CC, CB, and CA bits in ADCTL to select the
  channel to be converted
• Always set CD to 0 (CD1 is used for factory
  testing)
• Can use this with single or continuous conversion
• Allows you to sample a single input every 32
  cycles (62,500 samples per second with 2MHz
  E-clock)

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A2D operation on the HC11

• Multiple channels
• Conversions for 4 channels will be performed
• Set the MULT bit in ADCTL to 1
• Use bit CC of the ADCTL to specify which group
  of 4 channels is to be converted
• CC 0 -- inputs 0-3 in ADR1-4
• CC 1 -- inputs 4-7 in ADR1-4
• CD should always be 0, CB and CA are dont cares
• Can use this with single or continuous conversion
• Each input is sampled every 128 clock cycles
  (15,625 samples per second)

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Sumary

• Transducers
• Signal conditioning
• D/A conversion
• Must use external converter for HC11
• A/D conversion
• HC11 has built-in A/D converter
• Uses port E
• Can convert 8 channels
• Operation
• Enable charge pump, select clock source during
  initialization
• Select single or continuous conversion, single or
  multiple channels
• Initiate conversion by writing to ADCTL
• Wait for CCF flag to indicate conversion is
  complete
• Read results