Microprocessors

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Microprocessors

• Data Converters
• Analog to Digital Converters (ADC)
• Convert an analog quantity (voltage, current)
  into a digital code
• Digital to Analog Converters (DAC)
• Convert a digital code into an analog quantity
  (voltage, current)
• Dr. Konstantinos Tatas and Dr. Costas Kyriacou

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Video (Analog - Digital)
3
Temperature Recording by a Digital System
Sampling quantization
Coding
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Need for Data Converters

• Digital processing and storage of physical
  quantities (sound, temperature, pressure etc)
  exploits the advantages of digital electronics
• Better and cheaper technology compared to the
  analog
• More reliable in terms of storage, transfer and
  processing
• Not affected by noise
• Processing using programs (software)
• Easy to change or upgrade the system
• (e.g. Media Player 7 ? Media Player 8 ? Real
  Player)
• Integration of different functions
• (p.?. Mobile phone watch camera games
  email

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Signals (Analog - Digital)

• Analog Signal
• can take infinity values
• can change at any time

• Digital Signal
• can take one of 2 values (0 or 1)
• can change only at distinct times

Reconstruction of an analog signal from a digital
one (Can take only predefined values)
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QUANTIZATION ERROR

• The difference between the true and quantized
  value of the analog signal
• Inevitable occurrence due to the finite
  resolution of the ADC
• The magnitude of the quantization error at each
  sampling instant is between zero and half of one
  LSB.
• Quantization error is modeled as noise
  (quantization noise)

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SAMPLING FREQUENCY (RATE)

• The frequency at which digital values are sampled
  from the analog input of an ADC
• A low sampling rate (undersampling) may be
  insufficient to represent the analog signal in
  digital form
• A high sampling rate (oversampling) requires high
  bitrate and therefore storage space and
  processing time
• A signal can be reproduced from digital samples
  if the sampling rate is higher than twice the
  highest frequency component of the signal
  (Nyquist-Shannon theorem)
• Examples of sampling rates
• Telephone 4 KHz (only adequate for speech, ess
  sounds like eff)
• Audio CD 44.1 KHz
• Recording studio 88.2 KHz

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Digital to Analog Converters

• The analog signal at the output of a D/A
  converter is linearly proportional to the binary
  code at the input of the converter.
• If the binary code at the input is 0001 and the
  output voltage is 5mV, then
• If the binary code at the input becomes 1001, the
  output voltage will become ......

D3 D2 D1 D0 Vout (mV)
0 0 0 0 0
0 0 0 1 5
0 0 1 0 10
0 0 1 1 15
0 1 0 0 20
0 1 0 1 25
0 1 1 0 30
0 1 1 1 35
1 0 0 0 40
1 0 0 1 45
1 0 1 0 50
1 0 1 1 55
1 1 0 0 60
1 1 0 1 65
1 1 1 0 70
1 1 1 1 75
45mV

• If a D/A converter has 4 digital inputs then the
  analog signal at the output can have one out of
  values.

16

• If a D/A converter has N digital inputs then the
  analog signal at the output can have one out of
  . values.

2?
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Characteristics of Data Converters

• Number of digital lines
• The number bits at the input of a D/A (or output
  of an A/D) converter.
• Typical values 8-bit, 10-bit, 12-bit and 16-bit
• Can be parallel or serial
• Microprocessor Compatibility
• Microprocessor compatible converters can be
  connected directly on the microprocessor bus as
  standard I/O devices
• They must have signals like CS, RD, and WR
• Activating the WR signal on an A/D converter
  starts the conversion process.
• Polarity
• Polar the analog signals can have only positive
  values
• Bipolar the analog signals can have either a
  positive or a negative value
• Full-scale output
• The maximum analog signal (voltage or current)
• Corresponds to a binary code with all bits set to
  1 (for polar converters)
• Set externally by adjusting a variable resistor
  that sets the Reference Voltage (or current)

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Characteristics of Data Converters (Cont)

• Resolution
• The analog voltage (or current) that corresponds
  to a change of 1LSB in the binary code
• It is affected by the number of bits of the
  converter and the Full Scale voltage (VFS)
• For example if the full-scale voltage of an 8-bit
  D/A converter is 2.55V the the resolution is
• VFS/(2N-1) 2.55 /(28-1) 2.55/255 0.01 V/LSB
  10mV/LSB
• Conversion Time
• The time from the moment that a Start of
  Conversion signal is applied to an A/D converter
  until the corresponding digital value appears on
  the data lines of the converter.
• For some types of A/D converters this time is
  predefined, while for others this time can vary
  according to the value of the analog signal.

• Settling Time
• The time needed by the analog signal at the
  output of a D/A converter to be within 10 of the
  nominal value.

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ADC RESPONSE TYPES

• Linear
• Most common
• Non-linear
• Used in telecommunications, since human voice
  carries more energy in the low frequencies than
  the high.

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ADC TYPES

• Direct Conversion
• Fast
• Low resolution
• Successive approximation
• Low-cost
• Slow
• Not constant conversion delay
• Sigma-delta
• High resolution, low-cost, high accuracy

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Interfacing with Data Converters

• Microprocessor compatible data converters are
  attached on the microprocessors bus as standard
  I/O devices.

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Programming Example 1

• Write a program to generate a positive ramp at
  the output of an 8-bit D/A converter with a 2V
  amplitude and a 1KHz frequency. Assume that the
  full scale voltage of the D/A converter is 2.55V.
  The D/A converter occupies the O/P address 0x6a0.
  

main() do for (i0ilt200i) Out32(0x6
a0,i) delayu(5) while (!_kbhit())
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D/A Converters example
Write a program to generate the waveform, shown
below, at the output of an 8-bit digital to
analog converter. The period of the waveform
should be approximately 8 ms. Assume that a time
delay function with a 1 µs resolution is
available. The full scale output of the converter
is 5.12 V and the address of the DAC is 63H.
Assuming that an 8-bit A/D converter is used to
interface a temperature sensor measuring
temperature values in the temperature range 0 -
51.2

• , specify The resolution in of the system in
• The digital output word for a temperature of 32.5
  
• The temperature corresponding to a digital output
  word of 01001110