Transcript SECT_12_AtoD_DtoA - Advanced Microcomputer Systems

A/D and D/A
Overview
• Introduction
o Definitions
o Op amps -- a quick review
• Digital-to-analog conversions
• Analog-to-digital conversions
• Reading: Text, Section 8.8, Chapter 12
Introduction to Analog I/O
• Physical phenomena typically involve analog signals (values taken
from the continuous domain)
o Examples include temperature, speed, position, pressure,
voltage, altitude, . . .
• Microprocessors work with digital quantities (values taken from
the discrete domain)
• To interact with analog signals, operate on continuous physical
values, conversion between analog and digital values is needed
• For a digital system to interact with analog systems, conversion
between analog and digital values is needed
• Building blocks to perform the conversions are:
o Digital to analog converters (DACs)
o Analog to digital converters (ADCs)
Cont..
Introduction to Analog I/O
Cont..
Introduction to Analog I/O
• Transducers convert energy from one form to
another
(E.g., convert mechanical motion to a voltage waveform)
o Sensors
 Potentiometer (position)
 Strain gauge, piezoelectric device (force)
 Thermistor, thermocouple (temperature)
 Photoconductive cell, phototransistor (light)
 Current transformer, SENSEFET (current)
 Microphone (sound)
o Actuators
 Solenoids, relays, speakers
 Darlington transistors, Triacs, SCRs, Thyristors
Cont..
Introduction to Analog I/O
• Transducers convert energy from one form to
another
(E.g., convert mechanical motion to a voltage waveform)
o Sensors
 Potentiometer (position)
 Strain gauge, piezoelectric device (force)
 Thermistor, thermocouple (temperature)
 Photoconductive cell, phototransistor (light)
 Current transformer, SENSEFET (current)
 Microphone (sound)
o Actuators
 Solenoids, relays, speakers
 Darlington transistors, Triacs, SCRs, Thyristors
Cont..
Introduction to Analog I/O
• Signal conditioning
o Most transducers (especially sensors) output low-level signals
 Usually less than 1V, may be millivolts or microvolts
o Signals are often noisy
o For these reasons, we usually need to apply adjust
(“condition”) a signal before A/D conversion
 Amplification
 Filtering
 Linearization
• Operational amplifiers (op amps)
o Useful in the design of DACs and ADCs because of their
performance characteristics
Cont..
Introduction to Analog I/O
 Open loop gain of
several hundred
thousand
 Input current
approximately 0,
output impedance
approximately zero
• Fig. 12.2 Op amp
circuits
Cont..
Introduction to Analog I/O
• Definitions (for analog electrical signals)
o Offset: minimum value
o Span: maximum value – minimum value
Some common spans:
range of 0 V to 5 V: span = 5 V
range of –12 V to 12 V: span = 24 V
range of 4 mA to 20 mA: span = 16 mA
• Definitions (for ADCs and DACs)
o Typically, the digital representation is an unsigned n-bit integer
o Resolution: span / 2n
(also known as the step size)
(this is the weight of the LSB)
Cont..
Introduction to Analog I/O
(sometimes resolution is simply given as n)
o Example:
Analog signal in range +5 to -5 volts, 8-bit ADC:
Offset = – 5 V
Span = 10 V
Step size = 10 / 256 = 39.1 mV
Notice that the MSB has a weight of 5 V (=span/2), and the
LSB has a weight of .0391 V
Digital to Analog Conversion
• A DAC takes an n-bit digital input and output a
corresponding analog voltage
• DAC systems normally consist of three components:
o A reference voltage
o The DAC itself
o An op amp for output buffering
• Ideal DAC would convert an n-bit code Bn-1 ... Bn-1
B1 B0 to output voltage as shown below:
o Vout= Span ( Bn-1 2-1 + Bn-2 2-2 + ... B0 2-n ) + Offset
o Vout will be a fractional value of the "full scale" voltage (spanoffset)
Maximum is the digital value of 111111…111 (all 1s)
Cont..
Digital to Analog Conversion
• DAC system relationship (Sho87)
Cont..
Digital to Analog Conversion
•
Weighted resistor
DACs
o Use an op amp and a
current divider network
to implement the
conversion function
Cont..
Digital to Analog Conversion
• Interfacing DAC to microprocessor system
o In principle, any DAC can be interfaced to any microprocessor
system
o In practice, some combinations of DACs and microprocessors
are easier than others and require much simpler hardware and
software in the interface
o Interfacing an 8-bit DAC to an 8-bit microprocessor is easy:
 Write to port connected to DAC and signal DAC to begin
o When DAC word > 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]
Cont..
Digital to Analog Conversion
• Double buffering example of DAC interface [Sho87]
Cont..
Digital to Analog Conversion
• D/A with the 68HC11
o No on-board D/A
o Must use an external converter
 Ex. DAC0808
• 8-bit D/A
• Connect to a parallel I/O port
• See example circuit on CEL web pages
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
• Counting converter [Sho87]
Cont..
Analog to Digital Conversion
• Successive
approximation is a
much faster method
• Figure 12.6 Successive
approximation ADC
The Analog to Digital subsystem in
the 68HC11
• The ADC system in the 68HC11 uses a variation of
the successive approximation converter
o 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
o Capacitors are charged during a sample period then held during
the approximation phase
o Each capacitor starting with the one that corresponds to the
MSB is switched in turn into the SAR circuit for the
comparison process
Cont..
The Analog to Digital subsystem in
the 68HC11
• Capacitor ladder network for A/D conversions
[HC11]
Cont..
The Analog to Digital subsystem in
the 68HC11
• 68HC11 A/D
o Supports 8 input ADC channels
o 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.
o In performing A/D conversions, 4 conversions are performed
as a "block," each taking 32 cycles -- 128 cycles total
o Control registers:
 OPTION ($1039)
• ADPU and CSEL bits
 ADCTL ($1030)
• Control and status information
Cont..
The Analog to Digital subsystem in
the 68HC11
 ADR1 - ADR4 ($1031 - $1034)
• Result registers
• To enable A/D operations on the HC11
o 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 successiveapproximation circuit
 Disabled by default to conserve power
 After enabling charge pump, the MCU should wait at least
100 usec before initiating A/D conversion (This allows
capacitor voltages to stabilize)
Cont..
The Analog to Digital subsystem in
the 68HC11
• To enable A/D operations on the HC11
o 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)
o 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)
Cont..
The Analog to Digital subsystem in
the 68HC11
• Single vs. continuous conversion
o 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
Cont..
The Analog to Digital subsystem in
the 68HC11
o 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)
o When you read a register, value may be up to 128
cycles old
Cont..
The Analog to Digital subsystem in
the 68HC11
• Single channel vs. multiple channel
o 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 (CD=1 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)
Cont..
The Analog to Digital subsystem in
the 68HC11
o 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 don’t cares
 Can use this with single or continuous conversion
• Each input is sampled every 128 clock cycles (15,625
samples per second)
Summary
o
o
o

o

•
•
•
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