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Data acquisition and manipulation
Chapter Eleven11.1 - 11.3
Dr. Gheith Abandah 1
Outline
• Introduction• The main features of a data acquisition system• The characteristics of an analog-to-digital
converter• The characteristics of the 16F873A analog-to-
digital converter• Summary
Dr. Gheith Abandah 2
Analog vs. Digital
Property Analog Digital
Representation Continuous voltage or current Binary Number
Precision Infinite range of values
Limited by the number’s length
Resistance to Degradation Weak Tolerant to signal
degradationProcessing Limited PowerfulStorage Impossible Possible
Dr. Gheith Abandah 3
Analog to Digital Conversion
• Most physical signals are analog.• Analog signals are captured by sensors or
transducers.• Examples: temperature, sound, pressure, …• Need to convert to digital signals to facilitate
processing by the microcontroller.• The device that does this is analog-to-digital
converter (ADC).
Dr. Gheith Abandah 4
ADC Types
1. Dual ramp (slow with very high accuracy, for precision measurements)
2. Flash converter (fast, lesser accuracy, for video or radar)
3. Successive approximation (medium speed and accuracy, for general-purpose industrial applications, commonly found in embedded systems)
Dr. Gheith Abandah 5
Conversion characteristicVoltage Range
Vr = Vmax – 0
Resolution = Vr / 2n
Quantization error = ± Resolution / 2
= Vr / 2n+1
Dr. Gheith Abandah 6
Conversion Steps
1. Get Sample2. Start conversion3. Wait4. Read digital value (in parallel or serially)
• Usually need a voltage reference
Dr. Gheith Abandah 7
Elements of a data acquisition system
Dr. Gheith Abandah 8
1. Transducers: physical to electrical2. Amplify and offset circuits
– The input voltage should traverse as much of its input range as possible
– Voltage level shifting may also be required
3. Filter: get rid of unwanted signal components4. Multiplexer: select one of multiple inputs5. Sampler: the conversion rate must be at least twice
the highest signal frequency (Nyquist sampling criterion)
6. ADC
Elements of a data acquisition system
Dr. Gheith Abandah 9
Sample and hold, and acquisition time
Dr. Gheith Abandah 10
Sample and hold, and acquisition time
Dr. Gheith Abandah 11
Example• What should be the acquisition time for a 10-bit ADC?The voltage should rise to
≥ Vs – quantization error = Vs - Vs / 2n+1
≥ Vs(2047/2048) = 0.9995 Vs
VC = Vs {1 − exp(−t/RC)}
0.9995Vs = Vs {1 − exp(−t/RC)}
exp(−t/RC) = 1 − 0.9995−t = RC ln(0.0005)t = 7.6RC
Dr. Gheith Abandah 12
Typical timing requirement of one A-to-D conversion
Dr. Gheith Abandah 13
Data acquisition in the microcontroller environment
• To operate to a good level of accuracy, an ADC needs a clean power supply and ground and no electromagnetic interference.
• When an ADC is integrated inside a microcontroller, it will be affected by the noisy internal power sources.
• So, integrated ADCs are not very accurate, typically 8- or 10-bit.
Dr. Gheith Abandah 14
The PIC 16 Series
Device Pins Features
16F873A16F876A
28 3 parallel ports,3 counter/timers,2 capture/compare/PWM,2 serial,5 10-bit ADC,2 comparators
16F874A16F877A
40 5 parallel ports,3 counter/timers,2 capture/compare/PWM,2 serial,8 10-bit ADC,2 comparators
Dr. Gheith Abandah 15
The PIC® 16F87XA ADC module
Dr. Gheith Abandah 16
Controlling the ADCThe ADC is controlled by two SFRs:•ADCON0 (1Fh)•ADCON1 (9Fh)
The result of the conversion is placed in: •ADRESH (1Eh)•ADRESL (9Eh)
Dr. Gheith Abandah 17
ADCON0: A/D Control Register 0 (address 1Fh)
• ADCS1:0: conversion clock select• CHS2:0: analog channel select• GO/DONE’: conversion status
The ADC interrupt flag ADIF and interrupt
enable ADIE bits can also be used
• U: unimplemented• ADON: A/D On
Dr. Gheith Abandah 18
A/D Conversion Clock Select bits
Dr. Gheith Abandah 19
•A full 10-bit conversion takes around 12 TAD cycles
•TAD should be equal to or just greater than 1.6 μs
•Minimum 2TAD between two successive conversions
•Maximum conversion rate is 30 kHz, higher rate by switching to higher
TAD after starting the conversion
ADCON1: A/D Control Register 1 (address 9Fh)
• ADFM: result format select1 = Right justified0 = Left justified
• ADCS2: conversion clock select• U: unimplemented• PCFG3:0: port configuration control
Dr. Gheith Abandah 20
A/D result format
Dr. Gheith Abandah 21
A/D Port Configuration Control
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The analog input model
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Calculating acquisition time
tac = Amplifier settling time
+ Hold capacitor charging time + Temperature coefficient
tac = 2 μs
+ 7.6RC for 10-bit accuracy + (Temperature − 25◦C)(0.05 μs/◦C)
Dr. Gheith Abandah 24
Calculating acquisition time - Example
RSS = 7kΩ, RIC = 1kΩ (VDD = 5V), RS = 0,
Temp = 35 ◦C, TAD = 1.6 μs
tac = 2 μs
+ 7.6(7kΩ + 1kΩ + 0)(120pF) + (35 − 25)(0.05 μs/◦C) = 2 + 7.3 + 0.5 = 9.8 μs
Total time = tac + 12TAD = 9.8 + 19.2 μs = 29 μsDr. Gheith Abandah 25
A/D Example – Page 1
...
bsf status,rp0
movlw B'00001011' ;set port A bits,
movwf trisa ;ADC set as inputs
movlw B'10000100' ;bits 0,1,3 analog input
movwf adcon1 ;right justify result
...
Dr. Gheith Abandah 26
A/D Example – Page 2
...
bcf status,rp0
movlw B'01000001' ;set up ADC: clock Fosc/8,
;switch ADC on but not converting,
;channel selection now is irrelevant
movwf adcon0
Dr. Gheith Abandah 27
A/D Example – Page 3main_loop
movlw B'01000001' ;select channel 0
movwf adcon0
call delay20u ;acquisition time
bsf adcon0,go ;start conversion
btfsc adcon0,go_done ;conversion ended?
goto $-1
movf adresh,0 ;read ADC output data high
movwf ldr_left_hi
bsf status,rp0
movf adresl,0 ;read ADC output data low
bcf status,rp0
movwf ldr_left_lo
Dr. Gheith Abandah 28
Summary - 1• Most signals produced by transducers are analog in
nature, while all processing done by a microcontroller is digital.
• Analog signals can be converted to digital form using an analog-to-digital converter (ADC). The ADC generally forms just one part of a larger data acquisition system.
• Considerable care needs to be taken in applying ADCs and data acquisition systems, using knowledge among other things of timing requirements, signal conditioning, grounding and the use of voltage references.
Dr. Gheith Abandah 29
Summary - 2• The 16F873A has a 10-bit ADC module that contains
the features of a data acquisition system.• Data values, once acquired, are likely to need further
processing, including offsetting, scaling and code conversion. Standard algorithms exist for all of these, and Assembler libraries are published.
• A simple interface between the analog and digital world is the comparator, which is commonly used to classify an analog signal into one of two states.
Dr. Gheith Abandah 30