Data acquisition and manipulation

1
Data acquisition and manipulation

• Chapter Eleven
• 11.1 - 11.3

2
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

3
Analog vs. Digital
4
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).

5
ADC Types

• Dual ramp (slow with very high accuracy, for
  precision measurements)
• Flash converter (fast, lesser accuracy, for video
  or radar)
• Successive approximation (medium speed and
  accuracy, for general-purpose industrial
  applications, commonly found in embedded systems)

6
Conversion characteristic

• Voltage Range
• Vr Vmax 0
• Resolution
• Vr / 2n
• Quantization error
• Resolution / 2
• Vr / 2n1

7
Conversion Steps

• Get Sample
• Start conversion
• Wait
• Read digital value (in parallel or serially)
• Usually need a voltage reference

8
Elements of a data acquisition system

• Transducers physical to electrical
• Amplify and offset circuits
• The input voltage should traverse as much of its
  input range as possible
• Voltage level shifting may also be required
• Filter get rid of unwanted signal components
• Multiplexer select one of multiple inputs
• Sampler the conversion rate must be at least
  twice the highest signal frequency (Nyquist
  sampling criterion)
• ADC

9
Elements of a data acquisition system
10
Sample and hold, and acquisition time
11
Sample and hold, and acquisition time
12
Example

• What should be the acquisition time for a 10-bit
  ADC?
• The voltage should rise to
• Vs quantization error Vs - Vs / 2n1
• 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

13
Typical timing requirement of one A-to-D
conversion
14
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.

15
The PIC 16 Series
16
The PIC 16F87XA ADC module
17
Controlling the ADC

• The ADC is controlled by two SFRs
• ADCON0 (1Fh)
• ADCON1 (9Fh)
• The result of the conversion is placed in
• ADRESH (1Eh)
• ADRESL (9Eh)

18
ADCON0 A/D Control Register 0 (address 1Fh)

• ADCS10 conversion clock select
• CHS20 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

19
A/D Conversion Clock Select bits

• 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

20
ADCON1 A/D Control Register 1 (address 9Fh)

• ADFM result format select
• 1 Right justified
• 0 Left justified
• ADCS2 conversion clock select
• U unimplemented
• PCFG30 port configuration control

21
A/D result format
22
A/D Port Configuration Control
23
The analog input model
24
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)

25
Calculating acquisition time - Example

• RSS 7kO, RIC 1kO (VDD 5V), RS 0,
• Temp 35 ?C, TAD 1.6 µs
• tac 2 µs
• 7.6(7kO 1kO 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 µs

26
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
• ...

27
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

28
A/D Example Page 3

• main_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

29
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.

30
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.