Chapter 8 Analog-to-Digital Converter (ADC)
Why Analog-to-Digital Conversion?
• In the physical world, everything is analog (continuous).
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• We need an analog-to-digital converter to translate analog signals to digital numbers so that the microcontroller can read and process them.
Example: 2-bit ADC
n-resolution ADC
Analog voltage (V)
Digital represen- tation
Or in terms of VREF- & VREF+:
e.g., V = 0, V = 5V 10-bit resolution REF- REF+ ,
ADC. Suppose the ADC results are as follows. What are the corresponding voltages?
V20 = 20×5/(210-1) = 0.0977V
V499 = 499×5/(210-1)= 2.43V
V898 = 898×5/(210-1)=4.38V
How can ADC be done?
Successive Approximation
Examples of Successive Approximation
PIC18 ADC Module
Block Diagram of PIC18 ADC
SFR involving in ADC
• A/D Control Register 0 (ADCON0)
• A/D Control Register 1 (ADCON1)
• A/D Control Register 2 (ADCON2)
• A/D Result High Register (ADRESH) • A/D Result Low Register (ADRESL)
• CHS3:CHS0: Analog Channel Select Bits
– PIC18F4520 has only 1 Analog-to-Digital unit
– Can only select 1 input from AN0:AN12 to be sampled and converted
• GO/DONE:
– Setting this bit starts A/D conversion – Cleared when A/D conversion is done
– Power up (1)/Shut off (0) the ADC module
Voltage Reference Configuration bits
• VCFG1: VREF- source 1 = VREF- (AN2)
• VCFG0: VREF+ source 1 = VREF+ (AN3)
A/D Port Configuration bits
PCFG<3:0> – The input analog signal must be configured as an analog input.
ADCON1: A/D Port Configuration bits
ADCON1: A/D Port Configuration bits
• Do the following two lines sound familiar?
movlw 0x0F
movwf ADCON1
• ADFM: A/D Result Format Select bit
A/D Conversion Time: T AD
• T = time required to complete 1-bit A/D AD
conversion
• Time required for one full 10-bit A/D conversion is 11 to 12 T – Successive
• From the PIC18F4520 specification, T
approximation
AD AD osc osc
must be between 0.7 – 25 μs.
• T can be specified from 2T to 64T by selecting the A/D Conversion Clock Select Bits (ADCS<0:2> in ADCON2).
How should T be set? AD
• e.g., If a 4MHz clock is installed, T =
• The minimum T that can be set is 4T .
64T (16μs). osc
• T can be chosen from 4T
osc (1μs) to
Sample-and-hold circuit
• This circuit ensures that the analog input voltage is stable throughout the A/D conversion process.
• The analog input voltage will charge up CHOLD to a steady state before ADC begins.
• The time required to reach steady state depends on Rs, RIC, Rss and CHOLD.
A/D Acquisition Time (ACQT)
• The minimum acquisition time (T ) for
PIC18F4520 is 1.4μs.
• ADC module will insert an acquisition time between the GO/DONE bit is set and when the conversion starts.
• Suppose T = 1μs. T >= 2 T AD ACQ AD
• T is set by setting ACQT<0:2> of ACQ
ADCON2 (chosen as a multiple of T ). AD
A/D Conversion Process
1. Set GO/DONE bit
2. Wait T to charge up C ACQ
3. CHOLD is disconnected and ADC begins
4. 11-12 T is required for a 10-bit ADC AD
5. When finished:
– ADRESH:ADRESL are loaded
– GO/DONE bit is cleared
– ADIF bit is set
– CHOLD is reconnected to the analog input
Example Use of the ADC
movlw movwf
movlw movwf
movlw movwf
b’00000001′ ADCON0
b’00001110′ ADCON1
b’00010100′ ADCON2
;Select ADC Channel 0, Enable ADC
;Set RA0 Analog Port, others Digital I/O
; ADFM Left justified, ACQT 4TAD, FOSC/4
; set PORTD output
; start Conversion
; adc_wait;wait ADC to be done
;display Top 8 bit
clrf TRISD
MainLoop: bsf ADCON0, GO adc_wait: btfsc ADCON0, GO
bra adc_wait
movff ADRESH, PORTD bra MainLoop
Assume fOSC = 4MHz, write a few instructions to turn on and configure a PIC ADC with the following requirements:
• ADC converts the analog signal fed into Channel AN3.
• ADC result is left justified.
• Select VDD and VSS as reference voltages.
• T must be at least 2.5 μs.
must be at least 18 μs.
You should be able to …
• Describe the roles of the following 5 registers: ADCON0, ADCON1, ADCON2, ADRESH and ADRESL in the A/D conversion. These registers specify the following options:
– Which channel to convert?
– Which channel should be set to accept analog input?
– How to store the conversion result?
– What is Vref+ and Vref-?
– What is the conversion time (T )?
– What is the acquisition time (T
You should be able to …
• Explain the process of sampling analog voltage before A/D conversion.
• Explain why an acquisition time is needed before A/D conversion.
• Describe the successive approximation algorithm.
• Describe the whole A/D conversion process.
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