ADC.1

Microprocessors and Microcontrollers

Analog-to-Digital Conversion

EE3954

by

Maarten Uijt de Haag, Tim Bambeck

ADC.2

PORTE

RE0 RE1 RE2 RF3 RE4 RE5 RE6 RE7

PORTA

RA0 RA1 RA2 RA3 RA4 RA5 RA6 RA7

Timer0 Timer1 Timer2 ADC

CCPs Comparators Synchronous Serial Port USARTs

Other Modules

Parallel Slave Port

LCD Drivers

Voltage Reference

Data EEPROM

8

Data Bus

Figure 4-2 in the Reference Manual

ADC.3

Analog-to-Digital Conversion (ADC) Example

PIC16F877

RA1/AN1 (ADC)

Electric Circuit

Microcontroller

Transducer Signal Conditioner

t

x(t)Analog Signal

ADC.4

Analog-to-Digital Conversion

t

v(t) Analog Signal

k

x’[k] Digital Signal

t

v’(t) Discrete-Amplitude Signal

k

x[k] Discrete-Time Signal

ADC.5

Digital-to-Analog Conversion (DAC) Example

PIC16F877

DAC

Microcontroller

Transducer

t

x(t)Analog Signal

k

x’[k]Digital Signal

PIC Microcontroller does NOT have a built-in DAC!!!

ADC.6

ADC Comparator

Basic Component:

+ _ Output, Vout

Input signal, Vin

Reference Voltage, Vref

If Vin > Vref , Vout = 5VDC (Logic ‘1’) If Vin ≤ Vref , Vout = 0VDC (Logic ‘0’)

∞

ADC.7

ADC 1-bit Example

0V

5V

2.5V

‘1’

‘0’

Vin > 2.5V: Signal is ‘1’

Vin ≤ 2.5V: Signal is ‘0’

1-bit => 21 = 2 levels => 1 threshold

ADC.8

ADC 2-bit Example

5.00 v

3.75 v

2.50 v

1.25 v

0.00 v

1.25 < Vin ≤ 2.50 V : Signal is ’01’

0.00 V ‹ Vin ≤ 1.25 V : Signal is ’00’

2.50 < Vin ≤ 3.75 V : Signal is ’10’

3.75 < Vin ‹ 5.00V : Signal is ’11’’

5 v / 4 levels = 1.25 each increment (thresholds)

2-bit => 22 = 4 levels => 3 thresholds

ADC.9

ADC 3-bit Example

3-bits => 23 = 8 levels => 7 thresholds (5v/8 = 0.625)

0.000 V 0.000V ‹ Vin ≤ 0.625V: Signal is 000’ ‘000’

‘001’ ‘010’ ‘011’ ‘100’ ‘101’ ‘110’ ‘111’

0.625 V 1.250 V 1.875 V 2.500 V 3.125 V 3.750 V 4.375 V 5.00 V

0.625V < Vin ≤ 1.250V: Signal is 001 1.250V < Vin ≤ 1.875V: Signal is 010 1.875V < Vin ≤ 2.500V: Signal is 011 2.500V < Vin ≤ 3.125V: Signal is 100 3.125V < Vin ≤ 3.750V: Signal is 101 3.750V < Vin ≤ 4.375V: Signal is 110 4.375V < Vin ≤ 5.00V: Signal is 111

A/D

Con

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Bina

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Inpu

t Vo

ltag

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To A

/D C

onve

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ADC.10

ADC 3-bit Example

Examples: Analog Input Voltage:

2.8756V -0.234V

4.9876V

1.1V

3.2V

Examples: Digital Output: 100

000 111 001 101

ADC.11

ADC The PIC16F877 Microcontroller •  10-bits:

–  210 = 1024 levels => 1023 thresholds –  Resolution = 5V/1024 = 0.0048828125 V –  Thresholds:

•  0.0048828125 V •  0.009765625 V •  0.0146484375 V •  Etc.

ADC.12

ADC Implementation: Parallel-Encoded or Flash ADC

Vref = 5V Vin

Priority Encoder

Digital Value out

I7 I6 I5 I4 I3 I2 I1 I0

O0 O1 O2

Example: 3-bit ADC

4.375V

3.125V

2.500V

1.875V

0.625V

1.250V

3.750V

ADC.13

Digital Logic – Revisited (?/) Priority Encoder

H H H H H H H L H H H

H H H H H H L X H H L

H H H H H L X X H L H

H H H H L X X X H L L

H H H L X X X X L H H

H H L X X X X X L H L

H L X X X X X X L L H

L X X X X X X X L L L

7I 6I 5I 4I 3I 2I 1I 0I 2O 1O 0O

Note: if one uses active low, LLH = 110

ADC.14

ADC Implementation: Parallel-Encoded or Flash ADC

Vref = 5V Vin

Priority Encoder

Digital Value Out

I7 I6 I5 I4 I3 I2 I1 I0

O0 O1 O2

Major drawback: An n-bit ADC Requires 2n-1 comparators, thus expensive Advantage: Highest speed ADC available

Example: 3-bit ADC

4.375V

3.125V

2.500V

1.875V

0.625V

1.250V

3.750V

ADC.15

ADC in 16F877 - Successive Approximation ADC

Vin

Clock Oscillator

Successive Approximation

Register

8-bit D/A converter

D7 D6 D5 D4 D3 D2 D1 D0

Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0

D- Flip Flops

(Latche)s

Clear Start

V

Digital Output

Data ready – End of conversion

Note: for an n-bit ADC it will take n clock-cycles to find an output.

ADC.16

ADC Successive Approximation – HW Flowchart

V < Vin

Set all bits D7…D0 to ‘0’

Set D7 to ‘1’ and compare

V to Vin

Clear D7

V < Vin

Set D6 to ‘1’ and compare

V to Vin

Clear D6

V < Vin

Set D5 to ‘1’ and compare

V to Vin

Clear D5

V < Vin

Set D0 to ‘1’ and compare

V to Vin

Clear D0

N

Y Y

N

Y

N

ADC.17

Successive Approximation Example – 4.3V – Step 1 – Bit 7 (D7)

0V

5V

D7 = Most Significant Bit

4.3V

Threshold = 2.5V

D7 = 0 (0XXX XXXX)

D7 = 1 (1XXX XXXX)

Result: 1XXX XXXX

ADC.18

Successive Approximation Example – 4.3V – Step 2 – Bit 6 (D6)

0V

5V

D7 = Most Significant Bit

4.3V

D6 = 0 (10XX XXXX)

D6 = 1 (11XX XXXX)

Result: 11XX XXXX

Threshold = 3.75

ADC.19

Example Voltage input range: 0V – 5V

Number of bits: 8 => 28 = 256 levels (= 255 thresholds)

Resolution = 5V / 256 = 0.01953125 V

Thresholds at : 0.01953125 V

2x0.01953125 V = 0.0390625 V 3x0.01953125 V = 0.05859375 V

… …

4.3V / resolution = 220.16 => 220 = binary value = 1101 1100

ADC.20

ADC As part of the microcontroller …

•  Reference Manual: –  Section 21 & 22 (8-bit ADC), –  Section 23 (10-bit ADC)

•  Datasheet: –  Section 11 (10-bit ADC)

ADC.21

ADC As part of the microcontroller …

PORTF

RF0 RF1 RF2 RF3 RF4 RF5 RF6 RF7

PORTG

RG0 RG1 RG2 RG3 RG4 RG5 RG6 RG7

Timer0 Timer1 Timer2 ADC*

CCPs Comparators Synchronous Serial Port USARTs

Other Modules

Parallel Slave Port

LCD Drivers

Voltage Reference

Data EEPROM

8

Data Bus

*or A/D

ADC.22

ADC The configuration …

ADC.23

ADC.24

ADC Register (SFR) settings

ADC.25

ADC Register (SFR) settings

ADC.26

Digital input: the standard input as we used it before!!

ADC.27

ADC The result …

ADC.28

ADC The result …

ADC.29

ADC Outside & Inside

ADC.30

ADC Timing …

ADC.31

Sample/Hold Acquisition Time Tacq = Amplifier Settling Time (Tamp)

+ Holding Capacitor Charging Time (Tc) + Temperature Coefficient Time (Tcoff)

Tacq = Tamp + Tc + Tcoff

Tamp = 2 µs for PIC 16F877

Next few slides find Tc & Tcoff:

ADC.32

Calculations for Tc:

Capacitor Charges through Series Resistances:

Rs = External signals source resistance (use 10,000 in lab) Ric = Interconnect Resistance (Ric ≤ 1,000 ohm) Rss = Sample Switch Resistance (function of Vdd) (for Vdd = 5v, Rss = 7,000 ohm)

CHOLD = Capacitor = 120 * 10-12 farads

So Time for capacitor to charge:

Tc = - CHOLD*( Rs + Ric + Rss) * ln (1/2047)

ADC.33

Calculations for Tc (continued):

Tc = - CHOLD( Rs + Ric + Rss) * ln (1/2047)

Tc = - (120 * 10-12)( 10,000 + 1,000 + 7,000) * ln (1/2047)

Tc = - (120 * 10 -12)*(18,000)*(-7.624)

Tc = 16.5 µs Therefore:

ADC.34

Finding Time for Temperature Coefficient Term (Tcoff) :

Tcoff = (Operating Temp in °C - 25°C)*(0.05µs/°C)

Lets assume worst case Operating Temperature is 50°C (= 122°F)

Tcoff = (50°C - 25°C)*(0.05µs/°C)

Therefore: Tcoff = 1.25 µs

ADC.35

So total Acquisition time Tacq =

Tacq = Tamp + Tc + Tcoff

Tacq = 2 µs + 16.5 µs + 1.25 µs

Tacq = 19.75 µs

With our 4 MHz PIC’s in lab – (1 µs instruction cycle) We would delay 20 µs before we start conversion (GO is set)

ADC.36

ADC Timing …

19.75 µs (example)

ADC.37

ADC Conversion Time for Successive Approx. Converter

Successive Approximation

TAD must be ≥ 1.6µs

Takes 11.5 TAD for conversion

ADC.38

Selecting TAD Clock Source Value Using PIC’s in Lab: Minimum TAD must be ≥ 1.6 µs

PIC’s in Lab use 4 MHz oscillator, so Tosc = 0.250 µs

We need to set ADCS1, ADCS0 bits in ADCON0 to select TAD:

ADCS1,ADCS0 Operation TAD Time 0 0 2 * Tosc 0.50 µs 0 1 8 * Tosc 2.00 µs 1 0 32 * Tosc 8.00 µs 1 1 use internal ‘RC’ ( ≈ 2 – 6 µs )

Select ADCS1, ADCS0 to 0,1 since this gives 2.0 µs time for TAD.

So A/D conversion Time = 11.5 * 2.0 µs = 23 µs

ADC.39

Time for A/D Conversion - Summary

19.75 µs (example)

11.5 * TAD = 23 µs

Note – You must wait at least 2 * TAD before next acquisition is started.

ADC.40

ADC The configuration …

ADC.41

ADC Use … Flow Chart

Configure A/D Module

Configure A/D Interrupt*

*if desired

Wait for TACQ

Start Conversion (set GO)

Wait for conversion to complete

Read A/D Result

Wait for at least 2TAD before next acquisition

•  Configure analog pins/voltage reference and digital I/O (ADCON1)

•  Select ADC input channel (ADCON0)

•  Select ADC conversion clock (ADCON0)

•  Turn on ADC module (ADCON0)

ADC.42

ADC Use … Flow Chart

Configure A/D Module

Configure A/D Interrupt*

*if desired

Wait for TACQ

Start Conversion (set GO)

Wait for conversion to complete

Read A/D Result

Wait for at least 2TAD before next acquisition

•  Clear ADIF bit •  Set ADIE bit •  Set PEIE bit •  Set GIE bit

ADC.43

ADC The interrupt …

ADC.44

ADC Use … Flow Chart

Configure A/D Module

Configure A/D Interrupt*

*if desired

Wait for TACQ

Start Conversion (set GO)

Wait for conversion to complete

Read A/D Result

Wait for at least 2TAD before next acquisition

Use POLLING: Keep checking the DO/DONE bit, if it is cleared the conversion is done. Use INTERRUPTS: Wait for the ADC interrupt

ADC.45

ADC An Example

PIC16F877

RA1/AN1

Write a program that makes continuous measurements of the output voltage of the microphone and writes the results to TEMPH (0x20) and TEMPL (0x21)

Assume that the Oscillator frequency clock is 4MHz

ADC.46

Example Configure A/D Module

Configure A/D Interrupt*

Wait for TACQ

Start Conversion (set GO)

Wait for conversion to complete

Read A/D Result

Wait for at least 2TAD before next acquisition

ADC.47

Example Configure ADC Module

0 1 0 0 1 0 0 1

RA1/AN1 thus PCFG3:PCFG0 = ‘0000’

RA1/AN1 thus CHS2:CHS0 = ‘001’

Conversion clock 4MHz < 5MHz thus

ADCS1:ADCS0 = ’01’

Turn on Module: ADON = ‘1’

1 0 0 0 0 0 0 0

Result is Right Justified thus ADFM = ‘1’

ADC.48

Code Configure ADC Module

ADCON0 equ 0x1F ADCON1 equ 0x1F

org 0x000 INIT_ADC: movlw B’01001001’

movwf ADCON0 ; Configure ADC via ADCON0 bsf STATUS, RP0 ; Access bank 1 movlw B’10000000’ movwf ADCON1 ; Configure ADC via ADCON1 bcf STATUS, RP0 ; Switch back to bank 0 … … … …

ADC.49

Configure A/D Module

Configure A/D Interrupt*

Wait for TACQ

Start Conversion (set GO)

Wait for conversion to complete

Read A/D Result

Wait for at least 2TAD before next acquisition

Example

ADC.50

Example Wait for TACQ

Need a delay for about/at least 20μseconds:

DELAY: movlw 0x05 ; 1 movwf COUNT ; 1

NXT: decfsz COUNT ; 1/2 goto NXT ; 2 return ; 2

DELAY duration including ‘call’ and return’: 5 + 3*COUNT 5 + 3*COUNT = 20 => 3*COUNT = 15 => COUNT = 5

ADC.51

Code Main Loop

… MAIN: call DELAY ; Wait for TACQ

bsf ADCON0,2 ; Start conversion (set GO) CHECK: btfsc ADCON0,2 ; Wait for conversion to complete*

goto CHECK ; use POLLING movf ADRESH,W ; Read A/D result (high byte) movwf TEMPH ; Write to TEMPH bsf STATUS,RP0 movf ADRESL,W ; Read A/D result (low byte) bcf STATUS,RP0 movwf TEMPL ; Write to TEMPL nop ; 1 Wait for 4 instruction cycles nop ; 1 goto MAIN ; 2

*GO/DONE = 0 2TAD = 2*8TOSC = 2*8*0.25μs = 4μs = 4 instruction cycles

ADC.52

Code Main Loop - Sample Period

… MAIN: call DELAY ; 20

bsf ADCON0,2 ; 1 CHECK: btfsc ADCON0,2 ; ~12 Tad = 24

goto CHECK ; movf ADRESH,W ; 1 movwf TEMPH ; 1 bsf STATUS,RP0 ; 1 movf ADRESL,W ; 1 movwf TEMPL ; 1 bcf STATUS,RP0 ; 1 nop ; 1 nop ; 1 goto MAIN ; 2

Sample Period = Ts ≈ 20 + 1 + 24 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 2 = 55TCY= 55μsec Sample Frequency = 1/Ts ≈ 1/55μsec = 18,182Hz ≈ 18kHz

ADC.53

Nyquist Criterion Why is this Sample Frequency Important?

The Nyquist criterion states that, in order to prevent undesired aliasing, one must sample a signal at a rate equal to at least twice its bandwidth.

fs > 2B

Sample frequency

Signal bandwidth

ADC.54

Nyquist Criterion Example

PIC16F877

RA1/AN1

Range of human hearing: 20 to 20,000 Hz

Must sample at fs > 2*(20,000 – 20) = 39,960Hz ≈ 40kHz

For example: CD – 44.1kHz sampling rate DVD – up to 96 kHz

And CD – 16 bits per sample

ADC.55

ADC Example … Using Interrupts

ADCON0 equ 0x1F ADCON1 equ 0x1F

org 0x000 goto INIT_ADC org 0x004 goto AD_ISR

INIT_ADC: movlw B’01001001’ movwf ADCON0 ; Configure ADC via ADCON0 bsf STATUS, RP0 ; Access bank 1 movlw B’10000000’ movwf ADCON1 ; Configure ADC via ADCON1 bcf STATUS, RP1

INIT_IRQ: bcf PIR1, ADIF ; Clear the ADIF flag (bit 6) bsf STATUS,RP0 ; Access bank 1 bsf PIE1, ADIE ; Enable AADC interrupt bcf STATUS, RP0 ; Access bank 0 bsf INTCON, PEIE ; Enable peripheral interrupts bsf INTCON, GIE ; Enable global interrupts … … …

ADC.56

Code Main Loop & Interrupt Service Routine

… call DELAY ; Wait for TACQ bsf ADCON0,2 ; Start conversion (set GO)

MAIN goto MAIN …

AD_ISR: movf ADRESH,W ; Read A/D result (high byte)

movwf TEMPH ; Write to TEMPH movf ADRESL,W ; Read A/D result (low byte) movwf TEMPL ; Write to TEMPL call DELAY ; Wait for TACQ bsf ADCON0,2 ; Start conversion (set GO) bcf PIR1, ADIF ; Clear the interrupt retfie

*GO/DONE = 0

ADC.57

ADC An Example … with Push-button

PIC16F877

RA1/AN1

Vss

VDD

RA5/AN4

NOTE that in this case RA5 is a DIGITAL input and RA1 is an ANALOG input

ADC.58

INIT_ADC: movlw B’01001001’ movwf ADCON0 ; Configure ADC via ADCON0 bsf STATUS, RP0 ; Access bank 1 movlw B’10000100’ movwf ADCON1 ; Configure ADC via ADCON1 movlw B’11111111’ ; Configure RA5 as an input pin movwf TRISA bcf STATUS, RP1