Tutorial — Practice 7: A/D converter

Learn by programming: follow each step, write your own code, then validate with the lab questions and exam. Adapt pins and routines to your board when needed.

What you will build

Read a potentiometer on AN0 with the ADC and display the value on LCD or LEDs.

Practice 7 schematic with potentiometer on AN0 and LCD — Potentiometer on RA0/AN0, LCD in 4-bit mode (RB0-RB3 data, RE0/RE1 control). — Practica 7 — Convertidor AD · UNEXPO

1
Configure ADCON1, ADCON2, and ADCON0

ADCON1: RA0 analog (AN0), rest digital (`0x0E`). ADCON2: `ADCS = 101` (16 Tosc @ 20 MHz), right-justified result (`ADFM = 1`). ADCON0: channel AN0, `ADON = 1`.
```
								        MOVLW   0x0E
        MOVWF   ADCON1
        MOVLW   B'10100010'
        MOVWF   ADCON2
        MOVLW   B'00000001'
        MOVWF   ADCON0
							
```
Tip: ADC theory: /en/adc/
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GO/DONE read routine

Set GO = 1 to start conversion. Wait for GO = 0 (polling) or use ADIF interrupt. Read ADRESL and ADRESH → 10-bit value (0–1023).


								leer_adc:
        BSF     ADCON0, GO
espera:
        BTFSC   ADCON0, GO
        GOTO    espera
        MOVF    ADRESL, W
        MOVWF   adc_l
        MOVF    ADRESH, W
        MOVWF   adc_h
        RETURN

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3
Scale to voltage or percentage

With Vref = 5 V: `V = reading × 5 / 1024`. You can show integer mV or map to PWM. Use software division or a lookup table if it does not fit in 8 bits.
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4
Display the value on LCD

Reuse 4-bit LCD routines (Practice 2). Convert the number to ASCII digit by digit (`÷ 1000`, `% 10`, etc.) and write on line 2. Update each main loop pass.
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5
Calibrate with the potentiometer

Rotate the pot from 0 to Vcc and verify 0–1023 (or 0.00–5.00 V). If readings jitter, average 4–8 samples. Capture Proteus screenshot for the report.
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Before you code — pre-lab

Pre-laboratory — reference research

Research for Practice 7 (A/D converter, UNEXPO). Links to course Topic 9.

Q 1 ADCON0, ADCON1 and ADCON2 registers

The PIC18F4550 10-bit ADC uses three registers:

| Register | Main role |
| --- | --- |
| **ADCON0** | Channel (`CHS`), ADC on (`ADON`), start conversion (`GO/DONE`) |
| **ADCON1** | Analog vs digital pins (`PCFG`) |
| **ADCON2** | Conversion clock (`ADCS`), result format (`ADFM`), acquisition time |

Typical flow: set ADCON1/ADCON2 once → select channel in ADCON0 → set `GO=1` → wait `GO=0` → read `ADRESH:ADRESL`.

Q 2 AN0 channel and potentiometer in the lab

The potentiometer divides voltage between VDD and VSS. The wiper connects to RA0/AN0. The digital value represents wiper position:

`V_in = (ADRES / 1023) × V_ref`

With `V_ref = VDD = 5 V`, ADRES = 512 ≈ 2.5 V. Set ADCON1 so RA0 is analog and other pins used by the LCD are digital.

Q 3 GO/DONE bit and conversion time

`GO/DONE` in ADCON0:

- Write 1 to GO → starts conversion.
- Hardware clears to 0 when done.

Time depends on ADCS (Tad) and acquisition time. At 20 MHz with ADCS = 101 (16 Tosc), Tad meets the datasheet minimum. Do not read the result while GO = 1.

With a 20 MHz crystal use ADCS = 101 (16 Tosc) in ADCON2 per the course guide.

Design your solution

Write your pseudocode

Include: ADC register setup, GO/DONE wait, 10-bit read and LCD display routine.

0 characters (minimum 80)

My pseudocode draft

My flowchart draft

Reference pseudocode

START
    Configure ADCON1 (AN0 analog)
    Configure ADCON2 (ADCS=101, ADFM=1)
    ADCON0 ← channel 0, ADON=1
    Init LCD
REPEAT FOREVER
    GO ← 1
    WHILE GO = 1 DO ; wait
    value ← ADRESH:ADRESL
    voltage ← scale(value)
    display voltage on LCD
END REPEAT

Reference flowchart

Practice 7 A/D converter flowchart — One-time ADC setup; loop: convert → scale → show on LCD.

Build and adapt the code

Practica · UNEXPO

Lab Practice 7

Implement analog-to-digital conversion: read a potentiometer or sensor, process the value, and display on LCD or LEDs.

Required steps in your code

Configure ADCON1 (analog pins)
Configure ADCON2 (clock, format)
Select channel in ADCON0
Start conversion and wait for GO/DONE
Read and scale to voltage or units

20 MHz crystal

Select ADCS = 101 (16 Tosc) in ADCON2 per Topic 9 ADC.

Reference source code

After building — post-lab

Post-laboratory — reference research

Closing questions on ADC and display.

Q 1 Why right-justify the result (ADFM = 1)?

With ADFM = 1 (right-justified), the 10 useful bits sit in the low positions of `ADRESH:ADRESL`, making it easier to treat the value as a 16-bit integer. Left-justified format spreads bits differently and changes scaling code.

Q 2 Sources of error in ADC readings

Common errors: noise on the analog line (missing 100 nF on AN0), high impedance, unstable reference (noisy VDD), reading before GO = 0, or supply noise shared with the LCD. Mention at least two in your report and how you mitigated them.

Plot voltage vs ADRES in your report if possible.

Check what you learned

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Tutorial — Practice 7: A/D converter

Configure ADCON1, ADCON2, and ADCON0