Analog-to-Digital Converter (ADC)

The PIC18F4550 has an integrated 10-bit ADC with 13 analog channels (AN0–AN12). It converts an analog voltage (potentiometer, LM35, resistor divider) into a digital number your program reads from ADRESH:ADRESL.

This page completes Topic 9 ADC: ADCON0/1/2 registers, conversion flow, and links to GPIO and config bits. The official lab is Practice 7 — A/D converter.

Analog channel map AN0 to AN12 on ports A, B and E — 13 ADC channels. RA4 is not analog. — Topic 9 ADC — based on PIC18F4550 datasheet

Figure 9.1: basic functions in a measurement system — Sensor → conditioning → ADC → processor. The ADC digitizes the analog signal. — Topic 9 — A/D Converter · UNEXPO

Figures 9.2 and 9.3: signal conditioning and ADC transfer characteristic — Conditioner before ADC and quantization curve (successive approximation). — Topic 9 — A/D Converter · UNEXPO

What the ADC does

The A/D converter measures voltage on an analog pin and represents it as integer 0–1023 (10 bits).

Step 1 — From analog world to digital

  Sensor (potentiometer, LM35…)
       ↓  voltage 0 V – Vref (typically 5 V)
  PIC ANx pin (RA0, RE0…)
       ↓  ADC module (successive approximation)
  ADRESH:ADRESL  →  value 0..1023 in your program

Step 2 — Resolution and LSB

Concept	PIC18F4550 value
Resolution	10 bits → 1024 levels (0 to 1023)
1 LSB	Vref / 1024 ≈ 4.88 mV with Vref = 5 V
Estimated voltage	`V ≈ (ADRES / 1023) × Vref`

Example: ADRES = 512 → V ≈ 2.5 V with Vref = 5 V.

Step 3 — Three configuration registers

Register	Main function
`ADCON1`	Which pins are analog vs digital (`PCFG`)
`ADCON2`	Conversion clock (`ADCS`), format (`ADFM`), acquisition time
`ADCON0`	Channel (`CHS`), on (`ADON`), start/done (`GO/DONE`)

Configure ADCON1/2 once at startup; each read touches ADCON0 (channel + GO).

Step 4 — Relation to GPIO and config bits

TRISx — ANx pin must be input.
ADCON1 — declares pin as analog; rest of port can stay digital (LCD, LEDs).
PBADEN = OFF in CONFIG — avoids RA/RB starting as analog unintentionally (Config bits).

Analog channels AN0–AN12

Port	Analog pins	Notes
A	RA0/AN0, RA1/AN1, RA2/AN2, RA3/AN3, RA5/AN4	RA4 is not analog (T0CKI digital)
B	RB0/AN12, RB1/AN10, RB2/AN8, RB3/AN9, RB4/AN11	Share port with GPIO and interrupts
E	RE0/AN5, RE1/AN6, RE2/AN7	Common when LCD uses RE

Practice 7: potentiometer on AN0 (RA0); LCD and control pins digital via ADCON1.

ADCON0 — channel, enable, and GO/DONE

Field	Function
`CHS3:CHS0`	Selects channel AN0–AN12
`ADON`	1 = ADC module on
`GO/DONE`	1 = conversion running; 0 = finished

Read flow:

  BSF ADCON0, GO     ; start conversion
       ↓
  Wait GO = 0        ; hardware done
       ↓
  Read ADRESH:ADRESL

ADCON1 — analog vs digital pins

PCFG in ADCON1 defines ANx vs digital GPIO. Typical Practice 7 — AN0 only analog:

        MOVLW   0x0E                ; RA0 analog, rest digital
        MOVWF   ADCON1

GPIO-only labs (Practice 1, keyboard LCD) often use ADCON1 = 0x0F for all pins digital.

ADCON2 — clock, format, and acquisition

Field	Function
`ADCS2:0`	Conversion clock (depends on Fosc)
`ADFM`	0 = right justified; 1 = left justified
`ACQT2:0`	Acquisition time (sample & hold)

20 MHz crystal (UNEXPO course): ADCS = 101 in ADCON2. Reference code Practice 7:

        MOVLW   B'10100010'         ; ADFM=1, ADCS=101
        MOVWF   ADCON2
        MOVLW   B'00000001'         ; channel AN0, ADON=1
        MOVWF   ADCON0
        BSF     TRISA, 0            ; RA0 input

Full walkthrough — read AN0 (potentiometer)

Subroutines in sandwich style:

config_adc:
        MOVLW   0x0E
        MOVWF   ADCON1
        MOVLW   B'10100010'
        MOVWF   ADCON2
        MOVLW   B'00000001'
        MOVWF   ADCON0
        BSF     TRISA, 0
        RETURN

read_an0:
        BSF     ADCON0, GO          ; start conversion
wait_adc:
        BTFSC   ADCON0, GO          ; skip if GO=0 (done)
        GOTO    wait_adc            ; jump → loop until finished
        MOVF    ADRESL, W           ; read result (per ADFM)
        MOVWF   adc_l
        MOVF    ADRESH, W
        MOVWF   adc_h
        RETURN

From programa:: CALL config_adc once; in loop CALL read_an0 and display on LCD or convert to voltage.

ADC interrupt (optional)

You can use ADIE in PIE1 + PEIE + GIE to read the result in an ISR when ADIF sets — same chain as Timer1. Practice 7 usually uses polling on GO/DONE.

Common mistakes

Mistake	Symptom
Skip ADCON1	Erratic reads or LCD pins affected
Pin as output (`TRIS=0`)	ADC does not measure correctly
GO without ADON	Conversion never starts
Wrong ADCS for Fosc	Slow or inaccurate conversion
Treat RA4 as analog	AN4 is on RA5, not RA4

UNEXPO Practice 7

Practica · Laboratory

Practice 7 — A/D converter

Read a potentiometer or sensor on AN0, convert the value and show on LCD or LEDs. 20 MHz crystal: ADCS = 101 in ADCON2. Full tutorial: Practice 7.

Next step

PWM / CCP — another Topic 9 module (analog-like output via duty cycle)
UART — send ADC value to PC (Practice 8)
Partial IV — ADC + serial review

Exam-style questions

How many bits does the PIC18F4550 ADC have?

10 bits → values 0–1023.

What does BSF ADCON0, GO do?

Starts a conversion; the bit stays 1 until done (GO/DONE = 0).

What is ADCON1 for?

Configures which pins are analog input vs digital GPIO.

Which PORTA pin is NOT analog?

RA4 — it is T0CKI (Timer0 digital input).

Ejercicio · Parcial

Calculate voltage

If Vref = 5 V and ADRES = 768, approximately how many volts? (768/1023 × 5 V)