Interrupts

You already master GPIO and buttons with polling (BTFSS PORTB in a loop). Interrupts let the PIC jump to a service routine (ISR) when an event occurs — Timer0, keyboard, UART — without checking flags all the time. They are required in Practice 2 (keyboard), Practice 3 (frequency meter), and Practice 5 (timers + ISR).

You saw the base registers in Registers — interrupts. Here you complete Topic 6: vectors, enable chain, correct ISR, and link to Timer 0.

Flow diagram of an interrupt from event to RETFIE — Full sequence: detect event, save context, run ISR, clear flag, return with RETFIE. Vectors: high **0x0008 (08H)**, low **0x0018 (18H)**. — Topic 6 Interrupts — UNEXPO · Mechatronics · Microcontrollers

What an interrupt is

An interrupt temporarily stops the main program to run urgent code. When done, the PIC returns exactly where it left off using the stack and RETFIE.

Step 1 — Polling vs interrupt

Approach	How it works	When to use
Polling	`BTFSS TMR0IF` / `PORTB` in infinite loop	Simple delays, one button
Interrupt	Hardware jumps to ISR when flag activates	Keyboard, precise timers, UART, frequency meter

In Buttons, debounce with CALL retardo_20ms is polling. In Practice 2, the matrix keyboard uses RBIE so the CPU is not blocked scanning columns.

Step 2 — Three enable layers

For a peripheral ISR to run (e.g. Timer1), turn on three switches:

  GIE = 1          ; global master (INTCON)
       ↓
  PEIE = 1         ; peripherals (INTCON) — timers 1/2/3, UART, ADC…
       ↓
  TMR1IE = 1       ; specific source (PIE1)
       ↓
  Peripheral ready ; Timer1 running, overflow → TMR1IF

Timer0 is special: TMR0IE and TMR0IF are in INTCON (not PIE1). For Timer0 you need GIE + TMR0IE; PEIE does not apply to Timer0 on PIC18.

Step 3 — Flag, vector, and ISR

The event sets a flag (TMR0IF, RBIF, ADIF…) to 1.
If the source is enabled and GIE = 1, the CPU jumps to the interrupt vector.
Your ISR handles the event, clears the flag, and runs RETFIE.

Priority	Address	Assembly notation
High	`0x0008`	`ORG 0x0008` or `ORG 08H`
Low	`0x0018`	`ORG 0x0018` or `ORG 18H`

In the course and datasheet you will see both forms: 0x0008 / 0x0018 (full hex) and 08H / 18H (short form used by MPASM in ORG). Both point to the same program memory address.

If you do not clear the flag, the PIC re-enters the ISR immediately.

Step 4 — Save context

The ISR may use WREG, STATUS, and other registers the main program needed. Course minimum: save WREG and STATUS on entry and restore before RETFIE. Details in the practical guide — context.

Vectors in the `.ASM` — sandwich style

Vectors go at the top, after CONFIG and before main code:

        ORG     0x0000
        GOTO    programa         ; reset → main

        ORG     0x0008            ; high priority (also: ORG 08H)
        GOTO    isr_high         ; jump → high-priority vector

        ORG     0x0018            ; low priority (also: ORG 18H)
        GOTO    isr_low          ; jump → low-priority vector

; ── Subroutines and ISRs ──────────────────────────────
isr_high:
        ; service high-priority sources
        RETFIE

isr_low:
        ; service low-priority sources
        RETFIE

programa:
        ; main code
        END

In the lab, many practices use high priority only (ORG 0x0008 / ORG 08H) with all sources in isr_high.

Key registers

Register	Function
`INTCON.GIE`	Global enable — no interrupts without GIE
`INTCON.PEIE`	Enables peripheral interrupts (Timer1/2/3, UART, ADC…)
`INTCON.TMR0IE` / `TMR0IF`	Timer0 enable and flag
`INTCON.RBIE` / `RBIF`	RB4–RB7 change interrupt
`PIE1`, `PIE2`	Enables each peripheral source (`TMR1IE`, `ADIE`…)
`PIR1`, `PIR2`	Event flags
`IPR1`, `IPR2`	High/low priority per source

Summary chain — Timer0:

        BCF     INTCON, TMR0IF   ; clear flag before start
        BSF     INTCON, TMR0IE  ; enable Timer0 interrupt
        BSF     INTCON, GIE     ; global master on
        BSF     T0CON, TMR0ON   ; start timer

Chain — Timer1 (peripheral):

        BCF     PIR1, TMR1IF
        BSF     PIE1, TMR1IE
        BSF     INTCON, PEIE
        BSF     INTCON, GIE

Main PIC18F4550 sources

Source	Pin / module	Typical flag	Course use
INT0–INT2	RB0, RB1, RB2	`INT0IF`…	External button
RB port	RB4–RB7	`RBIF`	Matrix keyboard — Practice 2
Timer0	Internal / RA4	`TMR0IF`	Delay, blink — Timer 0
Timer1/2/3	Internal	`TMR1IF`, `TMR2IF`…	Frequency meter, PWM
CCP	RC2, etc.	`CCP1IF`	Capture, compare, PWM
ADC	ANx channels	`ADIF`	Analog read
USART	RC6/RC7	`RCIF`, `TXIF`	Serial
USB	USB module	various	Advanced projects

Minimal ISR — Timer0 with context

Integrated example (vector + minimal save + reload):

        ORG     0x0008
        GOTO    isr_high

isr_high:
        BTFSS   INTCON, TMR0IF
        GOTO    isr_done         ; not Timer0 → exit

        ; ── Save context (minimum) ──
        MOVWF   temp_w           ; save WREG
        SWAPF   STATUS, W
        MOVWF   temp_status

        BCF     T0CON, TMR0ON    ; pause timer while reloading
        BCF     INTCON, TMR0IF   ; clear flag
        MOVLW   B'01100111'
        MOVWF   TMR0H            ; reload precalculated value
        MOVLW   B'01101010'
        MOVWF   TMR0L
        BSF     T0CON, TMR0ON

        ; ── Task: toggle LED ──
        BTFSC   LATB, 0
        GOTO    led_off
        BSF     LATB, 0
        GOTO    restore
led_off:
        BCF     LATB, 0

restore:
        SWAPF   temp_status, W
        MOVWF   STATUS
        MOVF    temp_w, W
isr_done:
        RETFIE                   ; return and re-enable GIE

Timer setup: see Timer 0 — 2 seconds.

RB4–RB7 interrupt — keyboard preview

For matrix keyboard (RB interrupt guide):

        BCF     INTCON2, RBPU    ; internal PORTB pull-ups (per lab)
        BSF     INTCON, RBIE     ; enable RB4–RB7 change
        BSF     INTCON, GIE

In the ISR: scan rows/columns, debounce, and clear RBIF by reading PORTB (PIC18 hardware requirement).

Common mistakes

Mistake	Consequence
Forget GIE	ISR never runs
Do not clear TMR0IF / RBIF	Infinite ISR re-entry
`RETFIE` without restoring WREG/STATUS	Corrupted main program
ISR too long	Missed events, timer jitter
Confuse polling with interrupt	CPU at 100% in empty loop

Next step

Practical guide — Interrupts — step by step in MPLAB/Proteus
Timer 0 — formula, T0CON, precise delay
Practice 5 — Timers and interrupts — integrated lab

Exam-style questions

What does INTCON.GIE do?

Enables or disables all interrupts globally. Without GIE = 1, no ISR runs.

Why clear TMR0IF before RETFIE?

If the flag stays 1, hardware considers the event still pending and re-enters the ISR.

Where are TMR0IE and TMR0IF?

In register INTCON, not PIE1/PIR1.

What does RETFIE do?

Returns from interrupt, restores PC from the stack, and re-enables GIE automatically.

Ejercicio · Parcial

Design the chain

List the registers you must set to 1 for Timer1 to generate an interrupt: GIE? PEIE? TMR1IE? TMR1ON?