Timer 0

Timer0 is the first timer in the course (Topic 8). It provides precise delays (replacing empty loops), counts external pulses on RA4/T0CKI, and generates periodic interrupts on overflow. You will combine it with Interrupts in Practice 3 (frequency meter) and Practice 5.

With a 20 MHz crystal and FOSC = HS (config bits), the internal timer clock is Fosc/4 = 5 MHz — the basis of all Topic 8 formulas.

What Timer0 does

Timer0 operates in two main modes:

Mode	`T0CON.T0CS`	Clock input	Typical use
Timer	0	Fosc/4 (internal)	Delays, LED blink
Counter	1	Pulses on RA4/T0CKI	Count external events

Step 1 — The counter increments automatically

You write an initial value in TMR0L (and TMR0H if 16 bit). With TMR0ON = 1, hardware increments the counter each clock cycle (divided by prescaler). On 255 → 0 (8 bit) or 65535 → 0 (16 bit), overflow occurs and TMR0IF = 1.

Step 2 — Prescaler

The prescaler slows the count. Values in T0CON.PS2:PS0: 1:2, 1:4, …, 1:256. Higher prescaler → longer time until overflow with the same initial value.

Step 3 — 8 bit vs 16 bit

Mode	Register	Max N	Max time @ 20 MHz, PS 1:256
8 bit (`T08BIT = 1`)	`TMR0L`	256	~13.1 ms
16 bit (`T08BIT = 0`)	`TMR0H:TMR0L`	65536	~3.36 s

For 2 seconds you need 16 bit + prescaler 1:256.

Step 4 — Polling or interrupt

Method	Code	When
Polling	`BTFSS INTCON, TMR0IF` in loop	One-shot delay, simple code
Interrupt	ISR at `ORG 0x0008`, `TMR0IE` + `GIE`	Background blink, frequency meter

ISR details: Interrupts — Timer0.

T0CON register

Bit	Name	Function
7	TMR0ON	1 = Timer active
6	T08BIT	1 = 8 bit; 0 = 16 bit
5	T0CS	0 = internal Fosc/4; 1 = external T0CKI
4	T0SE	Active edge in counter mode (0 = rising)
3	PSA	0 = prescaler assigned to Timer0
2–0	PS2:PS0	Prescaler 1:2 to 1:256

Example MOVLW B'00000111' → 16 bit, internal, prescaler 1:256:

  TMR0ON=0 (off while configuring)
  T08BIT=0 → 16 bit
  T0CS=0   → internal timer
  PSA=0, PS=111 → prescaler 1:256

Calculate N for a delay

Goal: 2 s with Fosc = 20 MHz, prescaler 1:256.

  Time = (4 / Fosc) × N × PS
  2 s    = (4 / 20×10⁶) × N × 256
  N      = 2 / [(4/20×10⁶) × 256] = 39062 counts

  Initial value = 65536 − 39062 = 26474 = 0x676A
  TMR0H = 0x67 , TMR0L = 0x6A

Check: N = 65536 − 26474 = 39062 ✓

Example: 2 seconds with 20 MHz crystal

Polling — reusable subroutine:

CONFIGURAR_TIMER_0:
        MOVLW   B'00000111'     ; 16 bit, internal, PS 1:256
        MOVWF   T0CON
        MOVLW   B'01100111'     ; TMR0H = 0x67
        MOVWF   TMR0H
        MOVLW   B'01101010'     ; TMR0L = 0x6A
        MOVWF   TMR0L
        RETURN

RETARDO_TMR0_2S:
        BCF     INTCON, TMR0IF  ; clear old flag
        BSF     T0CON, TMR0ON   ; start timer
espera_overflow:
        BTFSS   INTCON, TMR0IF  ; wait for overflow
        GOTO    espera_overflow ; jump → loop until TMR0IF=1
        BCF     T0CON, TMR0ON   ; stop timer
        BCF     INTCON, TMR0IF  ; clear flag
        RETURN

Use from programa::

        CALL    CONFIGURAR_TIMER_0
        CALL    RETARDO_TMR0_2S
        BSF     LATB, 0         ; action after 2 s

Counter mode — pulses on RA4

To count external pulses (e.g. frequency meter — Practice 3):

        BSF     TRISA, 4        ; RA4 / T0CKI input
        MOVLW   B'00100000'     ; T0CS=1, external counter
        MOVWF   T0CON
        CLRF    TMR0H
        CLRF    TMR0L
        BSF     T0CON, TMR0ON
        ; each pulse on T0CKI increments the counter

Measurement window: combine Timer0 as counter with Timer1 as time window — details in Practice 3.

Common mistakes

Mistake	Symptom
Forget to clear TMR0IF before waiting	Immediate or unpredictable delay
8 bit for long delay	Overflow too fast (~13 ms max with PS 1:256)
Wrong Fosc in formula	Times scaled wrong (check crystal and Proteus)
Do not stop TMR0ON when reloading in ISR	Extra counts during reload

Next step

Timer 1 and Timer 2 — 16 bit, PR2, PWM base
Practical guide — Interrupts — blink with ISR
Practice 3 — Frequency meter

Exam-style questions

What does TMR0IF = 1 indicate?

Timer0 overflowed (FF→00 or FFFF→0000 depending on mode).

What is the maximum time in 8 bit, Fosc = 20 MHz, PS = 1:256?

Time = (4/20×10⁶) × 256 × 256 = 13.1072 ms

Where is Timer0 prescaler configured?

In T0CON (PSA and PS2:PS0 bits).

Ejercicio · Parcial

Calculate maximum 8-bit time with Fosc=20MHz and prescaler 1:256

Time = (4/20MHz) × 256 × 256 = 13.1072 ms per Topic 8 of the Microcontrollers course (UNEXPO).