Timer 1 and Timer 2

After Timer 0 (8/16 bit, T0CON, delay with polling or ISR), Topic 8 continues with Timer1 and Timer2. Timer1 drives CCP capture and compare; Timer2 is the PWM time base with period register PR2. Both appear in Practice 3 (frequency meter), Practice 4/6 (PWM, stepper motor), and the PWM module.

Timer1/2 interrupts require PEIE + PIE1 plus GIE — unlike Timer0 which uses only INTCON (Interrupts).

Timer 1 — 16-bit timer

Feature	Detail
Registers	`TMR1H:TMR1L`, `T1CON`
Resolution	16 bit (65536 counts)
Prescaler	1, 2, 4, 8 (`T1CKPS1:0`)
Clock	Fosc/4 internal or external on T1CKI (RC0)
Interrupt	`TMR1IE` in PIE1, flag `TMR1IF` in PIR1

Course formula

  Time = (4 / Fosc) × (65536 − TMR1H:TMR1L) × Prescaler

Same idea as Timer0 in 16 bit, but max prescaler 1:8 (not 1:256). For long delays use high prescaler or chain overflows in the ISR.

T1CON register (summary)

Bit / field	Function
TMR1ON	1 = Timer1 active
T1CKPS1:0	Prescaler 1:1, 1:2, 1:4, 1:8
T1OSCEN	External low-power oscillator (LP)
T1SYNC	Sync in counter mode
T1CS	0 = Fosc/4; 1 = external source

Timer1 in CCP — capture and compare

The CCP module uses Timer1 as time reference:

CCP mode	What it measures / does	Practice
Capture	Latches `TMR1H:L` on edge at RC2	Pulse width, frequency
Compare	Triggers when `TMR1 == CCPR1H:L`	Timed signals

In Practice 3 (frequency meter), Timer1 often measures the time window while Timer0 counts pulses.

Example: enable Timer1 with interrupt

config_tmr1:
        MOVLW   B'00000001'     ; TMR1ON=1, prescaler 1:1 (adjust as needed)
        MOVWF   T1CON
        MOVLW   HIGH(.65536-.1000)   ; reload for N=1000 counts (example)
        MOVWF   TMR1H
        MOVLW   LOW(.65536-.1000)
        MOVWF   TMR1L
        BCF     PIR1, TMR1IF
        BSF     PIE1, TMR1IE
        BSF     INTCON, PEIE
        BSF     INTCON, GIE
        RETURN

Timer 2 — fixed period and PWM

Timer2 is an internal 8-bit timer only with period register PR2:

Feature	Detail
Counter	`TMR2` (8 bit, 0 → PR2 → reset)
Period	`PR2` defines match / overflow
Prescaler	1, 4, 16 (`T2CKPS1:0`)
Postscaler	1:1 to 1:16 (`TOUTPS3:0`) — delays interrupt
Interrupt	`TMR2IE` / `TMR2IF` in PIE1/PIR1

T2CON register (summary)

Bit / field	Function
TMR2ON	1 = Timer2 active
T2CKPS1:0	Prescaler 1, 4, 16
TOUTPS3:0	Postscaler for TMR2IF interrupt

PWM period formula (intro)

  PWM_Period ≈ (4 / Fosc) × (PR2 + 1) × T2_Prescaler

Duty percentage:

  Duty ≈ (CCPR1L : extensions) / (4 × (PR2 + 1)) × 100%

Full detail in PWM and CCP module.

Example: start Timer2

config_tmr2:
        MOVLW   D'249'          ; PR2 → defines period
        MOVWF   PR2
        MOVLW   B'00000111'     ; TMR2ON, prescaler 1:16 (adjust)
        MOVWF   T2CON
        CLRF    TMR2
        BCF     PIR1, TMR2IF
        BSF     PIE1, TMR2IE
        BSF     INTCON, PEIE
        BSF     INTCON, GIE
        RETURN

Quick comparison — Timer0, 1, and 2

	Timer0	Timer1	Timer2
Bits	8 or 16	16	8 (+ PR2)
Max prescaler	1:256	1:8	1:16 (+ post)
IE / IF	INTCON	PIE1/PIR1	PIE1/PIR1
Key use	Delay, RA4 counter	CCP capture/compare	PWM, multiplexing
Practice	3, 5	3	3, 4, 6

Timer 3

The PIC18F4550 includes Timer3 (similar to Timer1). See the datasheet for advanced projects; the UNEXPO course focuses on Timer0–2.

Common mistakes

Mistake	Consequence
Timer1/2 without PEIE	ISR never runs even if `TMR1IE=1`
Confuse PR2 with TMR2	PR2 = desired period; TMR2 counts up to PR2
PWM without active Timer2	No waveform on RC2 from CCP1
Wrong prescaler in formula	Incorrect calculated times

Next step

PWM and CCP module — duty cycle, CCP1CON
Practice 3 — Frequency meter
Practice 6 — Stepper motor

Exam-style questions

Which timer does CCP PWM use?

Timer2 with PR2 as period.

Is Timer1 8 or 16 bits?

16 bits — used in CCP capture and compare.

Which registers enable Timer2 interrupt?

TMR2IE (PIE1), PEIE and GIE (INTCON); clear TMR2IF (PIR1) in the ISR.

Ejercicio · Parcial

Pick the timer

To count pulses on RA4 for 1 s then read the total: Timer0 as counter or Timer2? Who marks the 1 s window?