Timer Programming for the PIC18 Microcontroller

Introduction

Timers are an integral part of any embedded system. They are used for generating precise time delays, measuring pulse widths, counting external events, and generating waveforms like PWM for motor control. The PIC18 family is equipped with multiple timers, each with unique capabilities. In this chapter, we will focus on the three core timers: Timer0, Timer1, and Timer2.

1. The Core Timers of PIC18

The PIC18 microcontroller typically contains up to five timers. However, Timers 0, 1, and 2 form the foundation. Their key features are summarized in Figure 1.

Figure 1: Comparison of PIC18 Timers (Based on PIC18F452)

2. Timer0: The 8/16-bit General Purpose Timer

Timer0 is the most flexible timer. It can operate as either an 8-bit or a 16-bit timer/counter. All its functions are controlled by the T0CON (Timer0 Control) register.

2.1. T0CON Register

The T0CON register is the heart of Timer0 programming. Figure 2 shows the bit structure of the T0CON register.

Figure 2: The T0CON Register (Address 0xFD5)

• TMR0ON (Bit 7): Timer0 On/Off bit.

  • 1 = Enables Timer0.

  • 0 = Disables Timer0.

• T08BIT (Bit 6): Timer0 8/16-bit mode select.

  • 1 = 8-bit timer/counter.

  • 0 = 16-bit timer/counter.

• T0CS (Bit 5): Clock Source Select.

  • 1 = External clock on RA4/T0CKI pin (Counter mode).

  • 0 = Internal instruction cycle clock (Fosc/4) (Timer mode).

• T0SE (Bit 4): Source Edge Select (only for counter mode).

  • 1 = Increment on high-to-low transition.

  • 0 = Increment on low-to-high transition.

• PSA (Bit 3): Prescaler Assignment.

  • 1 = Prescaler is NOT assigned to Timer0 (1:1). So we bypass the prescalar.

  • 0 = Prescaler is assigned to Timer0.

• T0PS2:T0PS0 (Bits 2:0): Prescaler Rate Select bits.

Figure 3: Timer0 Prescaler Ratios

2.2. Calculating Delay with Timer0

To generate a delay, we must load the TMR0 register (TMR0H and TMR0L) with an initial value. The timer increments from this value until it overflows (from 0xFF to 0x00 in 8-bit mode, or 0xFFFF to 0x0000 in 16-bit mode). The overflow sets the TMR0IF flag in the INTCON register.

Formula for Delay:

$\text{Delay}=\frac{(256-\text{TMR0 Value})\times \text{Prescaler}\times 4}{\text{Fosc}}(\text{for 8-bit mode})$

2.3. Programming Steps for Timer0

1. Configure the T0CON register (size, clock source, prescaler).

2. Load the initial value into TMR0L (and TMR0H for 16-bit mode).

3. Clear the TMR0IF flag.

4. Turn on the timer by setting TMR0ON = 1.

5. Wait for the TMR0IF flag to become 1 (polling).

Figure 4 shows a flowchart for a simple blocking delay using Timer0 polling.

2.3. Programming Steps for Timer0

1. Configure the T0CON register (size, clock source, prescaler).

2. Load the initial value into TMR0L (and TMR0H for 16-bit mode).

3. Clear the TMR0IF flag.

4. Turn on the timer by setting TMR0ON = 1.

5. Wait for the TMR0IF flag to become 1 (polling).

Assembly Code Example:

; Program to generate 1ms delay using Timer0 (Fosc = 10 MHz)
DELAY_1MS
; Configure Timer0
MOVLW B’11000111′ ; T08BIT=1 (8-bit), T0CS=0 (Int Clock),
; PSA=0 (Prescaler assigned), PS=111 (1:256)
MOVWF T0CON
MOVLW 0xD9 ; Load initial value for 1ms (approx)
MOVWF TMR0L
BCF INTCON, TMR0IF ; Clear overflow flag
BSF T0CON, TMR0ON ; Start Timer0

WAIT
BTFSS INTCON, TMR0IF ; Check if overflow occurred
BRA WAIT
BCF T0CON, TMR0ON ; Stop Timer0
RETURN

3. Timer1: The 16-bit Counter with Gate Control

Timer1 is a 16-bit timer/counter with an optional gated input, making it ideal for measuring the duration of external events. It also has a dedicated low-power oscillator input for Real-Time Clock (RTC) applications using a 32.768 kHz crystal.

3.1. T1CON Register

Timer1 is controlled by the T1CON register, shown in Figure 5.