How to use Timers in PIC18F4550 Microcontroller

For basic Timer operations, refer the Tutorial on Timers. PIC18F4550 is equipped with four Timers namely, Timer0, Timer1, Timer2 and Timer3. Before going for the details of Timer configurations, it is important to learn how time delay is calculated by the timer since exact delay generation is the most common application of Timers.

 

Example: Given that a time delay of 1 sec is to be generated and a 12MHz crystal oscillator is connected with PIC. Please note that this example considers external clock source for the controller, however, PIC18F4550 has provision for both external as well as internal clock source.

 

Timer is related to the internal frequency which is always Fosc/4.

 

Clock source frequency (Crystal)

 

Therefore, Timer frequency :

 

 

If Prescaler = 1:256, then

 

 

So, the time interval :

 

 

This means that when Timer runs, it will take 85 ?secs to increment its value at every count.

To calculate the value to be filled in Timer rolling over register to generate 1 sec delay :

 

 

These values are filled in the Timer register and it rolls over up to FFFF. The values are reloaded again to start timer for same delay. Before filling this value timer registers should be configured as we shall see.

 

Each of the four Timers of PIC18F4550 has certain special features some of which are explained below. The detailed list of these features can be obtained from PIC18F4550 datasheet.

 

·         Timer0 can work as Timer/Counter in both 8-bit and 16-bit modes

·         Dedicated 8-bit, software programmable prescaler

·         Selectable clock source (internal or external)

·         Interrupt on overflow

 

·         Timer1 can work as 16-bit timer or counter

·         Readable and writable 8-bit registers (TMR1H and TMR1L)

·         Selectable clock source (internal or external)

·         Alternate clock source can be provided at Timer1 oscillator pins (T1OSO & T1OSI) 

·         Interrupt on overflow

·         Timer1 can be multiplexed with other peripherals like ADC etc. and generates special event triggering for CCP (Capture, Compare and PWM) events.

 

·         8-bit Timer and Period registers (TMR2 and PR2, respectively)

·         Software programmable prescaler (1:1, 1:4 and 1:16)

·         Software programmable postscaler (1:1 – 1:16)

·         Interrupt on TMR2 to PR2 match

·         Optional use as the shift clock for the MSSP (Master Synchronous Serial Port) module

 

·         Timer3 can work as 16-bit timer or counter

·         Readable and writable 8-bit registers (TMR3H and TMR3L)

·         Selectable clock source (internal or external)

·         Alternate clock source can be provided at Timer1 oscillator pins (T1OSO & T1OSI) 

·         Interrupt on overflow

·         Timer3 can be multiplexed with other peripherals like ADC etc. and generates special event triggering for CCP (Capture, Compare and PWM) events.

 

Every Timer has certain registers related to it which must be configured for desired operations. The registers of Timer0 have been explained below.
 

 

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
TMR0ON	T08BIT	T0CS	T0SE	PSA	T0PS2	T0PS1	T0PS0

Fig. 2: Bit Configuration of PIC18F4550 Timer0

 

T0PS2: T0PS0: These are prescaler selection bits. The bits setting is shown in the following table to choose desired prescaler.

 

 

 

PSA: This bit is set to high if there is no need to assign a prescaler value.

        1 = Timer0 prescaler is not assigned. Timer0 clock input bypasses prescaler.

0 = Timer0 prescaler is assigned. Timer0 clock input comes from prescaler output.

 

T0SE: This bit is used when external source is selected for the Timer. This bit is used to select external clock edge to increment the Timer.

1 = Increment on high-to-low transition on T0CKI pin (Pin6)

0 = Increment on low-to-high transition on T0CKI pin (Pin6)

 

T0CS: This bit is used to select the proper clock source for Timer0.

1 = Transition on T0CKI pin (Pin6)

0 = Internal instruction cycle clock (CLKO)

Project Source Code

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// Program to use Timer0 of PIC18F4550 Microcontroller

void T0_init();
void main()
{
TRISB=0; // COnfigure PortB as output Port.
LATB=0x01;
T0CON=0x07; // Prescaler= 1:256, 16-bit mode, Internal Clock
while(1)
{
T0_init(); // Initialize Timer0
LATB=(LATB<<1)|(LATB>>7); // Circular right shift at PortB
}
}

void T0_init()
{
TMR0H=0xD2; // Values calculated for 1 second delay with 12MHz crystal
TMR0L=0x39;
T0CON.TMR0ON=1; // Timer0 On
while(INTCON.TMR0IF==0); // Wait until TMR0IF gets flagged
T0CON.TMR0ON=0; // Timer0 Off
INTCON.TMR0IF=0; // Clear Timer0 interrupt flag
}

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Project Components

Project Video