GCBASIC documentation
Timers

Syntax:

    InitTimer0 source, prescaler

Command Availability:

Available on all microcontrollers with a Timer 0 module.

See also see: InitTimer0 8bit/16bit for support for microcontrollers with a 8 bit/16 bit Timer 0 module.

Explanation:

InitTimer0 will set up timer 0.

Parameters are required as detailed in the table below:

When the timer overflows from 255 to 0, a Timer0Overflow interrupt will be generated. This can be used in conjunction with On Interrupt to run a section of code when the overflow occurs.

Microchip PIC microcontrollers:

On Microchip PIC microcontrollers where the prescaler rate select bits are in the range of 2 to 256 you should use one of the following constants. If the prescaler rate select bits are in the range of 1 to 32768 then see the subsequent table.

These correspond to a prescaler of between 1:2 and 1:256 of the oscillator speed where the oscillator speed is (FOSC/4). The prescaler applies to both the internal oscillator or the external clock.

Atmel AVR microcontrollers:

On Atmel AVR microcontrollers prescaler must be one of the following constants:

The prescaler will only apply when the timer is driven from the Osc the internal oscillator - the prescaler has no effect when the external clock source is specified.

Example 1 for 8-bit timer 0:

This code uses Timer 0 and On Interrupt to generate a Pulse Width Modulation signal, that will allow the speed of a motor to be easily controlled.

    #chip 16F88, 8

    #define MOTOR PORTB.0

    'Call the initialisation routine
    InitMotorControl

    'Main routine
    Do
        'Increase speed to full over 2.5 seconds
        For Speed = 0 to 100
            MotorSpeed = Speed
            Wait 25 ms
        Next
        'Hold speed
        Wait 1 s
        'Decrease speed to zero over 2.5 seconds
        For Speed = 100 to 0
            MotorSpeed = Speed
            Wait 25 ms
        Next
        'Hold speed
        Wait 1 s
    Loop

    'Setup routine
    Sub InitMotorControl
        'Clear variables
        MotorSpeed = 0
        PWMCounter = 0

        'Add a handler for the interrupt
        On Interrupt Timer0Overflow Call PWMHandler

        'Set up the timer using the internal oscillator with a prescaler of 1/2 (Equates to 0)
        'Timer 0 starts automatically on a Microchip PIC microcontroller, therefore, StartTimer is not required.
        InitTimer0 Osc, PS0_2          ' <<< the InitTimer0 instruction

    End Sub

    'PWM sub
    'This will be called when Timer 0 overflows
    Sub PWMHandler
        If MotorSpeed > PWMCounter Then
            Set MOTOR On
        Else
            Set MOTOR Off
        End If
        PWMCounter += 1
        If PWMCounter = 100 Then PWMCounter = 0
    End Sub

Key line: InitTimer0 Osc, PS0_2 — sets Timer 0’s clock source to the internal oscillator with a 1:2 prescaler; Timer 0 free-runs on a Microchip PIC microcontroller once initialised, so StartTimer is not needed.

Example 1 for 18-bit timer 0 operating an 8-bit timer:

The same example for a 16-bit timer 0 operating as an 8-bit timer.

#chip 16f18855,32
#option Explicit
'timer test Program

dim speed, MotorSpeed, PWMCounter as byte

#define MOTOR PORTb.0
dir MOTOR out

'Call the initialisation routine
InitMotorControl

'Main routine
Do
    'Increase speed to full over 2.5 seconds
    For Speed = 0 to 100
        MotorSpeed = Speed
        Wait 25 ms
    Next
    'Hold speed
    Wait 1 s
    'Decrease speed to zero over 2.5 seconds
    For Speed = 100 to 0
        MotorSpeed = Speed
        Wait 25 ms
    Next
    'Hold speed
    Wait 1 s
Loop

'Setup routine
Sub InitMotorControl
    'Clear variables
    MotorSpeed = 0
    PWMCounter = 0

    'Add a handler for the interrupt
    On Interrupt Timer0Overflow Call PWMHandler

    InitTimer0(Osc, TMR0_FOSC4 + PRE0_1 , POST0_1)          ' <<< the InitTimer0 instruction (16-bit timer in 8-bit mode)
    StartTimer 0

End Sub

'PWM sub
'This will be called when Timer 0 overflows
Sub PWMHandler

    If MotorSpeed > PWMCounter Then
        Set MOTOR On
    Else
        Set MOTOR Off
    End If
    PWMCounter += 1
    If PWMCounter = 100 Then PWMCounter = 0

End Sub

Key line: InitTimer0(Osc, TMR0_FOSC4 + PRE0_1 , POST0_1) — on this 16-bit-capable Timer 0, explicitly selects the FOSC/4 clock source with a 1:1 prescale and postscale, then calls StartTimer since this variant does not start automatically.

See Also:

Supported in <TIMER.H>