InitTimer0

InitTimer0 8bit/16bit

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:

Parameter Description

source

The clock source for this specific timer. Can be either Osc or Ext where`Osc` is an internal oscillator and Ext is an external oscillator.

Osc - Selects the clock source in use, as set by the microcontroller specific configuration (fuses or #config).  This could be an internal clock or an external clock source ( external clock sources are typically attached to the XTAL pins).

Ext - Selects the clock source attached to a specific exernal interrupt input port.  This allows a different clock frequency than the main clock to be used, such as 32.768 kHz crystals commonly used for real time circuits.

prescaler

The value of the prescaler for this specific timer. See the tables below for permitted values for Microchip PIC or the Atmel AVR microcontrollers.

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.

Prescaler Value Primary GCB Constant Constant Equates
to value

1:2

PS0_2

0

1:4

PS0_4

1

1:8

PS0_8

2

1:16

PS0_16

3

1:32

PS0_32

4

1:64

PS0_64

5

1:128

PS0_128

6

1:256

PS0_256

7

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.

Prescaler Value Primary GCB Constant Secondary GCB Constant Constant Equates
to value

1:1

PS_1

PS_0_1

1

1:8

PS_8

PS_0_8

2

1:64

PS_64

PS_0_64

3

1:256

PS_256

PS_0_256

4

1:1024

PS_1024

PS_0_1024

5




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

    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

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)
    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

Supported in <TIMER.H>

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