GCBASIC documentation
Timers

Syntax: (MicroChip PIC)

    InitTimer4 prescaler, postscaler

or, where you required to state the clock source, use the following

    InitTimer4 clocksource, prescaler, postscaler

Syntax: (Atmel AVR)

    InitTimer4 source, prescaler

Command Availability:

Available on all microcontrollers with a Timer 4 module.  As shown above a Microchip microcontroller can potentially support two types of methods for initialisation.

The first method is:

    InitTimer4 prescaler, postscaler

This the most common method to initialise a Microchip microcontroller timer.  With this method the timer has only one possible clock source, this mandated by the microcontrollers architecture, and that clock source is the System Clock/4 also known as FOSC/4.


The second method is much more flexible in term of the clock source.  Microcontrollers that support this second method enable you to select different clock sources and to select more prescale values.  The method is shown below:

    InitTimer4 clocksource, prescaler, postscaler

How do you determine which method to use for your specific Microchip microcontroller ?

The timer type for a Microchip microcontroller can be determined by checking for the existance of a T2CLKCON register, either in the Datasheet or in the GCBASIC "dat file" for the specific device.

If the Microchip microcontroller DOES NOT have a T4CLKCON register then timers 2/4/6/8 for that specific microcontroller chip use the first method, and are configured using:

    InitTimer4 (PreScale, PostScale)

If the microcontroller DOES have a T2CLKCON register then ALL timers 2/4/6/8 for that specific microcontroller chip use the second method, and are configured using:

    InitTimer4 (Source,PreScale,PostScale)

The possible Source, Prescale and Postscale constants for each type are shown in the tables below.  These table are summary tables from the Microchip datasheets.

Period of the Timers

The Period of the timer is determined by the system clock speed, the prescale value and 8-bit value in the respective timer period register.  The timer period for timer 4 is held in register PR4.  

When the timer is enabled, by starting the timer, it will increment until the TMR4 register matches the value in the PR4 register.  At this time the TMR4 register is cleared to 0 and the timer continues to increment until the next match, and so on.

The lower the value of the PR4 register, the shorter the timer period will be.  The default value for the PR4 register at power up is 255.

The timer interrupt flag (TMR4IF) is set based upon the number of match conditions as determine by the postscaler.  The postscaler does not actually change the timer period, it changes the time between interrupt conditions.

Timer constants for the MicroChip microcontrollers

Parameters for this timer are detailed in the tables below:

Table 1 shown above

prescaler can be one of the following settings, if you MicroChip microcontroller has the T4CKPS4 bit then refer to table 2:

Table 2

Note that a 1:64 prescale is only avaialable on certain midrange microcontrollers. Please refer to the datasheet to determine if a 1:64 prescale is supported by a spectific microcontroller.

Table 3

postscaler slows the rate of the interrupt generation (or WDT reset) from a counter/timer by dividing it down.

On Microchip PIC microcontroller one of the following constants where the Postscaler Rate Select bits are in the range of 1 to 16.

Explanation:(Atmel AVR)

InitTimer4 will set up timer 4, according to the settings given.

source can be one of the following settings: Parameters for this timer are detailed in the table below:

prescaler for Atmel AVR Timer 4 can be selected from the table below.

Prescaler Rate Select bits are in the range of 1 to 1024

Example:

This code uses Timer 4 and On Interrupt to generate a 1ms pulse 20 ms.

    #chip 18F25K80, 8

    #DEFINE PIN3 PORTA.1
    DIR PIN3 OUT

    #Define Match_Val PR4  'PR4 is the timer 2 match register
    Match_Val = 154        'Interrupt afer 154 Timer ticks (~20ms)

    On interrupt timer4Match call PulsePin3  'Interrupt on match
    Inittimer4 PS4_16, 15 'Prescale 1:64 /Postscale 1:16 (15)
    Starttimer 4

    Do
       'Waiting for interrupt on match val of 154
    Loop

    Sub PulsePin3
       pulseout Pin3, 1 ms
    End Sub