Spency's digital-diy 18F PIC micro Tutorial

18F Code Snippet - TMR0

TMR0 is an 8 byte timer/counter that can be pre-scaled to change vary its incrementing cycle. It is quite different in regards to setting up compared to the 16F's, but here's an example.

Pre-scaling is important as we may want the Timer Register to increment faster/slower in different applications. The speed in which it does cycle is dependent on both the external crystal speed and the pre-scaling options, or the Prescaler and external clock if an external source is used.

Now to make a timer that can be used to count mS very accurately. An easy way to find out how much time each clock cycle will take is to use a cycle calculator such as this. Make sure that the Internal Check Box is selected, now be sure that 20Mhz is the clock speed, and the pre-scaler is set to 1:32, you will see that the TMR0 period is now 1.638mS, so every time the TMR0 clock reaches 255, and rolls over to 0, 1.638mS has elapsed. To make things easier, I will work with a scale of 1000 for my mS counter. Now when mS = 10000, 10mS has elapsed, so there is no need to use Floats.

The sub routine "TMR0_Initialize" goes through the required steps to set up TMR0, follow the code at your own pace;

Device = 18F452
Clock = 20
      
Dim
    mS As Word,
    TMR0ON As T0CON.7,
    T08BIT As T0CON.6,
    T0CS As T0CON.5,
    T0SE As T0CON.4,
    PSA As T0CON.3,
    TMR0IF As INTCON.2,
    TMR0 As TMR0L,
    TMR0IE As INTCON.5,
    TMR0_Event As Boolean

Interrupt TMR0_Interrupt()
    Save(0)                  // Backup system variables
    If TMR0IF = 1 Then       // Check if a TMR0 Interrupt occurred
        TMR0IF = 0           // Clear the interrupt
        Inc(mS, 1638)        // Increment the mS counter (scale of 1000)
        If mS >= 10000 Then  // Working with a scale of 1000, so this 
            mS = mS - 10000  //  checks if 10mS has elapsed
            TMR0_Event = True
        EndIf
    EndIf                   
    Restore                  // Backup system variables
End Interrupt

Sub TMR0_Initialize() 
TMR0ON = 0               // Disable TMR0
T08BIT = 1               // Ensure TMR0 is working in 8 Bit mode
T0CS = 0                 // Ensure TMR increments from internal clock
T0SE = 0                 // Only used if external source is selected
PSA = 0                  // Ensure the Clock source uses the Prescaler
T0CON.0 = 0              // Set the Prescaler bits
T0CON.1 = 0              //
T0CON.2 = 1              //
TMR0 = 0                 // Clear the TMR0 register
TMR0IE = 1               // Enable TMR0 Interrupts
Enable(TMR0_Interrupt)   // Enable the TMR0 Interrupt Handler
TMR0ON = 1               // Enable TMR0 to increment
End Sub

// Start Of Main Program... 
mS = 0                       // Reset the mS counter
TMR0_Event = False           // Clear the TMR0 Event Flag
TMR0_Initialize              // Setup and enable TMR0
Low(PORTB.0)                 // Make PORTB.0 and output and set it low

While True
    While TMR0_Event = False // Wait for 10mS to elapse
    Wend                     // 
    TMR0_Event = False       // Reset the Event Flag
    Toggle(PORTB.0)          // Toggle PORTB.0
Wend                         // Loop forever

The result, a very accurate interrupt;