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Swordfish Code Snippet - TMR2 Print E-mail
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Written by Graham   
Saturday, 04 April 2009 10:22

tmr0.h11

Here's an extract from the 18F452 datasheet;


Timer2 is an 8-bit timer with a prescaler and a postscaler. It can be used as the PWM time base for the PWM mode of the CCP module(s). The TMR2 register is readable and writable and is cleared on any device Reset. The input clock (FOSC/4) has a prescale option of 1:1, 1:4 or 1:16, selected by control bits T2CKPS1:T2CKPS0 (T2CON).


The Timer2 module has an 8-bit period register, PR2. Timer2 increments from 00h until it matches PR2 and then resets to 00h on the next increment cycle. PR2 is a readable and writable register. The PR2 register is initialized to FFh upon Reset.


The match output of TMR2 goes through a 4-bit postscaler (which gives a 1:1 to 1:16 scaling inclusive) to generate a TMR2 interrupt (latched in flag bit, TMR2IF (PIR1)). Timer2 can be shut-off by clearing control bit, TMR2ON (T2CON), to minimize power consumption.


So basically the TMR2 will continue to cycle until it equals the register PR2, how fast it cycles depends on the pre and post scaler settings. At this stage an overflow flag is triggered, and the interrupt will be called. Great for small delays and PWM generation. Here's an example of how to set it up, the settings for PR2 and prescalers were found using the TMR2 calculator

 

18Ftmr21

 

Note the PIC's power supply/oscillator are not shown

 

Device = 18F452
Clock = 20
 
Dim TMR2IE As PIE1.1,         // TMR2 interrupt enable
    TMR2IF As PIR1.1,         // TMR2 overflow flag
    TMR2ON As T2CON.2,        // Enables TMR2 to begin incrementing
    mS As Word,               // mS register
    Last_mS As Word,          // mS polling register
    Signal_Pin As PORTB.0     // Signal output to frequency meter
 
 
Interrupt TMR2_Interrupt()
    Save(0)                   // Back up system variables
    If TMR2IF = 1 Then        // Check if the interrupt was from TMR2
        TMR2IF = 0            // Clear the TMR2 interrupt flag
        Inc(mS)               // Increment the mS counter
    EndIf                     //
    Restore                   // Restore system variables
End Interrupt
 
Private Sub TMR2_Initialize()
    TMR2ON = 0                // Disable TMR2
    TMR2IE = 0                // Turn off TMR2 interrupts   
 
    PR2 = 249                 // TMR2 Period register PR2
    T2Con = %00100001         // T2CON 0:1 = Prescale
                              //        00 = Prescaler is 1:1
                              //        01 = Prescaler is 1:4
                              //        1x = Prescaler is 1:16                                 
                              //      3:6 = Postscale              
                              //     0000 = 1:1 postscale
                              //     0001 = 1:2 postscale
                              //     0010 = 1:3 postscale...
                              //     1111 = 1:16 postscale
 
    TMR2 = 0                  // Reset TMR2 Value   
    TMR2IE = 1                // Enable TMR2 interrupts
    TMR2ON = 1                // Enable TMR2 to increment
    Enable(TMR2_Interrupt)
End Sub
 
// Start Of Program...
mS = 0                        // Reset mS register
Last_mS = 0                   // Reset mS polling register
 
TMR2_Initialize               // Setup and enable TMR2
Low(Signal_Pin)               // Make the signal an output, and set it low
 
 
While 1 = 1                   // Create an infinite loop
    Repeat                    // Poll the mS register and wait for a change
 
    Until mS <> Last_mS       //
 
    Last_mS = mS              // Update the polling register
 
    Signal_Pin = 1            // Create a pulse for the frequency counter
    Signal_Pin = 0
Wend

 


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Last Updated ( Sunday, 03 May 2009 12:47 )