Spency's digital-diy 16F PIC Tutorial

16F PIC TMR2 Example

Written With Proton

Here's an extract from the 16F87XA 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<1:0>).

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<1>)). Timer2 can be shut-off by clearing control bit, TMR2ON (T2CON<2>), to minimize power consumption.

So basically the TMR2 has a 2 bit prescaler and will continue to cycle until it equals the register PR2, 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

Device = 16F877A
Xtal = 4
      
Dim uS as Word
Dim mS as Word
Dim S as Word 

Symbol GIE = INTCON.7              ' Global Interrupt Enable Bit

Symbol TMR2_uS = 200               ' Set time period register of TMR2   
Symbol TMR2_Enable = PIE1.1        ' TMR2 interrupt enable
Symbol TMR2_Overflow = PIR1.1      ' TMR2 overflow flag
Symbol TMR2_On = T2CON.2           ' Enables TMR2 to begin incrementing

ON_INTERRUPT Int_Sub

Goto Initialization 

Int_Sub:

GIE = 0

If TMR2_Overflow = 1 And TMR2_Enable = 1 Then
   TMR2_Overflow = 0
   uS = uS + TMR2_uS
   If uS >= 1000 Then
     uS = uS - 1000
     mS = mS + 1
  	 If mS >= 1000 Then 
   	 	mS = mS - 1000
  	 	S = S + 1
  	EndIf
   EndIf
EndIf

GIE = 1

Context Restore

Initialization: 

TMR2_Enable = 0

INTCON.6 = 1    ' Peripheral Interrupts   

T2CON.0 = 0     ' 00 = Prescaler is 1
T2CON.1 = 0   	' 01 = Prescaler is 4
    		'1x = Prescaler is 16 
PR2 = 199   	' TMR2 Period    
T2CON.3 = 0     ' 0000 = 1:1 postscale
T2CON.4 = 0     ' 0001 = 1:2 postscale
T2CON.5 = 0     ' 0010 = 1:3 postscale
T2CON.6 = 0     ' 1111 = 1:16 postscale

TMR2 = 0    ' Reset TMR2 Value

TMR2_Enable = 1
TMR2_On = 1 

GIE = 1 

Main:

	 Repeat 	     ' Create a loop 

	 Until S = 60        '   that waits for 60 seconds

	                     ' 60 seconds has occurred

	 S = 0               ' Clear the seconds register

	 Goto Main           ' Loop for ever