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).
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 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
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