Spency's digital-diy 16F PIC Tutorial

16F PIC TMR0 Example

TMR0 is an 8 byte timer that can be pre-scaled to change vary its incrementing cycle. 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.

To change the pre-scaling for TMR0, the following bits of the OPTION Register are modified

OPTION_REG.0 = X   '000 - 1 : 2
OPTION_REG.1 = X   '001 - 1 : 4
OPTION_REG.2 = X   '010 - 1 : 8
                   '011 - 1 : 16
                   '100 - 1 : 32
                   '101 - 1 : 64
                   '110 - 1 : 128
                   '111 - 1 : 256

The OPTION register contains 8 bytes of information, the other 5 are for completely different options, so when I modify settings that are shared in Byte registers with other options, I access them like above, so I don't accidentally change anything I don't want to.

To change the pre-scaler too 1:4 (increment the TMR0 register every 4th clock cycle), the following would be used:

OPTION_REG.0 = 1
OPTION_REG.1 = 0
OPTION_REG.2 = 0

Now to make a timer that can be used to count mS and Seconds 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 4Mhz is the clock speed, and the pre-scaler is set to 1:2, you will see that the TMR0 period is now 512uS, so every time the TMR0 clock reaches 255, and rolls over to 0, 512uS has elapsed. Now we have a basis in which we can write our program.

Device = 16F877
Xtal = 4
   		   
Dim uS as Word
Dim mS as Word
Dim S as Word	

Symbol GIE = INTCON.7				' Global Interrupt Enable Bit
Symbol TMR0_uS = 512		  		' Set time interval of TMR0
Symbol TMR0_Enable = INTCON.5			' This option enables TMR0 interrupts
Symbol TMR0_Overflow = INTCON.2			' This is the TMR0 overflow flag
			
ON_INTERRUPT Int_Sub

Goto Initialization			
			
Int_Sub:

	GIE = 0

	If TMR0_Overflow = 1 And TMR0_Enable = 1 Then
	   TMR0_Overflow = 0
	   uS = uS + TMR0_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:
	
	TMR0_Enable = 0	   	   	 	' Disable TMR0 interrupts
	
	uS = 0		  			' Clear timer registers
	mS = 0		  			' 
	S = 0		  			'

	OPTION_REG.0 = 0   			'000	-	1 : 2
	OPTION_REG.1 = 0   			'
	OPTION_REG.2 = 0			
	OPTION_REG.5 = 0			' Select Internal Clock Source

	TMR0 = 0	   			' Clear the TMR0 register
	
	TMR0_Enable = 1				' Enable TMR0 Interrupts
	
	GIE = 1		  			' Enable Global Interrupts
		
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