Microcontrollers and YOU!

As your code gets more complex and so does the tasks you wish to perform its often important to take advantage of the build in hardware and gain the ability to multitask. One of the last modules to go over is the timer module. There are several timers build in but we will be focusing on the TMR0 module.

Using the module is fairly straight forward you can turn it “on” with about 3 lines of code. The TMR0 module is a 8 bit timer, thus after 256 clocks it carries over. We can also set a prescaler that sets what the ratio of the timer to the clock is. This prescale value has a max of 256 as well. With max prescaler our timer will reach a max time before rollover of 65536 instructions or .06 seconds. If you need a timer that lasts longer, the other modules hold larger values and thus rollover at a later time.

The Registers:

OPTION_REG: (bank 1) Holds timer configuration information.

The Code:

  #include (p16F690.inc)
     __config (_INTRC_OSC_NOCLKOUT & _WDT_OFF & _PWRTE_OFF & _MCLRE_OFF & _CP_OFF & _BOR_OFF & _IESO_OFF & _FCMEN_OFF)
org 0              ; Begginning of code
PORTS:
	bsf       STATUS,RP1
	clrf      ANSEL          ; All pins are digital I/O
 	clrf 	  ANSELH         ; All pins are digital I/O
	bcf       STATUS,RP1	
	bsf       STATUS,RP0
	movlw     b'0111'    	 ; Turn on 256 prescaler
	movwf     OPTION_REG
	bcf          OPTION_REG,3   ; Prescaler set to timer0
	bcf       OPTION_REG,5	 ; set timer for internal osc
	movlw     0x00           ; Move the hex value of 0 to multipurpose register W
              movwf     TRISC          ; make IO PortC all output
	bcf       STATUS,RP0
	clrf      PORTC
MAIN:
	movlw     0x01
	xorwf     PORTC
	btfss     INTCON,2  ; Check if timer overflow flag is set
	goto      $-1
	bcf       INTCON,2
	goto      MAIN
end

This code enables the timer and uses it to flash an LED when it rolls over.

Breaking it down:

movlw b'0111' ; Turn on 256 prescaler
movwf OPTION_REG
bcf OPTION_REG,3 ; Prescaler set to timer0
bcf OPTION_REG,5 ; set timer for internal osc

This is all the code we need for the timer module. It’s fairly straight forward. We mostly change OPTION_REG to set the timer for what we want.

OPTION_REG:

bit 7 RABPU: PORTA/PORTB Pull-up Enable bit
1 = PORTA/PORTB pull-ups are disabled
0 = PORTA/PORTB pull-ups are enabled by individual PORT latch values
bit 6 INTEDG: Interrupt Edge Select bit
1 = Interrupt on rising edge of RA2/INT pin
0 = Interrupt on falling edge of RA2/INT pin
bit 5 T0CS: Timer0 Clock Source Select bit
1 = Transition on RA2/T0CKI pin
0 = Internal instruction cycle clock (FOSC/4)
bit 4 T0SE: Timer0 Source Edge Select bit
1 = Increment on high-to-low transition on RA2/T0CKI pin
0 = Increment on low-to-high transition on RA2/T0CKI pin
bit 3 PSA: Prescaler Assignment bit
1 = Prescaler is assigned to the WDT
0 = Prescaler is assigned to the Timer0 module
bit 2-0 PS<2:0>: Prescaler Rate Select bits
(table in datasheet)

For the TMR0 module the bits we care about are 0,1,2,3 and 5. For basic operation we set bit 5 low, using the internal oscillator for the clock source. We assign the prescaler to the timer module and not the Watch Dog Timer. We also assign 111 to the prescaler which is the max value.

With that set, our basic code does a XOR operation with the current contents of PORTC, waits until the timer overflows by polling the TMR0 overflow flag bit (INTCON,2) and then returning to main and doing it all over again. In the end we get a blinking LED using highly efficient code.

There we have the TMR0 module in a nutshell. Most of the other timers work in a similar fashion. With this hardware under your belt the possibilities are endless.

Assembly Source

HEX