"TMR0" Experiment

TMR0 can be used in a number of different situations to keep track of the actual time instead of counting instruction cycles in an application. In TimeEnd, I showed how the timer could be kept from incrementing. In this experiment, the current value of the timer is continuously displayed on a set of eight LEDs.

The experiment uses the circuit shown below:

The parts needed for this experiment are listed in the table:

Part Description Required for the YAP-II/EMU-II?

PICmicro® MCU PIC16F84-04/P
PIC16F877-04/P In Socket

Vdd/Vss Decoupling Capacitor 0.1 uF (Any Type) No

_MCLR Pull Up Resistor 10K, 1/4 Watt No

4 MHz Ceramic Resonator Three Leaded Ceramic Resonator with Built in 27-33pF Capacitors No

PORTB LED Current Limiting Resistors 8x 220W, 1/4 Watt No - "LED1" Used

PORTA/PORTB LED 10 LED "Bargraph" Recommended No - "LED1" through "LED9" Used

Breadboard Any Type No

+5 Volt "Vcc" Power Supply Any Type No

Using a breadboard, the experiment is wired using the guide:

If the EMU-II or YAP-II is used, the experiment is wired as:

The source code listed below can be accessed from the CD-ROM by clicking Here.

 title  "TMR0 - Demonstrate the Operation of TMR0 with Prescaler"
;
;  This Code sets up TMR0 to run from the Instruction Clock and 
;   uses the Prescaler to divide the incoming clock.   
;
;
;  Hardware Notes:
;   PIC16F84 Running at 4 MHz
;   _MCLR is Pulled Up
;   All 8 bits of PortB are Pulled up and Connected to LEDs
;
;  Myke Predko
;  99.12.26
;
  LIST R=DEC
 ifdef __16F84
  INCLUDE "p16f84.inc"
 else
 ifdef __16F877
  INCLUDE "p16f877.inc"
 endif

;  Register Usage
 CBLOCK 0x020                   ;  Start Registers at End of the Values
 ENDC


 PAGE
 ifdef __16F84
 __CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON
 else
 __CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON & _DEBUG_OFF & _LVP_OFF & _BODEN_OFF
 endif

;  Mainline of TMR0
  org    0

  nop

  bsf    STATUS, RP0		
  clrf   TRISB ^ 0x080		  ;  Make All 8 PortB Bits Output
  movlw  0x0FF ^ ((1 << T0CS) | (1 << PSA) | 7)
  addlw  0				  ;  Put in Prescaler Value
  movwf  OPTION_REG ^ 0x080	  ;  Load the Option Register Value
  bcf    STATUS, RP0

Loop					  ;  Loop Here

  comf   TMR0, w			  ;  Output the TMR0 Value
  movwf  PORTB

  goto   Loop


  end

In the application code, note that the

  addlw  0

instruction can be given a different value, from 0 to 7, to demonstrate the effect the prescaler has on TMR0. When the prescaler value is small ("0"), you will not see any LEDs changing - this happens when the prescaler is larger; the most significant bits of TMR0 will change at a visible rate.

Click Here to look at the twenty eigth experiment - Random