"PinChg" Experiment

It is important to remember that the :

  bsf  Register, Pin

instruction actually executes as:

  Register = Register | (1 << Pin)

and the

  bcf  Register, Pin

instruction actually executes as:

  Register = Register & (0x0FF ^ (1 << Pin))

If an output (or input) pin is held at an undesireable state when one of these instructions is executed on another pin, then when the pin becomes an output, you will find that the pin will be at the state driven by the external hardware. This can be a problem if you are driving bipolar transistors directly as shown in the circuit 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

RA0 Pull Up 10K, 1/4 Watt No - "BUT1" Used

RA0 Push Button Momentary On/Modified for Breadboard No - "BUT1" Used

RB0 Transistor 2N3094 Yes

RB0 LED Current Limiting Resistor 220W, 1/4 Watt No - "LED1" Used

RB0 LED Any Type No - "LED1" Used

RB1 Transistor 2N3094 Yes

RB1 LED Current Limiting Resistor 220W, 1/4 Watt No - "LED1" Used

RB1 LED Any Type No - "LED1" 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  "PinChg - Change the State of an LED Inadvertently"
#define _version "1.00"
;
;  Update History:
;
;  99.11.23 - Created
;
;  This Application turns on a LED at RB0.  An input switch
;   at RA0 is then polled and when it is pressed, a LED at 
;   RB4 is then turned on using the "bsf" instruction.  The 
;   LEDs will be driven from PICmicro's I/O Ports directly 
;   (no current limiting resistor and to Ground), so when 
;   the button is pressed, the LED at RB0 should go out.  
; 
;  Myke Predko
;
;  Hardware Notes:
;  16F84 Running at 4 MHz
;  RA0 - Button Pulled up to Vcc and Active when pressed
;  RB0, RB1 - LED controlled by a NPN/N-Channel FET Transistor
;   with a 220 Ohm Pull Up
;
 LIST R=DEC
 ifdef __16F84
  INCLUDE "p16f84.inc"
 else
 ifdef __16F877
  INCLUDE "p16f877.inc"
 endif

;  Variable Register Declarations

;  Macros


 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

 org     0
Mainline

  nop

  clrf   PORTB			;  Initialize PortB to Nothing On
  bsf    STATUS, RP0
  movlw  0x0FC			;  Set RB0 & RB4 as Outputs
  movwf  TRISB ^ 0x080
  bcf    STATUS, RP0

  bsf    PORTB, 0			;  Turn on RB0 LED

Loop
  btfsc  PORTA, 0			;  Wait for Button to be Pressed
   goto  Loop

  bsf    PORTB, 1			;  Turn on RB1 LED
					; #### - RB0 LED Turned Off

  goto   $			;  Loop Forever


 end

For this experiment to work as desired (both LEDs on after the button is pushed), then 330® resistors should be put in series with the PORTB I/O pins and the 2N3904 transistors' bases.

Click Here to look at the twenty second experiment - TimeEnd