The circuit for Experiment 1

Any port-line can be changed from an input to an output or vice versa, at any time,  during the running of a program. 
If you are going to change the direction of a port-line, you must take into account any devices connected to the line. For instance, if a switch is connected between an output of the chip and rail; if the line is an output and is made LOW, the chip will be damaged when the button is pressed.  
The program consists of two loops in Main. The first instruction in Main looks at the input line and determines if the button has been pressed. This where a decision is made. If is is  not pressed, the micro is taken to the next instruction in the program and a loop is executed that makes an output line LOW to turn off a LED. If it is pressed, the micro is taken to the third instruction in Main to turn ON an output bit to illuminate a LED.  
The micro never stops executing instructions. It must be executing one loop or the other.
Main must be a loop (or more than one loop) as this is where the LED is turned ON and OFF and the microcontroller will loop this routine many times per second. 
In Main, instructions will take the micro to other routines called sub-routines, carry out operations such as delays, produce beeps, etc and return to Main, as you will see in a moment. 
Study the following experiment then click HERE for a test. 

EXPERIMENT 1a
Turning on a LED for 0.5sec - without debounce
This experiment turns on a LED for 0.5 sec when button "A" is pressed. It does not matter how long button "A" is pressed. The LED turns on immediately button "A" is pressed and turns off after 0.5 sec. 
If the button is kept pressed after the LED turns off, the LED turns on AGAIN as the program detects noise from the switch. 
This highlights the need for DEBOUNCE. 
Debounce is shown in the next experiment. 
There are variations of this: The LED can turn on for 0.5sec AFTER the button has been released (Experiment 1d) or the LED can turn on for 0.5sec MORE than the button-press time (Experiment 1e).  
All these programs show how to control the output. You can extend the program to two flashes or a delay between flashes or a flash then turn on a LED or driver transistor to operate a globe or motor, or any other output device. 
This program is an extension of Experiment 1, with the introduction of a delay. 
The maximum delay for 2 files is 250mS. To achieve 500mS, the Delay sub-routine is called two times. This requires less instructions than creating a 500mS delay, and is simpler to understand. 
The program is shown below and an alternate program is shown that controls the driver transistor. 
The control of a single line of the output port is called BIT MANIPULATION. 
Two or more output lines can be controlled at the same time by loading W with a hex value and moving the value to file 06. If this is done, all the output lines will be affected and if a line is HIGH and the corresponding bit in the instruction is "0" the line will change state. This may not be wanted and that's why bit manipulation is the safest. 

EXPERIMENT 1b
Turning on a LED for 0.5sec - with debounce
This experiment turns on a LED for 0.5 sec when button "A" is pressed. The LED does not turn on when the button is released. The secret is the very short delay 7x256uS at Main 1. This is just long enough to cover the time when the switch is being released. Release the switch very slowly and you will see the LED illuminate. This proves the delay is just covering the noise from the release of the switch. 
Study the following experiment then click HERE for a test. 

EXPERIMENT 1c
Two pushes to turn on a LED for 0.5sec
This experiment turns on a LED for 0.5 sec after two presses of the push-button. The program shows how to take control of a push-button to recognise a particular sequence of presses or length of press etc.  A program must "poll" or "look-at" an input on a regular basis and hold the information in a file called a "flag file," then go to the Main routine and carry out other operations. 
The micro must not get stuck in a sub-routine, as it always has other things to do. The program below shows how to place the switch instructions in a sub-routine and the Main program looks at the state of the button via a flag file. 

EXPERIMENT 1d
LED turns on for 0.5sec after button released
This experiment shows how to turn on an output when a button has been released. 

EXPERIMENT 2
Toggle a LED
This experiment toggles the push button. Each time the button is pressed, a LED changes state. 
The program introduces a very simple DEBOUNCE feature. All switches must be debounced to prevent false counting or false detection. 
All switches produce noise and the micro is executing a program so quickly that it will pick up multiple pulses from a switch, each time it is pressed or released. To prevent this from happening we need to look at the push button at intervals that will prevent false counting. 
The simplest debounce is a delay routine. The micro only looks at the button at intervals, dictated by the length of the delay. As soon as the push button is detected, the micro executes an XOR instruction that toggles the output line to change the state of the LED. It then goes to a delay sub-routine and when it returns, it looks for the release of the button. 
This represents one cycle of the program and when the button is pressed again, the output line is toggled once more. 
The program requires two separate loops as the micro is executing instructions at 1 million per second and a loop is executed so quickly that the micro can see the press of a button many times if the loop is not extended. One loop is the "pressed loop" and the other is the "not pressed loop." 
The program introduces the "button-press" flag. This is the lowest bit (bit0) in file 1F. You can use any file but the PIC Programming Course uses files 1E and 1F for flags. There are three instructions relating to this flag: The instruction to test the flag: BTFSC 1F,0  the instruction to set the flag: BSF 1F,0  and the instruction to clear the flag: BCF 1F,0  They must be placed in the program in the correct order, for the flag-bit to work. The delay can be increased until absolutely no false-triggering is produced.  This will limit the speed of operation of the button and if a high-speed input is required, the pulses must be clean.

EXPERIMENT 3
Running LEDs
The program runs a single LED forward and back across the 8 LEDs.  When the push button is pressed, the action stops. Releasing the button enables the action.
This program shows the "shift-left, shift right" feature. The contents of any file can be shifted. We can detect a single bit "dropping off either end of a file" as it is shifted into a location called the CARRY - located in another file called the Status file. The address of the Carry is file 03, bit0. 
If we test for a bit in the carry, we can move a single bit back and forth across a file and thus output this result on a set of LEDs to get a RUNNING LED effect. 
The program also constantly looks for the press of the push button to halt the effect. 
The Delay is an extension of the sub-routine above. It uses the 1mS delay and loops it 100 times by decrementing file 1B. The interesting feature is the inclusion of two RLF 06,1. They shift the LED from the end so that all the LEDs illuminate equally. 

To Top