I'm trying to use the code provided by Microchip to debounce the switch, but I couldn't really understand the code itself:

// This lesson shows one method to software debounce a switch input.
// The switch is sampled once every millisecond and must be low for
// 5 samples in a row to register a "press". Each press rotates
// the LEDs on PORTD.

#include "p18f45k20.h"
#include "delays.h"
#include "04 Switch Input.h" // header file

/** V A R I A B L E S *************************************************/
#pragma udata // declare statically allocated uinitialized variables
unsigned char LED_Display; // 8-bit variable

/** D E C L A R A T I O N S *******************************************/
#pragma code // declare executable instructions

void main (void)
{
unsigned char Switch_Count = 0;

LED_Display = 1; // initialize

TRISD = 0b00000000; // PORTD bits 7:0 are all outputs (0)
INTCON2bits.RBPU = 0; // enable PORTB internal pullups
WPUBbits.WPUB0 = 1; // enable pull up on RB0
ANSELH = 0x00; // AN8-12 are digital inputs (AN12 on RB0)
TRISBbits.TRISB0 = 1; // PORTB bit 0 (connected to switch) is input (1)

while (1)
{
LATD = LED_Display; // output LED_Display value to PORTD LEDs

LED_Display <<= 1; // rotate display by 1

if (LED_Display == 0)
LED_Display = 1; // rotated bit out, so set bit 0


while (Switch_Pin != 1);// wait for switch to be released

Switch_Count = 5;
do
{ // monitor switch input for 5 lows in a row to debounce
if (Switch_Pin == 0)
{ // pressed state detected
Switch_Count++;
}
else
{
Switch_Count = 0;
}
Delay10TCYx(25); // delay 250 cycles or 1ms.
} while (Switch_Count < DetectsInARow);
}

}

1. I don't understand what do the #pragma udata and #pragma code work for.
2. For the first while(Switch_Pin != 1), could it be replaced by while(Switch_Pin == 0)?
3. I wonder why the Switch_Count=5 is declared as this way, because if it is to work as the comment stated above(sample 5 consecutive low), I thought it should be Switch_Count=0, because from my understanding of the code, it will immediately loop out of the while(Switch_Count < DetectsInARow) once it detected a pressed state by incrementing Switch_Count to 6, and if the user is fast enough to press the switch before the code loads tilll Switch_Count=5, hehehe...

Thanks

 

Sorry, I would also like to know how does the code work to rotate the LED. Thanks...

 

1. I don't understand what do the #pragma udata and #pragma code work for.

Check the C18 User's Guide for this.

2. For the first while(Switch_Pin != 1), could it be replaced by while(Switch_Pin == 0)?

Strictly speaking, these are not necessarily the same. i.e. if the value of Switch_Pin were other than 1 or 0. In your project, it may make no difference depending on the definition of Switch_Pin... which I do not see.

3. I wonder why the Switch_Count=5 is declared as this way, because if it is to work as the comment stated above(sample 5 consecutive low), I thought it should be Switch_Count=0, because from my understanding of the code, it will immediately loop out of the while(Switch_Count < DetectsInARow) once it detected a pressed state by incrementing Switch_Count to 6, and if the user is fast enough to press the switch before the code loads tilll Switch_Count=5, hehehe...

Try Switch_Count--;

Sorry, I would also like to know how does the code work to rotate the LED. Thanks...

This...LED_Display <<= 1; .... code says to shift the (binary representation) value of LED_Display left one bit. So, for 8-bit, 0b00000001 left shifted one bit becomes 0b00000010.

 

Hi,

Regarding Q1, I have read #pragma DIRECTIVE stated in C18 User's Guide.
for #pragma code, it is to tell the MPLAB 18 to compile the C language code following this directive into the "code" section of program memory. I thought even without this declaration, the code will also be programmed into the code section of the ROM?

and for #pragma udata, the unitialized variables defined after this declaration will use the General Purpose Registers for storage. Firstly, I do not really understand what is "uninitialized" here means. Secondly, what is the main purpose to store "unsigned char LED_Display" variable here into General Purpose Register?

Thanks!