Tricky Interrupt

[davidm]

Here is an extract from a trial application using 'change of state' interrupts on PORTB. I am trying to ascertain which input (RB7 or RB6) invoked the interrupt.

An oscilloscope check on PORTC.F0 confirms the interrupt is being generated however, there is no activity on F5 nor F4.

OPTION_REG.RBPU is cleared so that weak pullups are enabled.

//////////////////////////////////////////////////////////////
//
// Device: PIC16F876
// Fosc: 14.7456MHz
//
// Pulse width: 0.66uSec
//
// PORTB bits 6 and 7 are pulsed inputs.
// PORTC bits 0, 5 and 4 are debug outputs.
//

unsigned char ucLastPortB; // ucLastPortB set to PORTB in main().
unsigned char ucIndexer;

void interrupt()
{
if ( INTCON & ( 1 << RBIF ) )
{
PORTC.F0 = 1;

ucLastPortB ^= PORTB; // Extract the change(s)

if ( ucLastPortB.F7 )
{
PORTC.F5 = 1;
ucIndexer = 0; // Reset
PORTC.F5 = 0;
}

if ( ucLastPortB.F6 )
{
PORTC.F4 = 1;
++ucIndexer; // Incr
PORTC.F4 = 0;

}

ucLastPortB = PORTB;

INTCON &= ~( 1 << RBIF ); // Clear flag
PORTC.F0 = 0;
}
}

//////////////////////////////////////////////////////////////

Comments are sought.

Regards - davidm

[pizon]

1. You must disable weak pull-ups, otherwise you won't be able to detect any change:

Code: Select all

OPTION_REG.f7 = 1; // RBPU off

2. Insert a small delay (~5ms) between PORTC.F4/F5 state change, to be able to detect it;