Progress on the PIC-powered die

PIC-powered LED die

Having realised that I was indulging in some horrible premature optimisation, I stopped trying to make this circuit work using a three-bit output and cracked on with a 4-bit version. The circuit just took a few minutes to build on top of the flashing LED circuit:

I added extra LEDs on RA0-4, and added a push-to-close switch on RB0/INT. I also added a capacitor across the input to even out any voltage fluctuations, and included a diode to save me from my own stupidity – I’ve already zapped more than one IC by getting the polarity wrong.

In the code, I stripped out the delay stuff, and started by just turning all the LEDs on, which was pretty simple:

       movlw	0x0F
       movwf   PORTA

I also had to add some more setup stuff:

	bsf	STATUS,5       	  ; Switch to Bank 1
        bsf     OPTION_REG, INTEDG  ; Set RB0 to generate an interrupt on a rising edge
        bcf     INTCON, INTF      ; clear interrupt flag
        bsf     INTCON, INTE      ; mask for external interrupts
        bsf     INTCON, GIE       ; enable interrupts
 
        movlw   0x01		  ; Port B to input
        movwf   TRISB

Then I turned the bulk of the old program into a sleep routine:

; Turn the LED off 
start
       	movlw	0x00              ; Turn the LED on by first putting
        movwf   PORTA             ; it into the w register and then on the port
	sleep
 
	END                       ; directive 'end of program'

And added some code to the interrupt handling routine:

; isr 
        btfss   PORTB, 0          ; Only run if RB0 is high
        goto    end_isr           ; If it's not high, branch to the end
 
        btfss   PORTA, 0          ; Invert Port A - if bit 0 of Port A is set, 
        movlw	0x0F              ; skip the instruction that makes it high, 
        btfsc   PORTA, 0          ; and vice-versa
        movlw	0x00
 
        movwf   PORTA             ; Put the selected value into PORTA
        bcf     PORTB, 0          ; Clear PORTB now that we're done with it.
                                  ; I don't know if this works or if it's necessary

The full source code is here.

This nearly works. As in, when I press the button, the LEDs do all light up. And then pressing it again turns them off. However, they also come on by themselves and flash very quickly, and then turn off. I assume this is because of some other interrupt being called, and that my starting btfss doesn’t sufficiently differentiate between interrupts caused by RB0 going high, and other sorts of interrupts.

Or, it might be because the pin is not tied to ground when the pin is open, and floating voltage is enough to trigger the interrupt.

More fiddling needed.

Useful things found while doing this: