Can anyone clarify what's going on with RB0 INT on an F84
please

Whilst writing a zero-crossing detector I measured the voltage
threshholds for RB0, to find out when the interrupt would be
generated. The results I've got are nothing like the data specs.
If someone has documentation to back up what I've found I'd
be glad to see it. I'm sure I'm not the first to run into this. If I've
got it wrong I'd like to know where

The assumption was that with RB0 as a Schmitt Trigger input
the voltages would be 0.8V and 0.2V, but my measurements
don't support this. The data sheet does say that Vhys is To
Be Determined, but aside from that it seems there's a dv/dt
limit for RB0, which doesn't sound like a Schmitt Trigger. With
a pot in the range 500R to 100k and INTEDG set for falling
voltage IRQ, it's quite easy to lower the voltage slowly (0.01V/s)
on RB0 without the PIC generating an INT IRQ. If an IRQ is
generated it happens at 1.6V. The ISR will be exited if btfss in the
ISR is true at 1.5V. With INTEDG=1 (rising voltage IRQ), the IRQ
will happen at 3.11V and exit via btfsc at 1.48V. An IRQ is
always generated no matter how slowly VRB0 rises. (I've also
tried this with RB3, and that will flip at 1.44V whichever way
V is going)

Sequence is, after Reset (using negative-going IRQ) -

- LED2 on, pot at full CW, V on b0 = 5.00V
- Turn pot CCW (ie lower voltage)
- When VRB0=1.60V LED2 goes off, LED1 comes on. This shows
   that PC is in the ISR
- When VRB0=1.50V LED1 goes off, LED2 comes back on,
   indicating completion of ISR
- Repeat, back to step 1 (no h/w reset necessary)

volts equ portb.0   ;pot wiper. Pot has 0V CCW terminal, 5V CW terminal
led1 equ portb.1   ;LEDs are transistor-switched via diode/8k2 resistor
led2 equ portb.2

org irq
goto led

movlw 00h   ;porta all o/p, nothing connected except a4 10k p-u
tris porta
clrf porta
movlw 01     ;portb all o/p except b0. Pot on b0, LEDs on b1 and b2
tris portb
clrf portb
movlw 80h   ;pull-ups off, falling edge IRQ
option
bsf inte         ;enable INT IRQ
bsf gie

wait      bsf led2     ;LED2 on, wait for INT
             goto wait
             nop
             nop
             goto wait    ;loop after ISR

led        bcf intf        ;ISR entry point
             bcf led2      ;LED2 off
             bsf led1      ;LED1 on

low       btfsc volts     ;test RB0
             goto low       ;loop until voltage drops under hysteresis

pot       btfss volts    ;test RB0
            goto pot       ;loop until voltage rises above hysteresis
            bcf led1        ;led1 off
            retfie             ;back to wait

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