Intermittent incorrect button value latch

[scm6079]

I'm experiencing an interesting problem with this library -- in testing I'm receiving intermittent invalid readings of input states showing the first input button continuously depressed when it is not. This is similar to what I would expect in a hardware problem of a floating input (e.g. input resistors NOT pulling the input high). HOWEVER, this only occurs with the C library, not the python library leading me to believe this is a software issue not an issue with the PiFace. No hardware is attached to the piface digital - this is using it "out of the box".

I put together a couple very simple tests to confirm. You must rapidly depress and release the first input button very rapidly to observe the failure (sometimes many dozen attempts are necessary for a failure).

Sample Python Control Test Case (FUNCTIONAL, no problems)

#!/usr/bin/env python3
from time import sleep
import pifacedigitalio
pifacedigitalio.init()
pfd = pifacedigitalio.PiFaceDigital()

while(True):
        print(pfd.input_pins[0].value)
        sleep(.01)

Sample C Failure Case

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include "pifacedigital.h"

int main( int argc, char *argv[] )
{
    uint8_t inputs;
    int hw_addr = 0;

    pifacedigital_open(hw_addr);

    while(1) {
        inputs = pifacedigital_read_reg(0x11, hw_addr );
        printf("Inputs: 0x%x\n", inputs);
        usleep(10000);
    }

    pifacedigital_close(hw_addr);
}

Once failed, you will observe an output stuck at 0xfe instead of 0xff.

Inputs: 0xfe
Inputs: 0xfe
Inputs: 0xfe
...

The issue does not clear - even after waiting - but will clear with an additional button press. I did reproduce the issue with an externally wired momentary push button switch as well.

[scm6079]

After some in-depth review from the Microchip website and some more tests, I've found:

1. The problem ONLY occurs with the Interrupt driven latched value, not the GPIOB read value (e.g. register 0x11, NOT 0x13)
2. The Python code reads register 0x13 - thus is not impacted directly in my sample code above.
3. There appears to be an open bug in the Python version for PiFace dealing with interrupts - which may be related based on my research (and does use 0x11).

Overall - it is clear that the final interrupt should have been triggered when the input lines transitioned to the switch open state (0xff) - but that interrupt change was NOT latched. At this point I'm still unclear why that latch is missed.

[tompreston]

This is related to an old bug in the PiFace Digital Emulator too. When the emulator starts up, pin 0 would be set because an interrupt was fired. Pressing it once would clear it and then the Emulator would resume normal operation (though I haven't tested it extensively). I fixed it with a hack by reading the interrupt capture register during initialisation to clear that first interrupt before anyone could see anything.

I can replicate the issue in Python with:

#!/usr/bin/env python3
from time import sleep
import pifacedigitalio
pifacedigitalio.init()
pfd = pifacedigitalio.PiFaceDigital()

while(True):
        print(hex(pfd.intcapb.value))
        sleep(.01)

Prints out:

0xfe
0xfe
0xfe
0xfe
0xfe
0xfe
0xfe
0xfe
...