Weird problem with logic level on pin.

[HenrikOlsson]

Hi,
I just had weird problem that I can't really explain, hopefully someone else here can.

Basically I have an 18F2431 device. One of it's pin is used as an fault-input normally pulled high thru 4.7k and then two active LOW signal are OR'd thru diodes to this pin.

In my Main loop I had this (basically):

Main:
  If Fault_Input = 0 THEN
    Fault_Flag = 1
    GOSUB Abort
  EndIf
 
  If FaultFlag = 0 THEN
    If RunServo = 1 THEN GOSUB DoServo
  ENDIF
 
  Goto Main

RunServo is a flag that gets set by a timer interrupt, currently at 1.5kHz.

This has worked fine for a very long time but a user has a problem where I suspect electrical noice is tripping the fault input. Therefor I thought I'd implement a simple filter and did this:

Main:
  If RunServo = 1 THEN
 
  If FaultInput = 0 THEN
    Accumumlator = Accumulator + 1
    If Accumulator = Limit THEN
    FaultFlag = 1
    GOSUB Abort
  EndIf
 
  ELSE
    Accumulator = 0 
 
  EndIf
 
  If FaultFlag = 0 THEN GOSUB DoServo
 
ENDIF
Goto Main

Again, this is not exactly how it's done but the principle is the same.

To my surprise this didn't work. Well, it worked when the fault was triggered by one of the two OR'd signals but not by the other. The one that worked has a simple switch to GND making the voltage at the pin one diode drop (~0.6V). But the other input have a transistor (optoisolator output) as its "switch" to GND and there was about 0.55V voltage drop across that for a total "low" voltage at the pin of 1.15V.

This 1.15V is above the logic low thershold of the input which at Vdd=5V is about 0.75V. I fixed the problem by changing the diode to a schotky type with lower forward voltage drop and it now seems to be OK.

The big question I can't find the answer to is how and why it has worked before adding the filter code (there's been ano problem at all that I know of...). It doesn't matter if I set Limit to 1 which should make it trip the first time it sees the signal. The only difference with that is that it won't get checked untill the RunServo flag is set which happens 1500 times/second.

Anyone's got any ideas on what is happening here, it's clear that I was operating outside the electical specs but why does it matter WHERE in the code I actually check the input?

Thanks guys!
/Henrik.