What is the effect of intermittent output in crystals?

[rafayimran]

I am currently performing fault detection analysis for a PCB board. I would like to know what is the effect of a crystal if it has intermittent output? Also what would a general effect on the PCB would be if the crystal has 1) no output 2) incorrect output?

 

[2cool4me4]

Well, I'll see what I can chip in...

1) If there was no output, then chips like the ATMEGA probably would revert back to their own internal oscillator. Either that, or the board would grind to a halt because, well, the oscillator isn't oscillating.

2) If you have an RTC separate from the PCB, a microcontroller on the board could do something implementing something like a pause command. It could read the time (x), pause for what it thinks is one second, resume, and if the time isn't x + 1 second, the oscillator isn't working right.

This is what I can chip in, we'll see if anyone else has any more info.

- - - Updated - - -

Well, I'll see what I can chip in...

1) If there was no output, then chips like the ATMEGA probably would revert back to their own internal oscillator. Either that, or the board would grind to a halt because, well, the oscillator isn't oscillating.

2) If you have an RTC separate from the PCB, a microcontroller on the board could do something implementing something like a pause command. It could read the time (x), pause for what it thinks is one second, resume, and if the time isn't x + 1 second, the oscillator isn't working right.

This is what I can chip in, we'll see if anyone else has any more info.

 

[betwixt]

It depends on what the crystal does. In filter applications an intermmittent crystal would mean intermittent output because the signal either has to pass through if in series or resonate it if in parallel. In RF oscillator circuits, if designed properly a 'dead' crystal would stop the oscillator completely (= no output) although it isn't unknown for some designs to oscillate at much higher frequency due to parasitic capcitances. In MCU clocks, if there is a fail safe "clock fail" system a different clock source may take over so the frequency would change to whatever it's frequency was. If there is no clock fail detection it simply stops dead in it's tracks. If it's aPLL reference oscillator the system would unlock and probably drive it's correcton voltage hard in one direction.

Brian.