How to troubleshoot a logic device
No matter what your device type is (discrete logic, logic function, microcontroller, microprocessor), the following are the fundamental troubleshooting steps:
- Check all power rails
- Check resets
- Check clocks
If these are all OK, then go in-depth:
- What is the current draw for each rail?
- What is the condition of the clock signal? Is it clean of noise and spikes?
- What is the condition of ground? Is there any noise carried over ground?
To check noise on the ground, you obviously cannot base your reference to the scope's GND lead connected to the same ground the top of the probe is. Remember also that the scope's grounded lead is exactly that -- GROUNDED to chassis and to EARTH GROUND of the wiring system of the building.
In order to measure the condition of a ground such as DGND, reference to another ground, or to a ground star. For example, chassis ground may be cleaner than DGND, and can be used as a reference point. You should also float the scope chassis by using an isolation transformer (one which does NOT connect the earth lug of the output plug to the earth lug of the input plug).
If your troubleshooting is for a non-recurring, or an infrequently recurring fault, try the following:
- Are you using a ground star with a system of isolated grounds? Are loads / circuits which belong to AGND connected to DGND?
- How clean is the ground?
- How clean is the clock?
- How clean are the signal lines?
- Are there any floating pins? All pins other than those designated NC (not internally connected) must be pulled up or down as appropriate.
- Does operation of the chip / signal improve if the scope probe is placed in the trace / pin? This happens very often because the scope tip is a small (10pF for an expensive probe, more for cheapo ones) capacitance to ground, which "smoothes out" a bad signal and makes the chip operate better.