I have a piece of test equipment that requires the clock to be stopped before it can be used on the board your working on. It tests IC in circuit and I am using it on a 68000 cpu controlled system. One book said to ground each leg of the crystal and to remove any timer chips from the circuit to stop the clock. Is this correct or is there some other way to go about this. Can you ground a leg of one of the pins on the 68000? will grounding both legs of the crystal cause any damage?
Thanks Russ
Seems to me you would want to GATE the clock, not stop it.
Restarting a crystal takes many hundreds to many thousands of unpredictable cycles.
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controlled system.
The 68000 is not stable with its clock stopped (it needs the clock active for dynamic RAM-like registers, and because there are charge pump biasing networks that use the clock). There IS a pin for HALT on this CPU, which brings all operations to a stop. Probably you want to HALT the processor, instead of not clocking it.
A few CMOS processors can be halted by gating the clock off, but it's not generally good practice.
If you are using a crystal just connected to an X1 and X2 pin pair then it will be sufficient to ground the X1 pin (using the typical naming practice where X1 is the input to the onboard oscillator inverter). On the other hand if you are using an external crystal oscillator device these often have an output enable pin which is typically held in the "enabled" state on the board. Overcome this enabled state and you can get the oscillator to stop outputting its clock signal.
- mkaras
controlled system.
A bit OT here but before a reader thinks "oh, mine is all static so it'll be fine to short the crystal": Even modern uCs can become unpredictable or do unexpected stunts. A clock fault often is muffled by an internal "pinch hitter" clock kicking in and that can throw a curve during such ICT procedures. Also, timers could run into a dead end and all that stuff.
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