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Forum Index : Microcontroller and PC projects : Crystal Oscillator query

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bfwolf
Senior Member

Joined: 03/01/2025
Location: Germany
Posts: 153
Posted: 02:43pm 28 Jan 2026
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Of course, that depends a lot on the application.

I assume that these crystal oscillators aren't "silent" during the startup phase but often perform a frequency sweep.

For example, if you have a microcontroller with an internal (calibrated) RC oscillator, you can run it on the RC clock for a while and then switch to the external clock once it's stable.

It's more complicated with microcontrollers (like many AVRs) where you have to define the clock source in fuses during programming and can't later switch from the RC oscillator to an external clock via a register.
 
EDNEDN
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Joined: 18/02/2023
Location: United States
Posts: 281
Posted: 04:08am 29 Jan 2026
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  Mixtel90 said  
We used to temporarily stop the Z80 clock to let DMA happen. The Z80 knows nothing about it and just carries on when the clock is restored.


In fact, that was one of the great things about the Z80.   I wire wrapped up a CP/M Z80 machine.    And I was able to debug it with a logic probe.   I made the clock circuit feeding it the 4 MHz able to feed it clock pulses via a (debounced) toggle switch.

I could toggle the switch back and forth and use the logic probe to see exactly what I/O ports or memory addresses were being accessed.  

It was really nice of the Zilog guys to make the Z80 able to work and hold state at 0 Hz.
 
lizby
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Joined: 17/05/2016
Location: United States
Posts: 3577
Posted: 02:47pm 29 Jan 2026
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  EDNEDN said  
  Mixtel90 said  
We used to temporarily stop the Z80 clock to let DMA happen. The Z80 knows nothing about it and just carries on when the clock is restored.


In fact, that was one of the great things about the Z80.   I wire wrapped up a CP/M Z80 machine.    And I was able to debug it with a logic probe.   I made the clock circuit feeding it the 4 MHz able to feed it clock pulses via a (debounced) toggle switch.

I could toggle the switch back and forth and use the logic probe to see exactly what I/O ports or memory addresses were being accessed.  

It was really nice of the Zilog guys to make the Z80 able to work and hold state at 0 Hz.


I did the same thing with the terrific Motorola 68000 (with 1MB flat memory capability)--also wire-wrapped. You could single-step at any rate you liked.

Oh, the times (but wouldn't want to go back).
PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed
 
Bowden_P
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Joined: 20/03/2019
Location: United Kingdom
Posts: 169
Posted: 09:38pm 29 Jan 2026
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Hi,
Weighing up all your advice, and some ploughing through the literature, I have decided that I woun't be able to determine good conditions for a crystal, so will use an oscillator module instead.

Probably a ECS-3951M-200-BN-TR, 5V supply, 50ppm, 7mm x 5mm package.

For all my past projects I have used an HC49U cased crystal alone without problems, with drive requirements up to the 1.2mW level. Now I wonder if even they were overstressed!

With best regards, Paul.
Nothing so constant as change.
 
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