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EEVblog 1614 - Circuit Design TIP: Crystal Oscillators

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You can easily replace any chip crystal circuit with a dedicated external crystal oscillator. Reducing your BOM count, and making your design easier, smaller, and potentially more robust.

Cyber City Circuits:

One of the things I've noticed, if you're thinking of using an oscillator with a microcontroller, is that oscillators have pretty high current consumption, compared to modern microcontrollers.  And it won't get shut off during the fancy low-power modes, either :-(

What would be nice is something covering how to correctly calculate the loading caps for a MCU crystal. I don't think it's uncommon to think I have a 12pF xtal so I'll use 12pF capacitors, or tha app note shows 22pF caps so I need a 22pF xtal etc.

For a lot of MCUs it's close-ish and Just Works, though for radio MCUs (WiFi Bluetooth etc) it can make a difference.

I'm pretty certain that I've seen some datasheets that tell you to feed an external clock into X2

Nominal Animal:
Yep; the i.MX RT1060 Processor Reference Manual (as used in Teensy 4.x) describes three different ways of replacing the 24MHz crystal with an external source, with the suggested one being driving XTALI at nominal 1.1V and leaving XTALO externally floating.  (The other two are overdriving both differentially, or overdriving XTALO and capacitively loading XTALI, which also happen to work because it uses a differential amplifier to detect oscillation internally.)

Even old ATmega32U4 (older AVR) datasheet explicitly mentions that you can leave XTAL2 floating and drive XTAL1 with a clock source (with less than 2% variation between consecutive clock cycle durations), although the CLKSEL fuses need to be programmed to a dedicated value (0000) in that case.

I too would prefer good advice on calculating/choosing proper loading caps, though.


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