topic Crystal selection in ColdFire/68K Microcontrollers and Processors

Crystal selection

sanket_patadiya — Thu, 25 Feb 2021 10:32:17 GMT

Hi, We are working with the MCF51QM128VLH 32bit MCU. Currently, We are using ECS-080-18-23A-JGN-TR crystal with no load capacitance. Now we are planning to define ECS-80-18-30-JGN-TR as alternate crystal whose ESR is 100ohm instead of 80ohms of ECS-080-18-23A-JGN-TR. Generally the RNEG(negative resistance) of the amplifier should be grater than or equal to 5*ESR.

So please let us know RNEG(Negative resistance) of the amplifier the MCU can provide to the crystal. If the MCU can provide the RNEG >= 500ohms or not?

Would you suggest ECS-80-18-30-JGN-TR as an alternate crystal ?

Re: Crystal selection

MikeC1 — Wed, 14 Sep 2022 23:27:55 GMT

I don't have an answer,

I have the same question, but for a different processor

need to know what the absolute negative resistance value (Rs) for DSP56F803?

Need this value to select an alternative 8Mhz crystal oscillator, ESR=100, without load caps.

need to verify Rs is at least 500 ohms

The Driver circuit inside the processor is what determines the negative resistance not the crystal supplier.

The NXT/Freescale Semi datasheets and manuals for this part to not mention or show this value.

Can anyone help with this?

Re: Crystal selection

TomE — Sun, 18 Sep 2022 23:29:42 GMT

Searching this site for Application Notes:

https://www.nxp.com/docs/en/application-note/AN1706.pdf

https://www.nxp.com/docs/en/application-note/AN1783.pdf

They're a bit hard-core. Here's the simple way to verify the oscillator margin:

https://www.murata.com/en-sg/products/timingdevice/crystalu/overview/basic/margin

The latter is very simple. Take the crystal ESR. Multiply by 5. Pick a resistor of that value. Solder that in series with the crystal. Does it oscillate? If it does you have a margin of "5". For the advanced case, put the device in an environmental chamber and make sure it keeps working over the full temperature range.

Tom