CC3301: External crystal ESR specifications

Hi Wolfgang,

The XTAL used in our reference design has an ESR of max 30ohm, this is why this is noted in the datasheet.

In general the ESR matters because you want to make sure that the oscillator will be stable at the chosen frequency (40MHz) with all the added board resistance and other factors that will limit the drive capability of the XTAL. If you have a higher ESR on the XTAL you chose, that just means you have less margin for extra added resistance before the XTAL stops working.   

Note that in the reference design for the CC3301 a 150 ohm resistor on the p-side of the XTAL is required. When we add this added resistance with the existing ESR from the XTAL and the board resistance this is the total resistance the oscillator has to overcome to drive the signal. Through in-depth testing with the reference design XTAL, we know that the 150 ohm + 30 ohm max ESR is reliable and promises datasheet performance. 

Now for your use-case, if the max ESR is greater, you might just need to make the additional 150 ohm resistor smaller to compensate for the extra added ESR. However from our testing of max resistance the reference XTAL can still drive, this change in ESR will most likely be negligible. However this depends on other XTAL factors like the drive strength.  

I recommend you reference this app note: https://www.ti.com/lit/an/slaa322d/slaa322d.pdf 

Although the document references a different family of devices and different XTAL, the theoretical parameters described are the same for the CC3301.

Regards,

Jonathan