RSET value

[jgaguitron7]

My design is based off STM32MP257F-EV1. In which a RTL8211F-CG Ethernet Transceiver is used. My question is on Pin 39 (RSET) of the transceiver. A 2K49 resistor is used. Where does this value come from? Is it a recommendation from Realtek supplier?

[PatrickF]

Hi @jgaguitron7 

Agree this value is missing from their datasheet. They poorly document their devices (and most of their documents are not public).

This is the value they put on the RTL8211F Reference Schematics (confidential document, provided by Realtek to customers with NDA).

This kind of external resistor is frequent on devices to generate accurate internal bandgap reference voltage (almost) independent of environmental variations (temperature, voltage) than silicon characteristics (or requiring less design complexity or factory calibration).

Regards.

[jgaguitron7]

Thank you, Patrick.

I was challenged by Plant to change this value to 2.4K for practical/available stock purposes. I think I should stick to the 2.49K recommendation since there is no known range of value selection.

[PatrickF]

Hi,

2.4k might probably be ok for a home made prototyping if you have no other choice, but for any other usages, I would stick to 2.49k 1% which is quite common (1% E48 and E96 series are now standard resistor without any availability and pricing concerns). 

I could understand the manufacturing requirement to reduce different part number count, but here it is a must have.

Change in this resistor might end up to electrical issues on the ethernet reliability.

Regards.