W5500 RSVD, SPI lines resistors

Question

Hi, I am using the W5500 with STM32F303 with external transformer due to POE. I need some confirmations regarding the layout: -are the 10K pulldown resistors useful on all RSVD lines? i have seen implementation with all RSVD pulled down with 10k but the official schematic https://wizwiki.net/wiki/lib/exe/fetch.php?media=wiznet_schematic:ethernet_chip:w5500_sch_v110_use_trans_.pdf leave them N.C. In my current implementation i leaved all RSVD N.C. except pin23 whic i tied to GND. -what is the purpose of the 33R resistors on all SPI lines? my system is 3.3V and i wonder if i could get rid of them. -are pull-up/down resistors needed to configure PMODE lines? i used solder jumpers and tied them directly to GND/3V3. -what is the right way to connect Tx and Rx center taps? i have seen various implementations, in my case the difficulty is that there is a POE. The schematic below is my current wiring, its on 4 layer PCB, all differential pairs are matched length and controlled impedance. There is an AGND but it may be replaced by a 0R or solder jumper since i have read it causes problems of excessive ESD sensitivity. i initially had VA1A and VA2A (POE) connected to center tap, its been removed when i changed the terminations on this side of transformaer, i wonder if and how i should conect them back. Any advise will be warmly welcome

Eugeny · Answer

Your questions are not related to W5500, but rather to general electronics. I will give you short answers, and you will find further information yourself. Hoel: but the official schematic You must read datasheets carefully and use official schematics. Reusing someone else’s circuits is at your own risk. If something will get wrong if you answer that you did not follow manufacturer’s guidelines, you will most probably be answered that you have unsupported configuration and referred back to reference schematic to do the homework. Hoel: what is the purpose of the 33R resistors on all SPI lines? Resistors in series are usually put for impedance matching. Hoel: are pull-up/down resistors needed to configure PMODE lines? Datasheet says that these pins do already have pull-ups. You must get some digital level on these pins, thus please go and read on voltage divider. Hoel: in my case the difficulty is that there is a POE POE is not well covered by the WIZnet, so you are in gray field now. I recommend you to look into reference schematics for other card/chipset manufacturers (e.g. Intel), you will most probably find some application/design notes on the subject.