0

I am working on a project that includes the W5500 circuit, this is the first time i had the opportunity to layout Ethernet Interface circuits.

Schematic: enter image description here

Whole board and W5500 Module Layout Pictures Here

My questions are :

  1. In general does it matter if i not calibrate the differential pair impedance 100 ohm for the pair and 50 ohm for the individual signals if the traces are very short and crowded with termination resistors and other components as shown in the schematic below ? if yes, which part exactly should i implement ( before / after the 33 ohm or after the TVS diode if that is any different.

  2. I am aware of the original chip manufacturer's HW Guide lines here but in order to implement proper impedance on a 2 layer board then it is going to be very thin and thus mechanically unsuitable for the entire board 20 x 10 cm ( i hope i'm wrong ). The original chip manufacturer is selling an open source W5500 Ethernet shield here has eagle source files, i checked it and as far as i understand it may not follow the strict impedance control guidelines ( maybe things are a bit forgiving ? maybe i'm wrong ).

  3. I have added TVS diodes (IC2, IC4) USBLC6-2SC6 in the differential pair path as shown in the schematics below, any notes on the selection of this unit or any notes on the layout would be very much appreciated.

  4. As shown in the layout screenshots below, I made a separation on top layer (red) between the differential pair and the ground plane. and left the bottom ground plane as clear as i can get for now, any suggestions or comments are welcome.

  5. Under the TVS diode at the differential pair path, i needed to connect a 3.3VA and that trace runs under the IC which is passing through it the differential pair, i’ve attached a picture here to further explain my question, is this a real problem or not ?.

  6. Does the center tap connections need to have any impedance controlled value ?

  7. This may be off topic , but any ideas on how to connect the RJ45 Metal shield to the ground plane , the board will be enclosed in a metal box, how can i connect them ( System ground ----- RJ45 Metal Chassis ------ Metal Enclosure ), I've searched around a bit in this site and there are several methods and apparently there is a debate on to implement it, i think i'd go with "Short them together directly via the mounting holes on the PCB" answered by user3624, where i got the idea.

Some useful notes :

  1. I am using this RJ45 Straight Connector MFG Part no. 74990101210 from Wurth Electronics.

  2. I found this Wiznet Wiki HW Guidelines page to do the layout here

  3. The trace width of the differential pair is 10 mils and the distance between the pair is 10 mils, I'm working on a 2oz copper weight PCB, the fab house i'm working with offers the limit of 10 mils trace width and clearance 10 mils.Also i'm currently using Eagle v6.6

  4. I'm using this tool to figure out the differential pair/trace impedance characteristics.

I would appreciate any feedback. Thanks in advance !

Regards

Sultan

Sultanpepper
  • 71
  • 7
  • 1
    It seems odd to me that you wouldn't have the RJ45 near the edge of the board, the position may be difficult to use depending on the tab placement (if on the bottom) and may interfere with the capacitor right in front of it. – Ron Beyer May 07 '18 at 18:06
  • 1
    sharp corners on traces is bad practice. – asndre May 07 '18 at 19:33
  • Suppose the RJ45 is on the PCB Edge and the capacitor on top is out of the question, what would be your opinion on the impedance matching issue in question 1 and 2 ? – Sultanpepper May 07 '18 at 19:37
  • @asndre You mean the ground plane cut ? I will fix right away thank you for the heads up here – Sultanpepper May 07 '18 at 19:39
  • 1
    with lengths on your board it (highly likely :-) does not matter... your possible goal there could be to keep both traces in a pair the same length (as it possible) and topology (e.g. if you change layer for one trace in a pair, change layer for the opposite one at the same place or at least as close as possible). – asndre May 07 '18 at 19:43
  • 1
    i mean traces under the RJ on red layer, two have sharp conners. – asndre May 07 '18 at 19:44
  • 1
    imo, tvs at rx pair is misplaced, it should be between the RJ and the caps. – asndre May 07 '18 at 19:46
  • 1
    6> no. 7> connect the ground to the case directly, try to mechanically connect the RJ shield to the case too, if it's not possible, connect it to the ground directly. – asndre May 07 '18 at 19:50
  • 1
    5> if some traces (on different layers of a two-layer pcb) are "crossed" unavoidably, try to trace them so they "cross" each other at right (90 deg) angle (i.e. orthogonally). – asndre May 07 '18 at 19:54
  • @asndre Thank you sir, i really appreciate your concern and useful advice! I’ll be forever grateful. – Sultanpepper May 07 '18 at 21:39

0 Answers0