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Didan , 06-03-2019, 12:52 PM

Dear forum, I received a task to review a board with ethernet controller W5100 from Wiznet and Ethernet Connector. It has SIM800L module as well ESP8266 uC. I cant give lot of details because confidentialitty.
As I know, for ETH controller and Conn, It has differential signal pairs because ETH interface with RXIN, RXIP and TXON, TXOP RJ45 connector. For this board, I have only 2 layers and I dont know if I need to decide and go through 4 layers because Ethernet Interface. I attached pictures from this layout and I would like suggestions how to improve it. Besides it theres a battery charger CI and holes where I will place a switch power supply module. I indicated So my questions:
1) Keep 2 layers or go through 4 layers? (1 gnd plane and 1 power plane, 2 signal layers)
2) How to make a correct differential pair for ethernet controller wiznet W5100, I did some serpentine to tune impedance on 50 ohms, and tune differential pair skew to 0mm.
3) Theres a need to use serpentine together with differential pair and which rules I should use, general rules, i find this reference design: https://wizwiki.net/wiki/doku.php?id...hardware:start
4) What to improve looking for this board?
5) I have SIM800L module, Esp8266 uController and Ethernet interface what to improve looking for this design and its reasonable this way or need to make rework, like place multilayers and its correct to use vias to do differential pair for ETH as was done? Well, whole board was done using auto router by coworker here...

robertferanec , 06-04-2019, 05:15 AM

Hi @Didan, I would recommend to use numbers for every question you ask and try to formulate short and clear questions. Honestly, I have got lost somewhere in the middle of your post. It would be very helpful if it's a little bit more clear to read.

PS: Possibly start with only 1 most important question

robertferanec , 06-04-2019, 07:00 AM

Thank you @Didan

Lakshmi , 06-07-2019, 08:14 AM

1. Why Auto-router is used? Do it Manually.
2. I would Prefer to go with 4 Layer. What about the Antenna section? The care has been taken right (Proper shielding and GND stitching via's.?)
3. Where are the Magnetics{Transformer} used in between the RJ45 and Ethernet Controller.
4. As I can see the Ethernet controller will interfacing with ESP8266 by SPI. Hope you're aware that speed will be reduced.
5. Since you have told that You have SIM800L I highly recommend you to go with 4 Layer Board.
6. Have you read this Layout guide: http://www.i-vis.co.jp/pdf/wiznet/tc...yout_Guide.pdf

Didan , 06-07-2019, 01:02 PM

Hello Lakshmi, I read their layout guide... Why I should have magnetics ? Up to know I didnt understand it because I dont wanna magnetics bro... Im trying to figure out it. Its not my design, was from coworker here and hes mechanical engineer. I was hired to develop hardware here and review boards done by company. Its Kicad software, there are many limitations about antenna section, why I should use antenna section? I found a ethernet shield only using 2 layers, what u think about it ? https://easyeda.com/hot/Arduino_Ethe...ield-Ka3XRxrJD

robertferanec , 06-09-2019, 02:02 AM

1) Keep 2 layers or go through 4 layers? (1 gnd plane and 1 power plane, 2 signal layers)
- I generally prefer to use 4 layer instead of 2, especially if it is going to be a real product sold to customers. I only use 2 layers rarely.

2) How to make a correct differential pair for ethernet controller wiznet W5100, I did some serpentine to tune impedance on 50 ohms, and tune differential pair skew to 0mm.
- For ethernet, I normally use 100OHS differental pair impedance and I will make all the ethernet signals similar length. If you are not sure, have a look and measure signals on our iMX6 Rex development baseboard: https://www.imx6rex.com/application/...lopment-board/

3) Theres a need to use serpentine together with differential pair and which rules I should use, general rules, i find this reference design: https://wizwiki.net/wiki/doku.php?id...hardware:start
- the link doesnt work. General rules for ethernet can be found for example in "COM Express design guide" .. for example here: https://docs.toradex.com/102492-layout-design-guide.pdf



4) What to improve looking for this board?
- that is very hard to answer, especially if only screenshots are available.

5) Well, whole board was done using auto router by coworker here...
- I never use autoruter, autorouter may not be able to correctly do layout.

@Lakshmi has very good points!

robertferanec , 06-11-2019, 01:50 AM

- maybe you will transfer data in small frequency, but the physical layer will run according to standards of Ethernet (or the other words, if you are going to transfer data in small frequency, that doesn't mean, that the whole Ethernet will run on that frequency ... it only means, that ethernet will transfer your data quickly and then it will be waiting for next data).

- if there is no other option, you can route differential pairs on 2 layer PCB, but, be aware, these signals will not follow recommended impedance and therefore you need to keep them as short as possible (not ideal, but in standard interfaces e.g. USB, Ethernet it will work). Personally, I prefer 4 layer PCBs at least.

robertferanec , 06-12-2019, 05:07 AM

@Didan, I am not sure what to answer. If you feel confident, go for 2 layers.