Layout design in a board with Ethernet Controller and Connector

[Didan]

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]

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

[Didan]

Hi Robert. So, I use numbers for questions, the rest I was updating about my problem here. Anyway I will try make it clear, im thinking how because all informations are important.

[robertferanec]

Thank you Didan

[Didan]

Hi Robert, what about my question? My mainly doubt is if gonna be ok route ethernet controller and rj 45 connector in a 2 layers board and about its differential pair matching. Im using winznet W5100 ethernet chip up to 100Mhz

[Lakshmi]

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]

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]

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!

[Didan]

Thanks Robert. I knew that we are going to use only up to 10Khz, maybe 50Khz, not 100Mhz that Ethernet provide. My question is also up to what frequency gonna start be important to worry about differential pair, lenght matching? I found this link with ethernet done with 2 layers. What do you think about this design? Is it correcT? Im not sure if its gonna work, probably yes but with 100Mhz? I would like to have more insights from who has more experience than me.
2 layers ethernet: https://easyeda.com/hot/Arduino_Ethe...ield-Ka3XRxrJD
I hope some answers from you, they are important to me. Thanks

[robertferanec]

we are going to use only up to 10Khz, maybe 50Khz, not 100Mhz that Ethernet provide

- 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.

[Didan]

I agree with you about 4 layers, but to interface only ethernet in a board which only it have mid frequency (up to 100Mhz) working with ESP-12E, SIM800L gonna be very expensive, my was if should work ethernet well at least up to 50Khz. Can you explain me consequences about a component not following right impedance? I know its gonna decrease its data rate but how much on average? If it works on 100Mhz in a 4 layer for 2 layers it wont work on 100Mhz because impedance matching? It needs to be 2 layers because if not gonna be very expensive but it still end user, for customers, we are going to make 800 boards and I cant make mistakes and I need to use 2 layers, thats the point. I cant follow all PCB techniques because you know as Hardware Engineer we need to reduce cost and if we can do it even if our circuit dont work 100%, we should try and need to meet customer specification. If we was going to use 50Mhz or above I should recommend /think 4 layers as well. Do you know up to what frequency we should have a good signal integrity for hight mid-high frequency? It means, I dont need to worry so much about matching impedance or length.

[robertferanec]

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