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Series resistors on high speed signal tracks

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peter-h:
I understand what a transmission line is.

However the PCB is 4-layer FR4, 1.6mm total thickness, and it can't be anything else. I can control the track width; that's all.

Also the Zout of the CPU is not documented. The only clue I have is a load of 22R resistors, and the PCB they use for their developer kit (also FR4, 1.6mm, probably 0.006" traces. It is this one: https://www.st.com/en/evaluation-tools/stm32f4discovery.html. This is an add-on board with the RJ45
https://www.st.com/content/st_com/en/products/evaluation-tools/product-evaluation-tools/mcu-mpu-eval-tools/stm32-mcu-mpu-eval-tools/stm32-eval-boards/stm3221g-eval.html
and you can see the traces to the RJ45 are going to be very long, due to the RJ45 being on a daughter board...

It is 100M ethernet, not gigabit.

My plan is to leave out the resistors and make all the relevant traces really short; under 2cm. Fortunately this is possible. It is 100mbps data, not gigabit. I can get all relevant chips right next to each other. Or even on top of each other; on opposite sides of the PCB. Not sure if the latter is a good idea because of the vias.

That's an interesting calculator; thank you. Interesting one needs a 0.13" width to get 50 ohms :)

T3sl4co1l:

--- Quote from: peter-h on September 04, 2019, 05:00:39 pm ---My plan is to leave out the resistors and make all the relevant traces really short; under 2cm. Fortunately this is possible. It is 100mbps data, not gigabit. I can get all relevant chips right next to each other. Or even on top of each other; on opposite sides of the PCB. Not sure if the latter is a good idea because of the vias.
--- End quote ---

Not really necessary for an (R)MII bus, but just as well, the Ethernet signals themselves are much more robust with respect to signal quality so are better to run the distance with.

Again, the pin driver rise times will be a few nanoseconds, give or take; trace lengths on the order of 5cm barely feel it.  Signal quality is something to be aware of, and can be an EMC problem, but is unlikely to be an outright functional problem.



--- Quote ---That's an interesting calculator; thank you. Interesting one needs a 0.13" width to get 50 ohms :)

--- End quote ---

You'll only be putting ground plane on the bottom side?  What's in the middle?

Tim

Gibson486:
I never saw what would happen if you did not do fixed impedance on Ethernet traces. You can still do fixed impedance on a 4 layer board (very common). Your PCB thickness really does not matter because they will control the distance between the adjacent layers to compensate.  The only requirement is that a ground plane exists under those traces.

You can do vias, you just have to make sure that the differential pairs match. If the plus side has a via, you need to do it to the negative side too.

peter-h:
The layers are

signals
+3.3V plane
GND plane
signals

AFAIK all three sheets are the same thickness i.e. about 0.5mm.

The software I am using is Protel PCB 2.8, 1995 vintage, under a winXP VM. Don't laugh - it works great :) I also have a very good autorouter for it: Cooper & Chyan Specctra, of same vintage. I hand route

analog signals
GND and VCC connections (to the planes)
sensitive RF stuff (e.g. clock lines) or differential pairs
high voltage isolated stuff (e.g. optoisolated items); I routinely work to a 2.5kV standard and sometimes 10kV

and then (if there is much left; often there isn't) I draw the constraining polygon for the autorouter and let it do the digital interconnects.

Re RF compliance, I think the best strategy is to keep RF stuff short, keep loop area small (in switching power supplies), be very careful with return currents (loop area again), drive any clock nets with a resistor and put a position for a "last resort for the day in the EMC lab" capacitor (to GND) right after the resistor, and generally keep circuitry really compact, squeezing everything together tightly. Also the ST CPUs have clock spread spectrum feature which helps, although probably only if driving external memory which with these chips one rarely needs to do. And of course one always does EMC testing with perfectly made cables, etc :)

c64:

--- Quote from: peter-h on September 04, 2019, 07:04:04 pm ---The software I am using is Protel PCB 2.8, 1995 vintage, under a winXP VM. Don't laugh - it works great :)

--- End quote ---
:-+
I'm using Altium 2009 (not as vintage as yours but still, 10 years old already) also under winxp VM. Works great, super fast. I don't want to upgrade because anything older doesn't work inside VM (requires 3d support)

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