Request : High speed design guidelines

[Yashodha D]

Dear Sir,

We are planning to design/implement A20-SOM-EVB on one of our board so Please provide the high speed signals design guidelines for the board A20-SOM-EVB.

Kindly provide the Impedance and PCB design guidelines for below mentioned connectors.
1.USB
2.ETHERNET

[LubOlimex]

The best course of action is investigating the design of A20-SOM-EVB which is an open-hardware design. The design (both PCB and schematic) can be investigated with old free version of Eagle (we used Eagle 4.16r2 for the design but you should be fine using last free version Eagle 7.0). The source files can be found here:

https://github.com/OLIMEX/SOM/tree/master/A20/A20-SOM-EVB

For the Ethernet routing we use the Micrel guidelines from AN-111 document, here:

https://www.eeweb.com/wp-content/uploads/articles-app-notes-files-general-pcb-design-and-layout-guidelines-1446782066.pdf

Specifics about A20-SOM:

1. A20-SOM-EVB has only GbE and our A20-SOM is designed to work with GbE by default. Hence we care about PA15's first function - GTXCK. Same for PA14 - it is not needed for GbE and there is no resistor, so we just routed it for GPIO operation. If you wish to use the board for MbE Ethernet function apply the modifications needed for Ethernet operation.

Now if I remember correctly only that CK pin is different between MbE and GbE and this specific wire PA15 is probably the cause of issues at your side. It is a lot shorter than the rest of the wires. Here is a length wires of of A20-SOM:

Length in mm

RX

ERXD0 96.68
ERXD1 96.69
ERXD2 96.57
ERXD3 96.62
ERXDV 96.73
ERXCK 96.77

TX

ETXD0 102.02
ETXD1 102.08
ETXD2 102.17
ETXD3 102.09
ETXEN 102.22
ETXCK 50.35

How TX length matching can be fixed if you made a mistake in your design? There are at least three different ways or combination of ways this can be fixed.

- Software method - I believe length differences and delays can be fixed by software means in the Ethernet driver and settings for the interface. If the hardware design is final, look into that direction.

- Lack of 22R resistor might be an issue so place such on your bottom board near the connector between the two boards. Maybe experiment with different values for resistor, this might fix the lenght difference without additional re-routing;

- Try to make the ETXCK wire longer on your bottom board so that it matches rest of the TX signals (around 102mm + length of wires of your bottom board).

Additionally, maybe take a look at some of our designs with MbE, like A20-OLinuXino-MICRO (it is also open source board, so you can find hardware design files in same repository):

https://github.com/OLIMEX/OLINUXINO/blob/master/HARDWARE/A20-OLinuXino-MICRO/1.%20Latest%20hardware%20revision/A20-OLINUXINO-MICRO_Rev_M.pdf

Despite that it has no 22R resistors (since wires are really short), the design is pretty similar.

Finally, follow the usual Ethernet routing rules. RX and TX lengths should be approximately the same (this might be hard for you to do since you lack information about A20-SOM), distance between MbE Ethernet wires and between rest of the wires should be at least three times wire width to avoid noise and so on. Then there should be differential pairs between your Ethernet chip and the Ethernet connector.

If the issues remain and time is a problem, maybe consider using GbE chip and routing instead, similar to A20-SOM-EVB.

2. C193 and L9 is an EMI reduction filter. It is not populated by default (NA) since during our tests and measurements it proved to be redundant, but we have left the pads as an option for the future or if somebody has EMI problems can populate it (or for bigger order we can populate it by default). It is usually only recommended for GbE operation.

Notice that if you use another Ethernet chip you'd probably need to add support in the Linux, this will probably require rebuilding the image.

[LubOlimex]

For the USB we use this document:

http://apachetechnology.in/KC/Multimedia/PCI/hs_usb_pdg_r1_0.pdf

[Yashodha D]

Hi,

Please provide the specification/description for TM211Q01FM22.

[LubOlimex]

We are reseller of this product you can find its datasheet here: https://www.olimex.com/Products/Components/Connectors/TM211Q01FM22/resources/TM211Q01FM22.pdf

[Yashodha D]

we got Datasheet sir. We need a description/parameter details of this part number.

[LubOlimex]

I have nothing extra. We are distributors of that part we don't manufacture it ourselves.

[Yashodha D]

Please provide the 3D model for A20-SOM-EVB_Rev_B.

[LubOlimex]

We don't have 3D models for the board, the main reason we lack such is that we don't need component height to manufacture the boards. So components in our designs have arbitrary height or lack height all together.

[Yashodha D]

Hi,

Please provide the Board Dimensions of A20-SOM-e16Gs16GM latest version board.

[LubOlimex]

I checked and compared older revisions and latest revision - there are no changes in dimensions in latest revisions. These dimensions still apply:

https://www.olimex.com/Products/SOM/A20/A20-SOM/resources/a20-som_dimensions.png