Hot!Connecting KSZ8863RLL to a MAC using RMII

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2018/08/10 01:28:18 (permalink)

Connecting KSZ8863RLL to a MAC using RMII

I'm adding a Lantronix xPico 250 to an embedded deivce to provide WiFi access. The device has an ARM based SOM which includes a PHY and there is also an external RJ45. The three are connected using a Microchip KSZ8663RLL 3-port switch. The SOM's PHY is capacitively coupled to one of the PHYs on the switch, and the external RJ45 is connected to the other. These can communicate with each other no problem. The Lantronix xPico 250 supports an RMII interface so this is connected to the RMII port on the switch. The RMII pins on the switch and the Lantronix are direcly mapped to each other by pin name. i.e. TX -> TX, RX -> RX.
It doesn't work though. I can access the xPico 250 wirelessly and accessing its internal webpages can see that the status of its ethernet link (eth0) is down. I don't have much else to go an at the moment.
I've checked the clock. The Lantronix xPico 250 expects a reference clock as an input which the switch is providing.
I've done some Googling and am aware that RMII isn't designed to work between two MACs. The datasheet for the Microchip switch refers to the RMII interface supporting both MAC and PHY modes but doesn't explain how to configure this. Every example reference design I've seen on the internet has it set up as a MAC. How can I be sure the switch is set up as an RMII PHY?
If it is actually a MAC and I'm connecting MAC to MAC, do I need to swap the signals round (ie. RX -> TX, TX -> RX etc)
Schematic attached.

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    Re: Connecting KSZ8863RLL to a MAC using RMII 2018/08/30 11:07:13 (permalink)
    I'm having the same problem with a very similar configuration.   In my case it is the KSZ8864 (4-port) switch and a different wireless module that supports RMII. 
    I've looked at your schematic and I am using the same connections, and think the Rx/Tx connections look correct.   But again, I'm having the same failure where the switch is working for other channels but not on this RMII channel.
    I have the switch pin strapped so its RMII interface is PHY Mode (MAC-to-MAC), and similar to your schematic their are no MDIO serial connections between the switch and wireless module.  I configured the switch for PHY Mode based on the fact that in this mode the switch's CRS_DV pin is an output, and on the WiFi module the CRS_DV line can only be an input.   Maybe this is a false assumption, and the switch should be set for MAC Mode (MAC-to-PHY). 
    To answer your question, I don't think there are any registers in the switch that will confirm that it is pin strapped correctly and in RMII mode.   At least I couldn't find anything with the KSZ8864 switch.
    Have you made any progress with this problem?
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