LPC54608 RMII Connections

[saveriomercurio]

We're designing a board using an LPC54608 and a LAN8720 to provide an Ethernet interface. We have an LPCXpresso54608 development board (OM13092) as a reference. We're confused about the connection of the LAN8720's CRS_DV pin. The pin's name is "Carrier Sense/Receive Data Valid". On the development board this pin is connected to the LPC54608's PIO4_10 pin, which is the ENET_RX_DV pin, which is described as "Ethernet receive data valid".

What's confusing is that the PIO2_2 pin is described as "ENET_CRS — Ethernet Carrier Sense (MII interface) or Ethernet Carrier Sense/Data Valid (RMII interface)". PIO5_1 has the same description. Why is the PHY's CRS_DV pin not connected to one of these two pins rather than PIO4_10, which seems to be only half of the multiplexed CRS_DV output from the PHY?

We've looked at the LPC54608's Ethernet registers (section 36.6 of the LPC54608 User Manual), and the only reference to CRS occurs with MII mode. Is CRS ignored if in RMII mode. If so, then should it be connected to a pin that has the "pure" ENET_RX_DV function, such as PIO4_10 or PIO5_0?

[Hui_Ma]

Hi,

From LAN8720A datasheet, which is RMII Ethernet PHY.

The CRS_DV signal provided function to indicate the receive medium is non-idle.

Please check below picture for the detailed info:

So, the LPCXpresso54608 board using P4.10 (ENET_RX_DV) to connect with LAN8720A's CRS_DV pin.

Wish it helps.

Have a great day,
Mike

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[saveriomercurio]

Thanks for the response. However, you're just reiterating what we already know. In RMII mode that signal is a multiplexed signal. But the signal goes to a pin on the MCU that, based on its description, is not for multiplexed signals. PIO4_10 is described as simply "Ethernet receive data valid". PIO2_2, on the other hand, is explicitly described as "ENET_CRS — Ethernet Carrier Sense (MII interface) or Ethernet Carrier Sense/Data Valid (RMII interface)". So why is the signal from the PHY connected to PIO4_10 instead of PIO2_2?

In our design PIO4_10 is not available, but PIO5_0 and PIO5_1 are. PIO5_0 has the same function/description as PIO4_10. PIO5_1 has the same function/description as PIO2_2. We found that if the PHY's CRS_DV signal is connected to PIO5_1, then Ethernet communication doesn't work. If, however, it's connected to PIO5_0 then it works.

When would you connect to PIO2_2/PIO5_1? This is not clear based on the descriptions and the PIO functions table in the LPC54608 User Manual.

[Hui_Ma]

Hi,

First of all, thank you for the patience.

I got below feedback from LPC product team:

The user manual is correct and the connection of LAN8720 CRS_DV pin to LPC54608 P4_10 pin is correct.

The datasheet description of the pin functions needs to be:

   ENET_CRS – Ethernet Carrier Sense (MII interface only)

   ENET_RX_DV – Ethernet Receive Data Valid (MII interface) or Ethernet Carrier Sense/Receive Data Valid (RMII interface)

I think that was a document issue.

We should provide more info about ENET_RX_DV function description for both MII and RMII interface.

Wish it helps.

Have a great day,
Mike

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Note: If this post answers your question, please click the Correct Answer button. Thank you!
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[Hui_Ma]

Hi,

First of all, sorry for the later reply.

I am checking with LPC product team about your query.

I will let you know when there with any feedback.

Thank you for the patience.

best regards,

Mike