lpcware

LPC1778 EMAC Module: Clarification Required

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by MikeSimmonds on Tue Nov 25 09:58:18 MST 2014
On the 1778 (and presumably other devices with the EMAC block), there is a 'Maximum Frame Register' to set the maximum size for
received frames. [NOTE this is with the HUGE FRAMES option bit disabled.]

The power on default is 1536 bytes, conveniently larger than the maximum size of 1518/1522 for Ethernet frames/ VLAN tagged frames.

If the size is set to say 576 bytes, what actually happens when a over large frame appears on the wire?

Is it transparently ignored?

Is part of the frame stored?

Does the large frame use up a receive descriptor? Can the DMA ever write past the descriptor size (corrupting the following buffer?)

If so, what flags are set (if any) in the receive status word?

And which (if any) interrupt status flags are set.

The UM is completely silent on this issue.


Further more, if I attempt to transmit from a buffer exceeding the Ethernet maximum frame size,
what happens?

Is the transmission automatically fragmented? Even if the 'Do Not Fragment' bit is set?
If not, what error and or interrupt status flags are raised.

Regards, Mike

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