Chapter 12: 10/100 IEEE802.3 Media Access Controller
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
12-44
before the frame ends.
•
When the MAC completes the reception of a frame and the current received
Descriptor has been closed
•
When the receive process is suspended because of a host-owned buffer, and a new
frame is received.
12.5.1.3.2
Receive Frame Processing
As incoming frames arrive, the MAC recovers the incoming data and clock pulses, and then
sends them to the reception engine. The reception engine strips the preamble bits and
stores the frame data in the RxFIFO. Concurrently, the reception section performs address
filtering depending on the perfect table or hash table. If the frame fails the address filtering, it
is ignored and flushed from the RxFIFO. Frames that are shorter than 64 bytes, due to
collision or premature termination, are also ignored and flushed from the RxFIFO.
The RxFIFO requires a similar handshake algorithm to that of the TxFIFO. When a new
frame is received, the first RxFrmReq is controlled by the reception frame threshold
(RxSOFTh). This RxSOFTh is set in the RFCReg (Receive Frame Configuration Register),
and determines the minimum frame required in the RxFIFO to initiate the DMA operation.
Once this threshold is met, the entire frame needs to be read out of the RxFIFO, regardless
of any error conditions and bits set in the RFCReg. If the MAC detects any errors before the
threshold is met, the frame will be discarded depending on the state of the appropriate
configuration bit in the RFCReg.
Once the threshold is met, each successive assertion of RxDMAReq is based on the PBL,
which may or may not be the same as the threshold.
If a frame ends before either PBL or threshold RxSOFTh is met, RxDMAReq will
automatically be asserted, regardless the PBL or the threshold.
12.5.1.3.3
Receive Frame Processing Suspended
If a receive frame arrives while the reception process is suspended, the MAC re-fetches the
current Descriptor in memory. If the Descriptor is now owned by the MAC, the reception
process reverts back to the running state and starts the frame reception. If the Descriptor is
still owned by the host, the MAC discards the current frame in the reception FIFO and
increments the Missed Frames counter.
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