NXP Semiconductors LPC24XX UM10237 - Manual (Page 274)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 274 of 792
NXP Semiconductors
UM10237
Chapter 11: LPC24XX Ethernet
After reset, the state machine is in the INACTIVE state. As soon as the TxEnable bit is set
in the Command register and the Produce and Consume indices are not equal, the state
machine transitions to the ACTIVE state. As soon as the TxEnable bit is cleared and the
transmit datapath has completed all pending transmissions, including committing the
transmission status to memory, the state machine returns to the INACTIVE state. The
state machine will also return to the INACTIVE state if the Produce and Consume indices
are equal again i.e. all frames have been transmitted.
For the state machine in Figure 11–35 , a soft reset is like a hardware reset assertion, i.e.
after a soft reset the transmit datapath is inactive until the datapath is re-enabled.
9.17 Transmission padding and CRC
In the case of a frame of less than 60 bytes (or 64 bytes for VLAN frames), the Ethernet
block can pad the frame to 64 or 68 bytes including a 4 bytes CRC Frame Check
Sequence (FCS). Padding is affected by the value of the ‘AUTO DETECT PAD ENABLE’
(ADPEN), ‘VLAN PAD ENABLE’ (VLPEN) and ‘PAD/CRC ENABLE’ (PADEN) bits of the
MAC2 configuration register, as well as the Override and Pad bits from the transmit
descriptor Control word. CRC generation is affected by the ‘CRC ENABLE’ (CRCE) and
‘DELAYED CRC’ (DCRC) bits of the MAC2 configuration register, and the Override and
CRC bits from the transmit descriptor Control word.
The effective pad enable (EPADEN) is equal to the ‘PAD/CRC ENABLE’ bit from the
MAC2 register if the Override bit in the descriptor is 0. If the Override bit is 1, then
EPADEN will be taken from the descriptor Pad bit. Likewise the effective CRC enable
(ECRCE) equals CRCE if the Override bit is 0, otherwise it equal the CRC bit from the
descriptor.
If padding is required and enabled, a CRC will always be appended to the padded frames.
A CRC will only be appended to the non-padded frames if ECRCE is set.
If EPADEN is 0, the frame will not be padded and no CRC will be added unless ECRCE is
set.
Fig 35. Transmit Active/Inactive state machine
ACTIVE
TxStatus = 1
INACTIVE
TxStatus = 0
TxEnable = 1
AND
TxProduceIndex <> TxConsumeIndex
TxEnable = 0 and not busy transmitting
OR
TxProduceIndex = TxConsumeIndex
reset
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