GR740-UM-DS, Nov 2017, Version 1.7
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GR740
13.4.5.5 The transmission process
When the transmit enable bit int the AMBA port DMA control/status register is set, the port starts
reading descriptors immediately. The number of bytes indicated are read and transmitted. Once a
transmission is finished, status information is written to the first field of the descriptor and a packet
sent bit is set in the DMA control register. If an interrupt was requested, it will also be generated.
Then, a new descriptor is read and - if enabled - a new transmission starts, otherwise the transmit
enable bit is cleared and the transmitter channel stays idle until it is enabled again.
13.4.5.6 The descriptor table address register
The internal pointer, which is used to keep the current position in the descriptor table can be read and
written through the APB interface. This pointer is set to zero during reset and is incremented each
time a descriptor is used. It wraps automatically when the 1024 bytes limit for the descriptor table is
reached or it can be set to wrap earlier by setting a bit in the current descriptor.
The descriptor table register can be updated with a new table anytime when no transmission is active.
No transmission is active if the transmit enable bit is zero and the complete table has been sent or if
the table is aborted (explained below). If the table is aborted one has to wait until the transmit enable
bit is zero before updating the table pointer.
13.4.5.7 Error handling
13.4.5.7.1 Abort Tx
The DMA control register contains a bit called Abort TX, which if set causes the current transmission
to be aborted, that is, the packet is truncated and an EEP is inserted. This is only useful if the packet
Table 149.
TXDMA transmit descriptor word 1 (address offset 0x4)
31
0
HEADERADDRESS
31: 0
Header address (HEADERADDRESS) - Address from where the packet header is fetched. Does not
need to be word aligned.
Table 150.
TXDMA transmit descriptor word 2 (address offset 0x8)
31
24 23
0
RESERVED
DATALEN
31: 24
RESERVED
23: 0
Data length (DATALEN) - Length of data part of packet. If set to zero, no data will be sent. If both
data- and header-lengths are set to zero no packet will be sent.
Table 151.
TXDMA transmit descriptor word 3(address offset 0xC)
31
0
DATAADDRESS
31: 0
Data address (DATAADDRESS) - Address from where data is read. Does not need to be word
aligned.