40.5.1.3 Transmit Buffer List

Transmit data is read from areas of data (the buffers) in system memory These buffers are listed in another data structure that also resides in main memory. This data structure (Transmit Buffer Queue) is a sequence of descriptor entries (as defined in Transmit Buffer Description Entry) that points to this data structure.

To create this list of buffers:

1. Allocate a number (n) of buffers of between 1 and 2047 bytes of data to be transmitted in system memory. Up to 128 buffers per frame are allowed.
2. Allocate an area 2n words for the transmit buffer descriptor entry in system memory and create N entries in this list. Mark all entries in this list as owned by EMAC, i.e., bit 31 of word 1 set to zero.
3. If fewer than 1024 buffers are defined, the last descriptor must be marked with the wrap bit — bit 30 in word 1 set to one.
4. Write address of transmit buffer descriptor entry to EMAC_TBQP register.
5. The transmit circuits can then be enabled by writing to the EMAC_NCR.