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Software Developer’s Manual 313
Register Descriptions
13.4.39 Transmit Descriptor Head
TDH (03810h; R/W)
This register contains the head pointer for the transmit descriptor ring. It holds a value that is an
offset from the base, and indicates the in–progress descriptor. It points to a 16-byte datum.
Hardware controls this pointer. The only time that software should write to this register is after a
reset (TCTL.RST or CTRL.RST) and before enabling the transmit function (TCTL.EN). If
software were to write to this register while the transmit function was enabled, the on-chip
descriptor buffers can be invalidated and indeterminate operation can result. Reading the transmit
descriptor head to determine which buffers have been used (and can be returned to the memory
pool) is not reliable.
Table 13-82. TDH Register Bit Description
LEN 19:7 0b
Descriptor Length.
Number of bytes allocated to the transmit descriptor circular
buffer.
Reserved 31:20 0b
Reserved
Reads as 0b. Should be written with 0b for future compatibility.
31 16 15 0
Reserved TDH