RM0090: Reference manual - STMicroelectronics

Ethernet (ETH): media access control (MAC) with DMA controller RM0090
Enhanced Tx DMA descriptors
Enhanced descriptors (enabled with EDFE=1, ETHDMABMR bit 7), must be used if time
stamping is activated (TSE=1, ETH_PTPTSCR bit 0) or if IPv4 checksum offload is
activated (IPCO=1, ETH_MACCR bit 10).
Enhanced descriptors comprise eight 32-bit words, twice the size of normal descriptors.
TDES0, TDES1, TDES2 and TDES3 have the same definitions as for normal transmit
descriptors (refer to Normal Tx DMA descriptors). TDES6 and TDES7 hold the time stamp.
TDES4, TDES5, TDES6 and TDES7 are defined below.
When the Enhanced descriptor mode is selected, the software needs to allocate 32-bytes (8
DWORDS) of memory for every descriptor. When time stamping or IPv4 checksum offload
are not being used, the enhanced descriptor format may be disabled and the software can
use normal descriptors with the default size of 16 bytes.
Figure 350. Enhanced transmit descriptor
TDES 0
TDES 1
TDES 2
TDES 3
TDES 4
TDES 5
TDES 6
TDES 7
31 0
O
W
N
Ctrl
[30:26]
Reserved
[31:29]
Res.
24
Ctrl
[23:20]
Buffer 2 byte count
[28:16]
● TDES4: Transmit descriptor Word4
Reserved
● TDES5: Transmit descriptor Word5
Reserved
● TDES6: Transmit descriptor Word6
Reserved
[19:18]
953/1416 Doc ID 018909 Rev 3
T
T
S
E
Reserved
[15:13]
Buffer 1 address [31:0]
Status [16:0]
Buffer 2 address [31:0] or Next descriptor address [31:0]
Reserved
Reserved
T
T
S
S
Time stamp low [31:0]
Time stamp high [31:0]
Buffer 1 byte count
[12:0]
ai17105b
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
TTSL
rw
Bits 31:0 TTSL: Transmit frame time stamp low
This field is updated by DMA with the 32 least significant bits of the time stamp captured
for the corresponding transmit frame. This field has the time stamp only if the Last segment
control bit (LS) in the descriptor is set.