register, offset 0xC00, so that the DMA operates with extended descriptor size. When this
control bit is reset to the default (0), the TDES4-TDES7 descriptor space is not valid and
only Alternate Descriptors are available, with a default size of 16 bytes (4 words).
Figure 20-4. Enhanced Receive Descriptor Structure
31
0
7
23
15
Status [30:0]
Byte Count Buffer 1
[12:0]
Byte Count
Buffer2 [28:16]
RDES0
RDES1
RDES2
RDES3
Buffer1 Address [31:0]
Buffer2 Address [31:0]/Next Descriptor Address [31:0]
OWN
Receive Timestamp High [31:0]
Receive Timestamp Low [31:0]
Reserved
Extended Status [31:0]
CTRL
CTRL
[15:14]
RDES4
RDES5
RDES6
RDES7
Reserved
Reserved
[30:29]
The following tables define the Enhanced Receive Descriptors. RDES0 contains the received frame
status, the frame length, and the descriptor ownership information. RDES1 contains the buffer sizes
and other bits that control the descriptor chain or ring. RDES2 and RDES3 contains the address
pointers to the first and second data buffers in the descriptor. The availability of the extended status
is indicated by Bit 0 of RDES0. RDES6 and RDES7 are available only when the Advanced Timestamp
or IP Checksum Full Offload feature is enabled.
Table 20-8. Enhanced Receive Descriptor 0 (RDES0)
Description
Bit
OWN: Own Bit
When set, this bit indicates that the descriptor is owned by the DMA. When this bit is reset, it indicates that the
descriptor is owned by the Host. The DMA clears this bit either when it completes the frame reception or when
the buffers that are associated with this descriptor are full.
31
AFM: Destination Address Filter Fail
When set, this bit indicates a frame failed in the DA filter in the MAC.
30
1419
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller