Specifications
RX62N Group Ethernet Transmit and Receive Settings
4.1.7 37BOverview of Receive Descriptor
Figure 4.6 shows the correspondence between a receive descriptor and a receive buffer.
A receive descriptor comprises, beginning from the start of the data, 32-bit units designated RD0, RD1, and RD2,
followed by padding. RD0 contains a bit indicating whether the receive descriptor is active or inactive as well as
descriptor configuration information and status information. RD1 indicates the size (RBL) of the receive buffer
referenced by the descriptor and the data length (RFL) of the received frame. RD2 indicates the start address of the
receive buffer. The length of the padding at the end is determined according to the descriptor length specified by the DL
bits in the EDMR register.
Depending on the receive descriptor settings, one descriptor can be used to store all the receive data in a single frame
in a receive buffer (single-frame/single-descriptor) or multiple descriptors can be used to store the receive data in a
single frame to multiple buffers (single-frame/multi-descriptor). To use single-frame/multi-descriptor operation,
multiple descriptors (a descriptor list) must be prepared beforehand. When the length of a received frame exceeds the
descriptor RBL, the EDMAC transfers the data it contains to consecutive receive buffers, continuing on to the next
descriptor as necessary. This would apply, for example, when the descriptor RBL is set to 500 bytes and a 1,514-byte
Ethernet frame is received. Beginning from the first descriptor, the data in the received Ethernet frame is saved 500
bytes at a time to successive buffers, with only the final 14 bytes transferred to the fourth buffer.
Receive descriptor
R
A
C
T
RFSRD0
RBL
RD1
RBARD2
31
30 29 28 27 26 0
31 0
R
F
E
R
D
L
E
R
F
P
1
R
F
P
0
015
31 16
Receive buffer
Valid receive data
RFL
Note: The padding is a redundant area whose size is adjusted according to the descriptor length
(16, 32, or 64 bytes).
Padding (4, 20, or 52 bytes)
Figure 4.6 Correspondence of Receive Descriptor and Receive Buffer
R01AN0629EJ0101 Rev.1.01 Page 23 of 52
Mar 31, 2011