RM0090: Reference manual - STMicroelectronics

RM0090 Ethernet (ETH): media access control (MAC) with DMA controller
Figure 331. Tagged MAC frame format
7 bytes
1 byte
6 bytes
6 bytes
QTag Prefix
4 bytes
2 bytes
42-1500 bytes
4 bytes
Each byte of the MAC frame, except the FCS field, is transmitted low-order bit first.
An invalid MAC frame is defined by one of the following conditions:
● The frame length is inconsistent with the expected value as specified by the length/type
field. If the length/type field contains a type value, then the frame length is assumed to
be consistent with this field (no invalid frame)
● The frame length is not an integer number of bytes (extra bits)
● The CRC value computed on the incoming frame does not match the included FCS
29.5.2 MAC frame transmission
Preamble
SFD
Destination address
Source address
Length/type = 802.1QTagType
Tag control information
MAC client length/type
MAC client data
Pad
Frame check sequence
MSB LSB
Bit transmission order (right to left)
bytes within
frame transmitted
top to bottom
MSB LSB
1 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0
User priority CFI
VLAN identifier (VID, 12 bits)
ai15630
The DMA controls all transactions for the transmit path. Ethernet frames read from the
system memory are pushed into the FIFO by the DMA. The frames are then popped out and
transferred to the MAC core. When the end-of-frame is transferred, the status of the
transmission is taken from the MAC core and transferred back to the DMA. The Transmit
FIFO has a depth of 2 Kbyte. FIFO-fill level is indicated to the DMA so that it can initiate a
data fetch in required bursts from the system memory, using the AHB interface. The data
from the AHB Master interface is pushed into the FIFO.
When the SOF is detected, the MAC accepts the data and begins transmitting to the MII.
The time required to transmit the frame data to the MII after the application initiates
transmission is variable, depending on delay factors like IFG delay, time to transmit
preamble/SFD, and any back-off delays for Half-duplex mode. After the EOF is transferred to
the MAC core, the core completes normal transmission and then gives the status of
transmission back to the DMA. If a normal collision (in Half-duplex mode) occurs during
transmission, the MAC core makes the transmit status valid, then accepts and drops all
further data until the next SOF is received. The same frame should be retransmitted from
SOF on observing a Retry request (in the Status) from the MAC. The MAC issues an
underflow status if the data are not provided continuously during the transmission. During
the normal transfer of a frame, if the MAC receives an SOF without getting an EOF for the
previous frame, then the SOF is ignored and the new frame is considered as the
continuation of the previous frame.
Doc ID 018909 Rev 3 914/1416