MSC8113 Reference Manual, Rev. 0
25-38
Freescale Semiconductor
Ethernet Controller
25.11 Flow Control
Because collisions cannot occur in Full-Duplex mode, the Ethernet controller can operate at the
maximum rate. If the rate becomes too fast for a station receiver, the station transmitter can send
flow-control frames to reduce the rate. Flow-control instructions are transferred by special frames
of minimum frame size. The length/type fields of these frames have a special value. Table 25-11
shows the flow-control frame structure.
If flow-control mode is enabled (MACCFG1R[RXFL] is set) and the Ethernet controller receiver
identifies a pause-flow control frame, transmission stops for the time specified in the control
frame. During this pause, only a control frame can be sent by the Ethernet controller. Normal
transmission resumes after the pause timer stops counting. If another pause-control frame is
received during the pause, the period changes to the new value received.
Before transmitting flow control frames, poll the Used Entry Count Register in the FIFO. If the
value is equal to or greater than the value in the Alarm Register, use the following recommended
steps to transmit flow control frames:
Table 25-11. Flow Control Frame Structure
Size [Octets]
Description
Value
Comment
7
Preamble
1
SFD
Start frame delimiter
6
Destination
address
01-80C2-00-00-01
Multicast address reserved for use in MAC frames
6
Source address
2
Length/type
88-08
Control frame type
2
MAC opcode
00-01
Pause command
2
MAC parameter
Pause time as defined by the PTV[PT] field. The pause period is
measured in pause_quanta, a speed independent constant of 512
bit-times (unlike slot time). The most-significant octet is transmitted
first. with a two pause_quanta resolution.
Note:
Because the pause period has a resolution of two
pause_quanta, the value programmed in this field is
rounded up to the nearest even number before it is used,
as follows:
MAC Parameter Value
Pause Period
0 none
1 or 2 2
×
pause_quanta
3 or 4 4
×
pause_quanta
2
Extended MAC
parameter
Pause time extended as defined by the PTV[PTE] field. The most
significant octet is transmitted first.
40
Reserved
—
4
FCS
Frame check sequence (CRC)
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