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CHAPTER 3 FUNCTIONAL DESCRIPTION
User’s Manual S14054EJ4V0UM
57
3.9.2 Flow control pause timer
The flow control pause timer is a 16-bit timer and stores the pause timer value in the received pause control
frame. If the pause timer value of the pause control frame is not 0, it indicates that a new frame should not be
transmitted. If the pause timer value of the received control frame is 0, normal transmission is resumed.
If the RXFC bit of the MACC1 register is 0, the value loaded to the pause timer is ignored. The setting of the
RXFC bit has nothing to do with the pause timer, and the pause timer is always updated when a valid control frame
has been received. If the setting of the RXFC bit is changed, software reset must be executed by using the MCRST
bit of the MACC2 register.
3.9.3 Transmitting pause control frame
The
µ
PD98431 automatically generates and transmits a pause control frame, depending on the relation between
the quantity of data in the receive FIFO and threshold values set by the RFDMH and RFDML fields of the RFIC1
register when the FLWCNT bit of the MACC3 register is 1.
When the FLWCNT bit is 1, and if the quantity of data in the receive FIFO exceeds the threshold level set by the
RFDMH field, the
µ
PD98431 automatically transmits the pause control frame. If a data frame is already being
transmitted when the
µ
PD98431 tries to transmit the pause frame, the
µ
PD98431 waits until transmission of the data
frame is completed, and then transmits the pause frame.
The pause control frame transmitted at this time is generated as follows. A reserved multicast address (01-80-C2-
00-00-01) is given as a destination address by hardware, and the station address set by the LSA1 and LSA2
registers is appended as a source address. 8808H is appended to the length/type field, and a pause op code of
0001H is appended to the control op code field. In addition, the value set to the PTIME field of the MACC3 register is
appended as the value of the pause timer.
After the quantity of data in the receive FIFO has exceeded the threshold value of the RFDMH field and the
µ
PD98431 has automatically transmitted the pause control frame, the data stored in the receive FIFO is transferred
to the upper layer. If the quantity of data in the receive FIFO falls below the threshold level set by the RFDML field as
a result, the
µ
PD98431 automatically generates and transmits a pause control frame with a pause timer value of 0,
prompting resumption of transmission by the other party.
3.10 Back Pressure
The
µ
PD98431 has a back pressure function. This function is valid only during half-duplex operation and is
enabled if the quantity of data in the receive FIFO exceeds the threshold level set by the RFDMH field of the RFIC1
register when the BACKPE bit of the MACC3 register is 1. If packet detection is detected with this function enabled,
a dummy packet is immediately transmitted, and collision is forcibly generated. When the data in the receive FIFO is
transferred to the host system and if the data quantity of the receive FIFO falls below the value of the RFDMH field of
the RFIC1 register as a result, this function is disabled.
3.11 Operation for VLAN Frames
The
µ
PD98431 detects a VLAN frame by comparing the value set in advance in a register with the TPID field in a
receive or transmit packet. This section explains the operation to be performed for a VLAN frame. The settings can
be performed separately for each port, by using the register provided for each port.
3.11.1 Detecting VLAN frames
The
µ
PD98431 always compares a 2-octet value of the TPID field that follows the source address in a transmit
packet with the value of the VLTP register in the
µ
PD98431. When the two values match it assumes the packet is a
VLAN frame.
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