Motorola PowerQUICC II MPC8280 Series Reference Manual page 999

0
Field
Reset 0 1 1
R/W
Addr
Figure 30-4. FCC Data Synchronization Register (FDSR)
30.5 FCC Transmit-on-Demand Registers (FTODRx)
If no frame is being sent by the FCC, the CP periodically polls the R bit of the next TxBD
to see if the user has requested a new frame/buffer to be sent. Polling occurs every 256 serial
transmit clocks. The polling algorithm depends on the FCC configuration, as shown in the
following equations:
Fast Ethernet:
10BaseT:
The user, however, can request that the CP begin processing the new frame/buffer without
waiting the normal polling time. For immediate processing, after setting TxBD[R], set the
transmit-on-demand (TOD) bit in the transmit-on-demand register (FTODR) twice to
activate. If TOD is set only once, the new frame/buffer will not be transmitted until the next
periodic polling request.
This feature, which decreases the transmission latency of the transmit buffer/frame, is
particularly useful in LAN-type protocols where maximum interframe GAP times are
limited by the protocol specification. Since the transmit-on-demand feature gives a high
priority to the specified TxBD, it can conceivably affect the servicing of the other FCC
FIFO buffers. Therefore, it is recommended that the transmit-on-demand feature be used
only for a high-priority TxBD and when transmission on this FCC has not occurred for a
given time period, which is protocol-dependent.
If a new TxBD is added to the BD table while preceding TxBDs have not completed
transmission, the new TxBD is processed immediately after the older TxBDs are sent.
0 1
Field TOD
Reset
R/W
Addr
Figure 30-5. FCC Transmit-on-Demand Register (FTODR)
MOTOROLA Chapter 30. Fast Communications Controllers (FCCs)
Freescale Semiconductor, Inc.
SYN2
1 1 1 1
0x1130C (FDSR1), 0x1132C (FDSR2), 0x1132C (FDSR3)
256 clocks / 25 MHz = 10 µs
256 clocks / 2.5 MHz = 100 µs
0000_0000_0000_0000
0x11308 (FTODR1), 0x11328 (FTODR2), 0x11348 (FTODR3)
For More Information On This Product,
Go to: www.freescale.com
FCC Transmit-on-Demand Registers (FTODRx)
7 8
0 0 1 1 1
R/W
—
R/W
15
SYN1
1 1 1 0
15
30-9