Freescale Semiconductor MPC8260 - Manual (Page 906)

Fast Communications Controllers (FCCs)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
29-8 Freescale Semiconductor
29.4 FCC Data Synchronization Registers (FDSR x)
Each FCC has a 16-bit, memory-mapped, read/write data synchronization register (FDSR) that specifies
the pattern used in the frame synchronization procedure of the synchronous protocols. In the totally
transparent protocol, the FDSR should be programmed with the preferred SYNC pattern. For Ethernet
protocol, it should be programmed with 0xD555. For the ATM protocol, FSDRx is used to generate a
constant byte for the HEC. It does not generate the HEC; instead it only outputs this constant byte as a
‘placeholder’ for the HEC. This byte is then replaced by the ATM PHY with the actual value.
At reset, FDSRx defaults to 0x7E7E (two HDLC flags), so it does not need to be written for HDLC mode.
The FDSR contents are always sent lsb first.
29.5 FCC Transmit-on-Demand Registers (FTODR x)
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:
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 7 8 15
Field SYN2 SYN1
Reset 0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 0
R/W R/W
Addr 0x0x1130C (FDSR1), 0x0x1132C (FDSR2), 0x0x1132C (FDSR3)
Figure 29-3. FCC Data Synchronization Register (FDSR)
Fast Ethernet: 256 clocks / 25 MHz = 10
µs
10BaseT: 256 clocks / 2.5 MHz = 100
µs