Freescale Semiconductor MPC8260 - Manual (Page 909)

Fast Communications Controllers (FCCs)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 29-11
The CP processes the TxBDs in a straightforward fashion. Once the transmit side of an FCC is enabled, it
starts with the first BD in that FCC’s TxBD table. When the CP detects that TxBD[R] is set, it begins
processing the buffer. The CP detects that the BD is ready either by polling the R bit periodically or by the
user writing to the FTODR. When the data from the BD has been placed in the transmit FIFO buffer, the
CP moves on to the next BD, again waiting for the R bit to be set. Thus, the CP does no look-ahead BD
processing, nor does it skip over BDs that are not ready. When the CP sees the wrap (W) bit set in a BD,
it goes back to the beginning of the BD table after processing of the BD is complete.
After using a BD, the CP normally clears R (not-ready); thus, the CP does not use a BD again until the BD
has been prepared by the core. Some protocols support continuous mode, which allows repeated
transmission and for which the R bit remains set (always ready).
The CP uses RxBDs in a similar fashion. Once the receive side of an FCC is enabled, it starts with the first
BD in the FCC’s RxBD table. Once data arrives from the serial line into the FCC, the CP performs the
required protocol processing on the data and moves the resultant data to the buffer pointed to by the first
BD. Use of a BD is complete when no room is left in the buffer or when certain events occur, such as the
detection of an error or end-of-frame. Regardless of the reason, the buffer is then said to be closed and
additional data is stored using the next BD. Whenever the CP needs to begin using a BD because new data
is arriving, it checks the E bit of that BD. This check is made on a prefetched copy of the current BD. If
the current BD is not empty, it reports a busy error. However, it does not move from the current BD until
it is empty. Because there is a periodic prefetch of the RxBD, the busy error may recur if the BD is not
prepared soon enough.
When the CP sees the W bit set in a BD, it returns to the beginning of the BD table after processing of the
BD is complete. After using a BD, the CP clears the E bit (not empty) and does not use a BD again until
the BD has been processed by the core. However, in continuous mode, available to some protocols, the E
bit remains set (always empty).
29.7 FCC Parameter RAM
Each FCC parameter RAM area begins at the same offset from each FCC base area. The protocol-specific
portions of the FCC parameter RAM are discussed in the specific protocol descriptions. Table 29-4. shows
portions common to all FCC protocols.
Some parameter RAM values must be initialized before the FCC is enabled; other values are
initialized/written by the CP. Once initialized, most parameter RAM values do not need to be accessed by
user software because most activity centers around the TxBDs and RxBDs rather than the parameter RAM.
However, if the parameter RAM is accessed, note the following:
• Parameter RAM can be read at any time.
• Tx parameter RAM can be written only when the transmitter is disabled—after a STOP TRANSMIT
command and before a RESTART TRANSMIT command or after the buffer/frame finishes
transmitting after a GRACEFUL STOP TRANSMIT command and before a RESTART TRANSMIT
command.
• Rx parameter RAM can be written only when the receiver is disabled. Note the CLOSE RXBD
command does not stop reception, but it does allow the user to extract data from a partially full Rx
buffer.