Motorola PowerQUICC II MPC8280 Series Reference Manual page 1001

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For frame-based protocols, a message can reside in as many buffers as necessary (transmit
or receive). Each buffer has a maximum length of (64K–1) bytes. The CP does not assume
that all buffers of a single frame are currently linked to the BD table. It does assume,
however, that unlinked buffers are provided by the core soon enough to be sent or received.
Failure to do so causes an error condition being reported by the CP. An underrun error is
reported in the case of transmit; a busy error is reported in the case of receive. Because BDs
are prefetched, the receive BD table must always contain at least one empty BD to avoid a
busy error; therefore, RxBD tables must always have at least two BDs.
The BDs and data buffers can be anywhere in the system memory.
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.
MOTOROLA Chapter 30. Fast Communications Controllers (FCCs)
Freescale Semiconductor, Inc.
Status and Control
Data Length
High-Order Data Buffer Pointer
Low-Order Data Buffer Pointer
Figure 30-7. Buffer Descriptor Format
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FCC Buffer Descriptors
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