Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 969

The following sections describe the transmit and receive mechanisms for the UTOPIA interface. The
expanded cell option is also discussed. Detailed information about UTOPIA mode can be found in
Chapter 42, "Interface Configuration,"
36.6.1 UTOPIA Transmit Overview
The UTOPIA transmit process begins with the ATM pace control unit (APC). The APC schedules the ATM
traffic using a scheduling table and a user-configured APC timer (CPM general-purpose timer 4) that
defines the maximum transmit bit rate (bandwidth). The APC maintains the traffic parameters for each
channel and divides the total bandwidth among the active channels. It can provide CBR, VBR and UBR
traffic services. ABR can also be supported through application software manipulation of APC parameters.
See Chapter 40, "ATM Pace Control,"
the APC.
With each tick of the APC timer, the APC prepares the channel(s) in the current time slot for transmission
by inserting the channel number(s) into the transmit queue. When the PHY asserts the transmit cell
available (TxClav) signal, the transmitter takes the next channel number from the transmit queue. The
transmitter uses the channel number to find the channel's transmit connection table (TCT).
For AAL5, the transmitter then copies 48 bytes (or up to 65 bytes for channels configured with expanded
cells) from the external buffer, performs CRC32, copies the cell header from the cell header entry of the
TCT, and sends the complete cell through the UTOPIA interface. For the last cell of an AAL5 frame, the
transmitter appends the trailer of the common part conversion sublayer-protocol data unit (CPCS-PDU) to
the user frame. It pads as required, appends the length (calculated during the frame transmit), and copies
the CPCS-UU and CPI from the TxBD. The transmitter also sets the PTI[1] bit in the header. An interrupt
can be optionally generated to declare the end of the transmit frame.
For AAL0, the transmitter simply copies the cell (except the HEC) prepared by the user from the channel's
buffer and sends it through the UTOPIA interface. The ATM controller can optionally generate CRC10 on
the cell payload and place the result at the end of the payload (CRC10 field). This feature is used to support
OAM CRC10; refer to the ITU specification I.610 for additional details.
If, however, the current active channel's buffer is not ready, the transmit process ends and no cell is sent
to the PHY. The PHY is responsible for generating an idle cell in an empty cell slot. An empty cell slot
will continue to be generated each time the APC schedules this channel in the transmit queue until either
a buffer is made ready or a
Controller," for additional information about ATM controller commands.
Note that the ATM controller does not generate the HEC in UTOPIA mode. The transmitter sends a
dummy byte value (0x00) in place of the HEC; the PHY is responsible for the actual calculation of the
HEC.
36.6.2 UTOPIA Receive Overview
The UTOPIA receive process begins when the PHY asserts the receive cell available signal (RxClav),
indicating that the PHY has a complete cell in its receive FIFO buffer. The ATM controller first receives
the cell header through the UTOPIA interface. The receiver translates the header address
(GFC/VPI/VCI/PTI) to a channel number using either a look-up table in dual-port RAM, address
Freescale Semiconductor
and in Chapter 43, "UTOPIA Interface."
for additional information about the operation and programming of
ACTIVATE CHANNEL
TRANSMIT DE
MPC885 PowerQUICC Family Reference Manual, Rev. 2
command is issued. See
ATM Overview
Chapter 39, "ATM
36-7