Transmit Cell Transfer Operation; Utopia Bus And Soc Drive - Freescale Semiconductor MPC850 User Manual

UTPClk
RxClav
RxEnb
UTPB
SOC

43.2.2 Transmit Cell Transfer Operation

Assertion of the TxClav signal generates a request for a cell to transmit. The MPC850SR's
UTOPIA interface implements the cell level handshake, and the PHY must be able to receive
a whole cell upon assertion of the TxEnb signal. Note that a RxClav signal assertion generates
a request that has higher priority than those caused by the assertion of the TxClav signal;
transmit cell requests are granted by the CP when it has completed all the cell receive requests.
The MPC850SR's UTOPIA controller divides the cell transfers into 1- to 4-byte groups and
uses the TxEnb signal to control the transfer. For example, a 53-byte cell transfer sequence is
divided into the following UTOPIA transfers:
• Header transfer (4 octets)
• UDF (HEC) transfer (1 octet)
• Cell payload transfers (total of 48 octets)
The MPC850SR asserts TxEnb, TxSOC, and TxPrty signals, and drives the UTOPIA bus
(through the UTPB signals) with data for each transfer. The cell header is transferred in a
4-octet group. The UDF is transferred as a single octet. The cell payload is transferred in
groups of 1 to 4 octets. The UDF (HEC) is not generated by the MPC850SR; during the UDF
transfer, the UTPB signals will be driven with 0x00.

43.2.2.1 UTOPIA Bus and SOC Drive

The UTOPIA bus (through the UTPB signals) and SOC signal are driven by the MPC850SR
only when TxEnb is asserted during transmit transfers; UTPB and SOC are three-stated when
TxEnb is not asserted.
The transmit start-of-cell sequence is shown in Figure 43-4. Note that the TxCav signal is not
sampled during the cell transfer.
P45 P46 P47
Figure 43-3. UTOPIA Receiver End of Cell
Chapter 43. UTOPIA Interface
P48
UTOPIA Single-PHY