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MPC7400 RISC Microprocessor Users Manual
MPX Bus Protocol
is stored and readable from the EMODE bit in MSSCR0. The state of MSSR0[EMODE] is
active high, meaning that if EMODE is detected as asserted at the negation of HRESET,
MSSR0[EMODE] = 1 and MPX bus mode is selected; otherwise, MSSR0[EMODE] = 0
and 60x bus mode is selected.
All signals for the MPX interface are speciTed with respect to the rising-edge of the
external system clock input (SYSCLK), and they are guaranteed to be sampled as inputs or
changed as outputs with respect to that edge.
Since the same clock edge is referenced for driving or sampling the bus signals, the
possibility of clock skew could exist between various modules in a system due to routing
or the use of multiple clock lines. It is the responsibility of the system to handle any such
clock skew problems that might occur.
The following sections provide a description of how the MPX interface operates and notes
or points to consider when designing a system with it.
In addition to the basic transfer protocol of the 60x bus interface, the MPX bus also supports
a data-only transfer
in order to support cache-to-cache transfers (or data intervention) and
a local bus slave. The HIT and DRDY signals are deTned in the MPX bus protocol to
support this type of transfer.
9.6.1 Address Tenure in MPX Bus Mode
The address tenure in MPX bus mode is very similar to that in 60x bus mode, except for
those differences outlined in the following subsections.
An address retry capability, similar to 60x bus mode, is provided to support the snoop
coherency protocol. In MPX bus mode, the MPC7400 additionally supports data
intervention for more efTcient coherency management. Address retry is used only when
HIT-style data intervention is not enabled or in those cases where HIT-style data
intervention is not allowed. An address retry response is issued by a snooping master in
order to interrupt another masters transaction on the bus, usually to write back that memory
which it has modiTed in its own cache. The address retry causes the original master to abort
the current transaction and rerun the transaction at a later time.
9.6.1.1 Address Arbitration Phase
Address bus arbitration in MPX mode differs from arbitration in the 60x interface in that
the MPC7400 can drive consecutive address tenures without a dead cycle on the address
bus in MPX bus mode if those address tenures are from the same processor. The MPC7400
does not use the ABB signal as an input (in 60x or MPX bus mode). When conTgured for
MPX bus mode, the MPC7400 does provide an indication when the address bus is busy on
the AMON output signal. Note that this signal is not a requirement of the MPX bus protocol
and may not be available on future products.