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MPC7400 RISC Microprocessor Users Manual
MPX Bus Protocol
9.6.1.2.1 Address Bus Driven Mode
In addition to selecting MPX bus mode at the negation of HRESET, the EMODE signal is
also used to select address bus driven mode after HRESET is negated. This mode provides
for improved electrical characteristics on the address and attributes signals by reducing the
time that these signals are not actively driven.
If EMODE is asserted after HRESET is negated, address bus driven mode is selected; if
EMODE is negated after HRESET is negated, normal address bus driving mode (address
bus not always driven) is selected. The read-only ABD bit in MSSCR0 indicates whether
the MPC7400 is in address bus driven mode. Note that address bus driven mode is only
available in MPX bus mode.
9.6.1.2.2 Address Bus Streaming
The 60x bus protocol forces a turn-around cycle on the bus between each address tenure
implying that 60x address tenures last at least three bus clock cycles (because the system
must provide AACK no earlier than the cycle following the assertions of TS).
In MPX bus mode, the MPC7400 can drive consecutive address tenures without a dead
cycle in between. So, two-cycle address tenures are possible if AACK is not delayed and
the same master receives a qualiTed bus grant to drive another address tenure. This is
referred to as address bus streaming.
9.6.1.2.3 Address Bus Parity
The MPC7400 address parity generation and reporting in MPX bus mode operates
identically to that in 60x bus mode.
9.6.1.2.4 Address Pipelining
Address pipelining in the MPX bus mode uses split transactions in the same way as the 60x
interface with the exception that data need not necessarily be returned in order. Generalized
data tenure reordering, beyond that allowed by the DBWO function, is possible in the MPX
interface. This generalized reordering protocol is described in Section 9.6.2.2.8, òData
Tenure Reordering in MPX Bus Only.ó
9.6.1.3 Transfer Attributes in MPX Bus Mode
The transfer attribute signals for the MPC7400 include the TT[0:4] and TSIZ[0:2] signals,
as well as TBST, CI, WT, and GBL. There is no change to the implementation of the GBL
signal in MPX bus mode. In MPX bus mode, the CI and WT signals function as inputs (as
well as outputs). They are used as inputs in determining illegal conditions for data-only
intervention where ARTRY must be asserted instead of HIT in response to a snooped
transaction.