
9-14
MPC7400 RISC Microprocessor Users Manual
60x Address Bus Tenure
Figure 9-5. Address Bus Arbitration Showing Bus Parking
9.3.1.3 Ignoring ABB
The 60x bus protocol allows for masters that do not implement the ABB signal as an input,
such as the MPC7400. The elimination of ABB from the MPC7400 interfaces removes
logic from critical timing paths in the processor interface, allowing higher frequency bus
operation. It does, however, put more responsibility on the system arbiter; the arbiter must
synthesize its own version of ABB and must not issue an address bus grant (BG) to a
processor when it would cause a collision on the address bus with an address tenure from
another processor.
The ABB signal may be ignored if ABB and TS are asserted simultaneously by all masters,
or where arbitration (through assertion of BG) is properly managed in cases where the
regenerated ABB may not properly track the ABB signal on the bus. If the MPC7400s ABB
signal is ignored by the system, it must be connected to a pull-up resistor to ensure proper
operation.
9.3.2 Address Transfer
During the address transfer, the physical address and all attributes of the transaction are
transferred from the bus master to the slave device(s). Snooping logic may monitor the
transfer to enforce cache coherency; see the description of bus snooping in Section 9.3.3,
òAddress Transfer Termination.ó
The MPC7400 supports a little-endian mode in which the low-order address bits are
operated on (or
munged
) based on the program-requested data transfer size. This munging
is performed internally before the address reaches the internal caches and bus units. When
little-endian mode is selected, the 60x bus interface
still operates in big-endian mode.
That
D1
0
1
need_bus
BG
ARTRY
qual BG
ABB
BR
SYSCLK
TS