External Bus Interface
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
13-33
•
RSV is low when the following is true:
— AT0 = 0 (Core access)
— AT2 = 1 (Data)
— AT3 = 0 (Reservation)
13.4.7.3.5
Burst Data in Progress (BDIP)
The master asserts BDIP to indicate to the slave that another data beat follows the current data beat.
13.4.8
Termination Signals
The following sections discuss the termination signals supported by the MPC885.
13.4.8.1
Transfer Acknowledge (TA)
TA indicates normal completion of the bus transfer. The slave asserts TA with every data beat returned or
accepted during a burst cycle.
13.4.8.2
Burst Inhibit (BI)
The slave asserts BI to indicate to the master that it cannot burst. If this signal is asserted, the master must
transfer in multiple cycles and increment the address for the slave to complete the burst transfer.
13.4.8.3
Transfer Error Acknowledge (TEA)
Terminates the bus cycle under a bus error condition for which the current cycle is aborted. TEA overrides
other cycle termination signals, such as TA.
Note that for burst transactions, TEA should be asserted externally only on the first or last beats. Assertion
of TEA on an intermediate beat may result in erratic operation, including lockup of the MPC885 requiring
hard reset.
13.4.8.4
Termination Signals Protocol
The transfer protocol was defined to avoid electrical contention on lines that can be driven by various
sources. To do that, a slave should not drive signals associated with the data transfer until the address phase
is completed and it recognizes the address as its own. The slave should disconnect from signals
immediately after it has acknowledged the cycle and no later than the termination of the next address phase
cycle. This indicates that termination signals should be connected to power through a pull-up resistor to
prevent a master from sampling undefined values in any of these signals when no real slave is addressed.
See
Figure 13-25
and
Figure 13-26
.
Содержание PowerQUICC MPC870
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