
9-24
G2 PowerPC Core Reference Manual
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MOTOROLA
Data Bus Tenure
cacheable (and write-back for data store operations) in the respective page or block
descriptor to take advantage of burst transfers.
The core output core_tbst_out indicates to the system whether the current transaction is a
single- or four-beat transfer (except during
eciwx
/
ecowx
transactions, when it signals the
state of EAR[28]). A burst transfer has an assumed address order. For load or store
operations that miss in the cache (and are marked as cacheable and, for stores, write-back),
the G2 core uses the double-word-aligned address associated with the critical code or data
that initiated the transaction. This minimizes latency by allowing the critical code or data
to be forwarded to the processor before the rest of the cache line is filled. For all other burst
operations, however, the cache line is transferred beginning with the eight-word-aligned
data.
The G2 core does not directly support dynamic interfacing to subsystems with less than a
64-bit data path. It does, however, provide a static 32-bit data bus mode; for more
information, see Section 9.1.3, “Optional 32-Bit Data Bus Mode.”
9.4.4
Data Transfer Termination
Four signals are used to terminate data bus transactions—core_ta, core_drtry (data retry),
core-tea (transfer error acknowledge), and core_artry_in. The core_ta signal indicates
normal termination of data transactions. It must always be asserted on the bus cycle
coincident with the data that it is qualifying. It may be withheld by the slave for any number
of clocks until valid data is ready to be supplied or accepted. core_drtry indicates invalid
read data in the previous bus clock cycle. core_drtry extends the current data beat and does
not terminate it. If it is asserted after the last (or only) data beat, the core negates
core_dbb_out but still considers the data beat active and waits for another assertion of
core_ta. core_drtry is ignored on write operations. core_tea indicates a nonrecoverable bus
error event. Upon receiving a final (or only) termination condition, the core always negates
core_dbb_out for one cycle.
If core_drtry is asserted by the memory system to extend the last (or only) data beat past
the negation of core_dbb_out, the memory system should three-state the data bus on the
clock after the final assertion of core_ta, even though it will negate core_drtry on that clock.
This is to prevent a potential momentary data bus conflict if a write access begins on the
following cycle.
The core_tea signal is used to signal a nonrecoverable error during the data transaction. It
may be asserted on any cycle during core_dbb_out, or on the cycle after a qualified core_ta
during a read operation, except when no-core_drtry mode is selected (where no-core_drtry
mode cancels checking the cycle after core_ta). The assertion of core_tea terminates the
data tenure immediately, even if in the middle of a burst; however, it does not prevent
incorrect data that has just been acknowledged with core_ta from being written into the G2
core cache or GPRs. The assertion of core_tea initiates either a machine check exception or
a checkstop condition based on the setting of the MSR.
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