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MOTOROLA
Chapter 8. PCI Bus Interface
8-11
PCI Bus Protocol
to the other devices on the PCI bus that the target has decoded the address and claimed the
transaction. DEVSEL may be driven one, two, or three clock cycles (fast, medium, or slow
device select timing) following the address phase. Device select timing is encoded into the
devices PCI status register. If no agent asserts DEVSEL within three clock cycles of
FRAME, the agent responsible for subtractive decoding may claim the transaction by
asserting DEVSEL.
A target must assert DEVSEL (claim the transaction) before or coincident with any other
target response (assert its TRDY, STOP, or data signals). In all cases except target-abort,
once a target asserts DEVSEL, it must not negate DEVSEL until FRAME is negated (with
IRDY asserted) and the last data phase has completed. For normal termination, negation of
DEVSEL coincides with the negation of TRDY or STOP.
If the Trst access maps into a targets address range, that target asserts DEVSEL to claim
the access. But, if the initiator attempts to continue the burst access across the resource
boundary, then the target must issue a target disconnect.
The MPC8240 is hardwired for fast device select timing (PCI status register[10D9] = 0b00).
Therefore, when the MPC8240 is the target of a transaction (local memory access or
conTguration register access in agent mode), it asserts DEVSEL one clock cycle following
the address phase.
As an initiator, if the MPC8240 does not see the assertion of DEVSEL within four clock
cycles after the address phase (that is, Tve clock cycles after it asserts FRAME), it
terminates the transaction with a master-abort.
8.3.5 Byte Alignment
The byte enable (C/BE[3D0], during a data phase) signals are used to determine which byte
lanes carry meaningful data. The byte enable signals may enable different bytes for each of
the data phases. The byte enables are valid on the edge of the clock that starts each data
phase and stay valid for the entire data phase. Note that parity is calculated on all bytes
regardless of the byte enables. See Section 8.6.1, òPCI Parity,ó for more information.
If the MPC8240, as a target, sees no byte enables asserted, it completes the current data
phase with no permanent change. This implies that on a read transaction, the MPC8240
expects that the data is not changed, and on a write transaction, the data is not stored.
8.3.6 Bus Driving and Turnaround
A turnaround cycle is required, to avoid contention, on all signals that may be driven by
more than one agent. The turnaround cycle occurs at different times for different signals.
The IRDY, TRDY, DEVSEL, and STOP signals use the address phase as their turnaround
cycle. FRAME, C/BE[3D0], and AD[31D0] signals use the idle cycle between transactions
(when both FRAME and IRDY are negated) as their turnaround cycle. The PERR signal
has a turnaround cycle on the fourth clock after the last data phase.