
MOTOROLA
Chapter 7. PCI Bus Interface
7-9
The terms ‘edge’ and ‘clock edge’ always refer to the rising edge of the clock. The terms
‘a(chǎn)sserted’ and ‘negated’ always refer to the globally visible state of the signal on the clock
edge, and not to signal transitions. ‘
’ represents a turnaround cycle in the timing
diagrams.
7.4.1 Read Transactions
Figure 7-1 illustrates a PCI single-beat read transaction. Figure 7-2 illustrates a PCI burst
read transaction. The transaction starts with the address phase, occurring when a master
asserts FRAME. During the address phase, AD[31–0] contain a valid address and
C/BE[3–0] contain a valid bus command.
The first data phase of a read transaction requires a turnaround cycle. This allows the
transition from the master driving AD[31–0] as address signals to the target driving AD[31–
0] as data signals. The turnaround cycle is enforced by the target using the TRDY signal.
The earliest the target can provide valid data is one cycle after the turnaround cycle. The
target must drive the address/data signals when DEVSEL is asserted.
During the data phase, the command/byte enable signals indicate which byte lanes are
involved in the current data phase. A data phase may consist of a data transfer and wait
cycles. The C/BE[3–0] signals remain actively driven for both reads and writes from the
first clock of the data phase through the end of the transaction.
A data phase completes when data is transferred, which occurs when both IRDY and TRDY
are asserted on the same clock edge. When either IRDY or TRDY is negated, a wait cycle
is inserted and no data is transferred. The master indicates the last data phase by negating
FRAME when IRDY is asserted. The transaction is considered complete when data is
transferred in the last data phase.
Figure 7-1. PCI Single-Beat Read Transaction
SYSCLK
AD[31–0]
C/BE[3–0]
FRAME
IRDY
DEVSEL
TRDY
ADDR
CMD
DATA
BYTE ENABLES