
7-4
MPC105 PCIB/MC User's Manual
MOTOROLA
All signals are sampled on the rising edge of the PCI bus clock (SYSCLK). Each signal has
a setup and hold aperture with respect to the rising clock edge, in which transitions are not
allowed. Outside this aperture, signal values or transitions have no significance.
7.3.1 Basic Transfer Control
The basic PCI bus transfer mechanism is a burst. A burst is composed of an address phase
followed by one or more data phases. Fundamentally, all PCI data transfers are controlled
by three signals—FRAME, IRDY, and TRDY. A master asserts FRAME to indicate the
beginning of a PCI bus transaction and negates FRAME to indicate the end of a PCI bus
transaction. A master negates IRDY (initiator ready) to force wait cycles. A target negates
TRDY (target ready) to force wait cycles.
The PCI bus is considered idle when both FRAME and IRDY are negated. The first clock
cycle in which FRAME is asserted indicates the beginning of the address phase. The
address and bus command code are transferred in that first cycle. The next cycle begins the
first of one or more data phases. Data is transferred between master and target in each cycle
that both IRDY and TRDY are asserted. Wait cycles may be inserted in a data phase by the
master (by negating IRDY) or by the target (by negating TRDY).
Once a master has asserted IRDY, it cannot change IRDY or FRAME until the current data
phase completes regardless of the state of TRDY. Once a target has asserted TRDY or
STOP, it cannot change DEVSEL, TRDY or STOP until the current data phase completes.
In simpler terms, once a master or target has committed to the data transfer, it cannot change
its mind.
When the master intends to complete only one more data transfer (which could be
immediately after the address phase), FRAME is negated and IRDY is asserted (or kept
asserted) indicating the master is ready. After the target indicates the final data transfer (by
asserting TRDY), the PCI bus may return to the idle state (both FRAME and IRDY are
negated) unless a fast back-to-back transaction is in progress. In the case of a fast back-to-
back transaction, an address phase immediately follows the last data phase.
7.3.2 PCI Bus Commands
A PCI bus command is encoded in the C/BE3–C/BE0 signals during the address phase of
a PCI transaction. The bus command indicates to the target the type of transaction the
master is requesting. Table 7-1 describes the PCI bus commands as implemented by the
MPC105.