7-4
MPC106 PCIB/MC User's Manual
MOTOROLA
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/BE[3–0] 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 MPC106.
Table 7-1. PCI Bus Commands
C/BE[3–0]
PCI Bus Command
MPC106
Supports
as a Master
MPC106
Supports
as a Target
Definition
0000
Interrupt-
acknowledge
Yes
No
The interrupt-acknowledge command is a
read (implicitly addressing the system
interrupt controller). Only one device on the
PCI bus should respond to the interrupt-
acknowledge command. Other devices
ignore the interrupt-acknowledge
command. See Section 7.4.6.1, “Interrupt
Acknowledge Transactions,” for more
information.
0001
Special-cycle
Yes
No
The special-cycle command provides a
mechanism to broadcast select messages
to all devices on the PCI bus. See
Section 7.4.6.2, “Special-Cycle
Transactions,” for more information.
0010
I/O-read
Yes
No
The I/O-read command accesses agents
mapped into the PCI I/O space.
0011
I/O-write
Yes
No
The I/O-write command accesses agents
mapped into the PCI I/O space.
0100
Reserved
*
No
No
—
0101
Reserved
*
No
No
—