MOTOROLA
Chapter 7. PCI Bus Interface
7-3
The target interface uses the fastest device selection timing and can accept burst writes to
system memory of up to 32 bytes with no wait states. Burst reads from system memory are
also accepted with wait states inserted depending upon the timing of system memory
devices. The target interface disconnects when a transaction reaches the end of a cache line
(32 bytes).
7.2 PCI Bus Arbitration
The PCI arbitration approach is access-based. Bus masters must arbitrate for each access
performed on the bus. PCI uses a central arbitration scheme where each master has its own
unique request (REQ) output and grant (GNT) input signal. A simple request-grant
handshake is used to gain access to the bus. Arbitration for the bus occurs during the
previous access so that no PCI bus cycles are consumed due to arbitration (except when the
bus is idle).
The MPC106 does not function as the central PCI bus arbiter. It is the responsibility of the
system designer to provide for PCI bus arbitration. Regardless of the implementation, the
arbitration algorithm must be defined to establish a basis for a worst-case latency guarantee.
Latency guidelines are provided in the
PCI Local Bus Specification
. There are devices
available that integrate the central arbiter, interrupt controller, and PCI-to-ISA bridge
functions into a single device.
7.3 PCI Bus Protocol
This section provides a general description of the PCI bus protocol. Specific PCI bus
transactions are described in Section 7.4, “PCI Bus Transactions.” Refer to Figure 7-1,
Figure 7-2, Figure 7-3, and Figure 7-4 for examples of the transfer-control mechanisms
described in this section.
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 (frame), IRDY (initiator ready), and TRDY (target ready). 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 to force wait
cycles. A target negates TRDY 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