Chapter 7. PCI Bus Interface
7-9
PCI Bus Protocol
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). An
initiator asserts FRAME to indicate the beginning of a PCI bus transaction and negates
FRAME to indicate the end of a PCI bus transaction. An initiator 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 initiator and target in each
cycle that both IRDY and TRDY are asserted. Wait cycles may be inserted in a data phase
by the initiator (by negating IRDY) or by the target (by negating TRDY).
Once an initiator 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 an initiator or target has committed to the data transfer, it cannot
change its mind.
When the initiator 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 initiator 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 initiator is
requesting. Table 7-2 describes the PCI bus commands as implemented by the MPC8240.
Summary of Contents for MPC8240
Page 1: ...MPC8240UM D Rev 1 1 2001 MPC8240 Integrated Processor User s Manual ...
Page 38: ...xviii MPC8240 Integrated Processor User s Manual TABLES Table Number Title Page Number ...
Page 48: ...xlviii MPC8240 Integrated Processor User s Manual Acronyms and Abbreviations ...
Page 312: ...6 94 MPC8240 Integrated Processor User s Manual ROM Flash Interface Operation ...
Page 348: ...7 36 MPC8240 Integrated Processor User s Manual PCI Host and Agent Modes ...
Page 372: ...8 24 MPC8240 Integrated Processor User s Manual DMA Register Descriptions ...
Page 394: ...9 22 MPC8240 Integrated Processor User s Manual I2O Interface ...
Page 412: ...10 18 MPC8240 Integrated Processor User s Manual Programming Guidelines ...
Page 454: ...12 14 MPC8240 Integrated Processor User s Manual Internal Arbitration ...
Page 466: ...13 12 MPC8240 Integrated Processor User s Manual Exception Latencies ...
Page 516: ...16 14 Watchpoint Trigger Applications ...
Page 538: ...B 16 MPC8240 Integrated Processor User s Manual Setting the Endian Mode of Operation ...
Page 546: ...C 8 MPC8240 Integrated Processor User s Manual ...
Page 640: ...INDEX Index 16 MPC8240 Integrated Processor User s Manual ...