PCI (Technical)

This section is currently based solely on the work by Mark Sokos.

This file is not intended to be a thorough coverage of the PCI standard. It is for informational purposes only, and is intended to give designers and hobbyists an overview of the bus so that they might be able to design their own PCI cards. Thus, I/O operations are explained in the most detail, while memory operations, which will usually not be dealt with by an I/O card, are only briefly explained. Hobbyists are also warned that, due to the higher clock speeds involved, PCI cards are more difficult to design than ISA cards or cards for other slower busses. Many companies are now making PCI prototyping cards, and, for those fortunate enough to have access to FPGA programmers, companies like Xilinx are offering PCI compliant designs which you can use as a starting point for your own projects.

For a copy of the full PCI standard, contact:

Signal Descriptions:

AD(x)

Address/Data Lines.

CLK

Clock. 33 MHz maximum.

C/BE(x)

Command, Byte Enable.

FRAME

Used to indicate whether the cycle is an address phase or a data phase.

DEVSEL

Device Select.

IDSEL

Initialization Device Select

INT(x)

Interrupt

IRDY

Initiator Ready

LOCK

Used to manage resource locks on the PCI bus.

REQ

Request. Requests a PCI transfer.

GNT

Grant. indicates that permission to use PCI is granted.

PAR

Parity. Used for AD0-31 and C/BE0-3.

PERR

Parity Error.

RST

Reset.

SBO

Snoop Backoff.

SDONE

Snoop Done.

SERR

System Error. Indicates an address parity error for special cycles or a system error.

STOP

Asserted by Target. Requests the master to stop the current transfer cycle.

TCK

Test Clock

TDI

Test Data Input

TDO

Test Data Output

TMS

Test Mode Select

TRDY

Target Ready

TRST

Test Logic Reset

The PCI bus treats all transfers as a burst operation. Each cycle begins with an address phase followed by one or more data phases. Data phases may repeat indefinitely, but are limited by a timer that defines the maximum amount of time that the PCI device may control the bus. This timer is set by the CPU as part of the configuration space. Each device has its own timer (see the Latency Timer in the configuration space).

The same lines are used for address and data. The command lines are also used for byte enable lines. This is done to reduce the overall number of pins on the PCI connector.

The Command lines (C/BE3 to C/BE0) indicate the type of bus transfer during the address phase.

The three basic types of transfers are I/O, Memory, and Configuration.

PCI timing diagrams:

            ___     ___     ___     ___     ___     ___
CLK     ___|   |___|   |___|   |___|   |___|   |___|   |___

        _______                                   _________
FRAME          |_________________________________|

                ______  _______  ______  ______  ______
AD      -------<______><_______><______><______><______>---
                Address  Data1    Data2   Data3   Data4

                ______  _______________________________
C/BE    -------<______><_______________________________>---
                Command   Byte Enable Signals

         ____________                                   ___
IRDY                 |_________________________________|

         _____________                                  ___
TRDY                  |________________________________|

         ______________                                 ___
DEVSEL                 |_______________________________|

PCI transfer cycle, 4 data phases, no wait states. Data is transferred on the rising edge of CLK.

                         [1]              [2]        [3]
            ___     ___     ___     ___     ___     ___     ___     ___
CLK     ___|   |___|   |___|   |___|   |___|   |___|   |___|   |___|   |__

        _______                                                  _________
FRAME          |________________________________________________|

                                   A               B               C
                ______           ______________  ______  _____________
AD      -------<______>---------<______________><______><_____________>---
                Address           Data1           Data2   Data3

                ______  ______________________________________________
C/BE    -------<______><______________________________________________>---
                Command   Byte Enable Signals

                                                         Wait
         ____________                                    _____         ___
IRDY                 |__________________________________|     |_______|

                        Wait            Wait
         ______________________         ______                         ___
TRDY                           |_______|      |_______________________|

         ______________                                                ___
DEVSEL                 |______________________________________________|

PCI transfer cycle, with wait states. Data is transferred on the rising edge of CLK at points labelled A, B, and C.

Bus Cycles:

Interrupt Acknowledge (0000)

The interrupt controller automatically recognizes and reacts to the INTA (interrupt acknowledge) command. In the data phase, it transfers the interrupt vector to the AD lines.

Special Cycle (0001)

I/O Read (0010) and I/O Write (0011)

Input/Output device read or write operation. The AD lines contain a byte address (AD0 and AD1 must be decoded). PCI I/O ports may be 8 or 16 bits. PCI allows 32 bits of address space. On IBM compatible machines, the Intel CPU is limited to 16 bits of I/O space, which is further limited by some ISA cards that may also be installed in the machine (many ISA cards only decode the lower 10 bits of address space, and thus mirror themselves throughout the 16 bit I/O space). This limit assumes that the machine supports ISA or EISA slots in addition to PCI slots.

The PCI configuration space may also be accessed through I/O ports 0x0CF8 (Address) and 0x0CFC (Data). The address port must be written first.

Memory Read (0110) and Memory Write (0111)

A read or write to the system memory space. The AD lines contain a doubleword address. AD0 and AD1 do not need to be decoded. The Byte Enable lines (C/BE) indicate which bytes are valid.

Configuration Read (1010) and Configuration Write (1011)

A read or write to the PCI device configuration space, which is 256 bytes in length. It is accessed in doubleword units. AD0 and AD1 contain 0, AD2-7 contain the doubleword address, AD8-10 are used for selecting the addressed unit a the malfunction unit, and the remaining AD lines are not used.

Address     Bit 32      16   15           0

00          Unit ID        | Manufacturer ID
04          Status         | Command
08          Class Code               | Revision
0C          BIST  | Header | Latency | CLS
10-24            Base Address Register
28          Reserved
2C          Reserved
30          Expansion ROM Base Address
34          Reserved
38          Reserved
3C          MaxLat|MnGNT   | INT-pin | INT-line
40-FF       available for PCI unit

Multiple Memory Read (1100)

This is an extension of the memory read bus cycle. It is used to read large blocks of memory without caching, which is beneficial for long sequential memory accesses.

Dual Address Cycle (1101)

Two address cycles are necessary when a 64 bit address is used, but only a 32 bit physical address exists. The least significant portion of the address is placed on the AD lines first, followed by the most significant 32 bits. The second address cycle also contains the command for the type of transfer (I/O, Memory, etc). The PCI bus supports a 64 bit I/O address space, although this is not available on Intel based PCs due to limitations of the CPU.

Memory-Read Line (1110)

This cycle is used to read in more than two 32 bit data blocks, typically up to the end of a cache line. It is more efficient than normal memory read bursts for a long series of sequential memory accesses.

Memory Write and Invalidate (1111)

This indicates that a minimum of one cache line is to be transferred. This allows main memory to be updated, saving a cache write-back cycle.

Bus Arbitration:

This section is under construction.

PCI BIOS:

This section is under construction.