The PCI Bus is a high performance bus for interconnecting chips, expansion boards, and processor/memory subsystems.
124 pin (98+22) PCI 5 volt EDGE connector at the motherboard
PCI Universal Card 32/64 bit ---------------------------------------------------------------- | PCI Component Side (side B) | | | | | | optional | | ____ mandatory 32-bit pins 64-bit pins _____| |___| |||||||--|||||||||||||||||--|||||||--|||||||||||||| ^ ^ ^ ^ ^ ^ ^ ^ b01 b11 b14 b49 b52 b62 b63 b94
PCI 5V Card 32/64 bit | optional | | ____ mandatory 32-bit pins 64-bit pins _____| |___| ||||||||||||||||||||||||||--|||||||--||||||||||||||
PCI 3.3V Card 32/64 bit | optional | | ____ mandatory 32-bit pins 64-bit pins _____| |___| |||||||--||||||||||||||||||||||||||--||||||||||||||
The PCI specification defines two types of connectors that may be implemented at the system board level: One for systems that implement 5 Volt signaling levels, and one for systems that implement 3.3 Volt signaling levels. In addition, PCI systems may implement either the 32-bit or 64-bit connector. Most PCI buses implement only the 32-bit portion of the connector which consists of pins 1 through 62. Advanced systems which support 64-bit data transfers implement the full PCI bus connector which consists of pins 1 through 94. Three types of add-in boards may be implemented: "5 Volt add-in boards" include a key notch in pin positions 50 and 51 to allow them to be plugged only into 5 Volt system connectors. "3.3 Volt add-in boards" include a key notch in pin positions 12 and 13 to allow them to be plugged only into 3.3 Volt system connectors. "Universal add-in boards" include both key notches to allow them to be plugged into either 5 Volt or 3.3 Volt system connectors.Universal PCI Bus Pinouts
Rear of Computer :------:------: -12V |- B1 A1 -| Test Reset Test Clock |- B2 A2 -| +12V Ground |- B3 A3 -| Test Mode Select Test Data Output |- B4 A4 -| Test Data Input +5V |- B5 A5 -| +5V +5V |- B6 A6 -| Interrupt A Interrupt B |- B7 A7 -| Interrupt C Interrupt D |- B8 A8 -| +5V PRSNT1# |- B9 A9 -| Reserved Reserved |- B10 A10 -| +V I/O PRSNT2# |- B11 A11 -| Reserved :------:------: :------:------: Reserved |- B14 A14 -| Reserved Ground |- B15 A15 -| Reset Clock |- B16 A16 -| +V I/O Ground |- B17 A17 -| Grant Request |- B18 A18 -| Ground +V I/O |- B19 A19 -| Reserved Address 31 |- B20 A20 -| Address 30 Address 29 |- B21 A21 -| +3.3V Ground |- B22 A22 -| Address 28 Address 27 |- B23 A23 -| Address 26 Address 25 |- B24 A24 -| Ground +3.3V |- B25 A25 -| Address 24 C/BE 3 |- B26 A26 -| Init Device Select Address 23 |- B27 A27 -| +3.3V Ground |- B28 A28 -| Address 22 Address 21 |- B29 A29 -| Address 20 Address 19 |- B30 A30 -| Ground +3.3V |- B31 A31 -| Address 18 Address 17 |- B32 A32 -| Address 16 C/BE 2 |- B33 A33 -| +3.3V Ground |- B34 A34 -| Cycle Frame Initiator Ready |- B35 A35 -| Ground +3.3V |- B36 A36 -| Target Ready Device Select |- B37 A37 -| Ground Ground |- B38 A38 -| Stop Lock |- B39 A39 -| +3.3V Parity Error |- B40 A40 -| Snoop Done +3.3V |- B41 A41 -| Snoop Backoff System Error |- B42 A42 -| Ground +3.3V |- B43 A43 -| PAR C/BE 1 |- B44 A44 -| Address 15 Address 14 |- B45 A45 -| +3.3V Ground |- B46 A46 -| Address 13 Address 12 |- B47 A47 -| Address 11 Address 10 |- B48 A48 -| Ground Ground |- B49 A49 -| Address 9 :------:------: :------:------: Address 8 |- B52 A52 -| C/BE 0 Address 7 |- B53 A53 -| +3.3V +3.3V |- B54 A54 -| Address 6 Address 5 |- B55 A55 -| Address 4 Address 3 |- B56 A56 -| Ground Ground |- B57 A57 -| Address 2 Address 1 |- B58 A58 -| Address 0 +5 I/O |- B59 A59 -| +V I/O Acknowledge 64-bit |- B60 A60 -| Request 64-bit +5V |- B61 A61 -| +5V +5V |- B62 A62 -| +5V :------:------: :------:------: Reserved |- B63 A63 -| Ground Ground |- B64 A64 -| C/BE 7 C/BE 6 |- B65 A65 -| C/BE 5 C/BE 4 |- B66 A66 -| +V I/O Ground |- B67 A67 -| Parity 64-bit Address 63 |- B68 A68 -| Address 62 Address 61|- B69 A69 -| Ground +V I/O |- B70 A70 -| Address 60 Address 59 |- B71 A71 -| Address 58 Address 57 |- B72 A72 -| Ground Ground |- B73 A73 -| Address 56 Address 55 |- B74 A74 -| Address 54 Address 53 |- B75 A75 -| +V I/O Ground |- B76 A76 -| Address 52 Address 51 |- B77 A77 -| Address 50 Address 49 |- B78 A78 -| Ground +V I/O |- B79 A79 -| Address 48 Address 47 |- B80 A80 -| Address 46 Address 45 |- B81 A81 -| Ground Ground |- B82 A82 -| Address 44 Address 43 |- B83 A83 -| Address 42 Address 41 |- B84 A84 -| +V I/O Ground |- B85 A85 -| Address 40 Address 39 |- B86 A86 -| Address 38 Address 37 |- B87 A87 -| Ground +V I/O |- B88 A88 -| Address 36 Address 35 |- B89 A89 -| Address 34 Address 33 |- B90 A90 -| Ground Ground |- B91 A91 -| Address 32 Reserved |- B92 A92 -| Reserved Reserved |- B93 A93 -| Ground Ground |- B94 A94 -| Reserved :------:------:Same with descriptions:
Notes: Pin 63-94 exists only on 64 bit PCI implementations.
+V I/O is 3.3V on 3.3V boards, 5V on 5V boards, and define signal rails on the Universal board.
PCI is a synchronous bus architecture with all data transfers being performed relative to a system clock (CLK). The initial PCI specification permitted a maximum clock rate of 33 MHz allowing one bus transfer to be performed every 30 nanoseconds. Later, Revision 2.1 of the PCI specification extended the bus definition to support operation at 66 MHz, but the vast majority of today"s personal computers continue to implement a PCI bus that runs at a maximum speed of 33 MHz.
PCI implements a 32-bit multiplexed Address and Data bus (AD[31:0]). It architects a means of supporting a 64-bit data bus through a longer connector slot, but most of today"s personal computers support only 32-bit data transfers through the base 32-bit PCI connector. At 33 MHz, a 32-bit slot supports a maximum data transfer rate of 132 MBytes/sec, and a 64-bit slot supports 264 MBytes/sec.
The multiplexed Address and Data bus allows a reduced pin count on the PCI connector that enables lower cost and smaller package size for PCI components. Typical 32-bit PCI add-in boards use only about 50 signals pins on the PCI connector of which 32 are the multiplexed Address and Data bus. PCI bus cycles are initiated by driving an address onto the AD[31:0] signals during the first clock edge called the address phase. The address phase is signaled by the activation of the FRAME# signal. The next clock edge begins the first of one or more data phases in which data is transferred over the AD[31:0] signals.
In PCI terminology, data is transferred between an initiator which is the bus master, and a target which is the bus slave. The initiator drives the C/BE[3:0]# signals during the address phase to signal the type of transfer (memory read, memory write, I/O read, I/O write, etc.). During data phases the C/BE[3:0]# signals serve as byte enable to indicate which data bytes are valid. Both the initiator and target may insert wait states into the data transfer by deasserting the IRDY# and TRDY# signals. Valid data transfers occur on each clock edge in which both IRDY# and TRDY# are asserted.
A PCI bus transfer consists of one address phase and any number of data phases. I/O operations that access registers within PCI targets typically have only a single data phase. Memory transfers that move blocks of data consist of multiple data phases that read or write multiple consecutive memory locations. Both the initiator and target may terminate a bus transfer sequence at any time. The initiator signals completion of the bus transfer by deasserting the FRAME# signal during the last data phase. A target may terminate a bus transfer by asserting the STOP# signal. When the initiator detects an active STOP# signal, it must terminate the current bus transfer and re-arbitrate for the bus before continuing. If STOP# is asserted without any data phases completing, the target has issued a retry. If STOP# is asserted after one or more data phases have successfully completed, the target has issued a disconnect.
Initiators arbitrate for ownership of the bus by asserting a REQ# signal to a central arbiter. The arbiter grants ownership of the bus by asserting the GNT# signal. REQ# and GNT# are unique on a per slot basis allowing the arbiter to implement a bus fairness algorithm. Arbitration in PCI is "hidden" in the sense that it does not consume clock cycles. The current initiator"s bus transfers are overlapped with the arbitration process that determines the next owner of the bus.
PCI supports a rigorous auto configuration mechanism. Each PCI device includes a set of configuration registers that allow identification of the type of device (SCSI, video, Ethernet, etc.) and the company that produced it. Other registers allow configuration of the device"s I/O addresses, memory addresses, interrupt levels, etc.
Although it is not widely implemented, PCI supports 64-bit addressing. Unlike the 64-bit data bus option which requires a longer connector with an additional 32-bits of data signals, 64-bit addressing can be supported through the base 32-bit connector. Dual Address Cycles are issued in which the low order 32-bits of the address are driven onto the AD[31:0] signals during the first address phase, and the high order 32-bits of the address (if non-zero) are driven onto the AD[31:0] signals during a second address phase. The remainder of the transfer continues like a normal bus transfer.
PCI defines support for both 5 Volt and 3.3 Volt signaling levels. The PCI connector defines pin locations for both the 5 Volt and 3.3 Volt levels. However, most early PCI systems were 5 Volt only, and did not provide active power on the 3.3 Volt connector pins. Over time more use of the 3.3 Volt interface is expected, but add-in boards which must work in older legacy systems are restricted to using only the 5 Volt supply. A "keying" scheme is implemented in the PCI connectors to prevent inserting an add-in board into a system with incompatible supply voltage.
Although used most extensively in PC compatible systems, the PCI bus architecture is processor independent. PCI signal definitions are generic allowing the bus to be used in systems based on other processor families.
PCI includes strict specifications to ensure the signal quality required for operation at 33 and 66 MHz. Components and add-in boards must include unique bus drivers that are specifically designed for use in a PCI bus environment. Typical TTL devices used in previous bus implementations such as ISA and EISA are not compliant with the requirements of PCI. This restriction along with the high bus speed dictates that most PCI devices are implemented as custom ASICs.
The higher speed of PCI limits the number of expansion slots on a single bus to no more than 3 or 4, as compared to 6 or 7 for earlier bus architectures. To permit expansion buses with more than 3 or 4 slots, the PCI SIG has defined a PCI-to-PCI Bridge mechanism. PCI-to-PCI Bridges are ASICs that electrically isolate two PCI buses while allowing bus transfers to be forwarded from one bus to another. Each bridge device has a "primary" PCI bus and a "secondary" PCI bus. Multiple bridge devices may be cascaded to create a system with many PCI buses.
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.
Clock. 33 MHz maximum.
Command, Byte Enable.
Used to indicate whether the cycle is an address phase or a data phase.
Initialization Device Select
Used to manage resource locks on the PCI bus.
Request. Requests a PCI transfer.
Grant. indicates that permission to use PCI is granted.
Parity. Used for AD0-31 and C/BE0-3.
System Error. Indicates an address parity error for special cycles or a system error.
Asserted by Target. Requests the master to stop the current transfer cycle.
Test Data Input
Test Data Output
Test Mode Select
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.
   ___ ___ ___ ___ ___ ___ ___ ___ 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.
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.Sources: "Inside the PCI Local Bus" by Guy W. Kendall, Byte, February 1994 v 19 p. 177-180
Sources: "The Indispensible PC Hardware Book" by Hans-Peter Messmer, ISBN 0-201-8769-3
For a copy of the full PCI standard, contact:PCI Special Interest Group (SIG)
PO Box 14070
Portland, OR 97214