It is difficult to pick up a trade journal on computing design or embedded systems and not find a mention of the Peripheral Component Interconnect (PCI) bus. PCI is rapidly becoming the local bus of choice for new computer designs, and its industrywide acceptance promises high-volume and low-cost PCI peripherals at the right performance levels. The driving force for this growth is the need for bridge chips, which connect various CPUs to the bus and connect the bus to a variety of system-expansion buses.

PCI's high bandwidth of 132 Mbytes/sec for 32-bit transfers and 264 Mbytes/sec for 64-bit transfers allows the bus to support high-speed transactions for multimedia and embedded applications. Because the PCI bus is a mezzanine bus, it requires bridge chips to accommodate the protocols of various CPUs and host buses. The CPU-independent PCI bus supports both CISC and RISC µPs, and bridge chips are available for the Advanced Micro Devices Am29000; the Digital Equipment Corp Alpha; the Intel Pentium, i960, and i486; the Mips 4000; and the Motorola PowerPC µPs. Chips are also available for bridging the PCI bus to the ISA, EISA, VME, VL, and PCMCIA buses. In addition, PCI-to-PCI bridge chips are available to expand the capabilities of the PCI bus.

A group of more than 300 companies, the PCI Special Interest Group, controls the PCI bus, now an open specification in its second revision. The revision specifies that the bus operates at a maximum 33 MHz rate and has a 32-bit address/data path. The specification also allows for multiple bus masters that arbitrate for control of the PCI bus. Bridge chips must match the byte lanes of the PCI bus to the byte lanes of the host or secondary bus to which the PCI bus connects.

The primary function of a bridge chip is to map the address space of one bus into the address space of another bus, so that every bus master in a system sees the same address map. PCI provides three independent physical-address spaces: memory, I/O, and configuration. Configuration space is unique to PCI. A host bridge must support the minimum set of configuration-space registers, which allow for plug-and-play operation.

The bridge must match byte-ordering of its primary and secondary buses, which may involve byte-swapping between byte lanes. Some µPs may expect data on a different byte lane from the natural byte lane for that address for certain byte or word sizes. If so, the bridge chip must move data to the proper byte lane for that bus. To perform byte-swapping, the bridge must know the transfer size, which is not encoded in the address lines; instead, the bridge determines the transfer size from the byte-enable lines.

The preferred method of decoupling the PCI bus from the host or secondary bus is through the use of bidirectional FIFO buffers. Because the primary means of data transfer on the PCI bus is a burst, FIFO buffers let each bus keep pace with asynchronous data transfers. A bridge can include two types of buffers: write posting and read prefetch. Either bus can implement either type of buffer.

Write-posting buffers accept write data from one bus and acknowledge reception to that bus, freeing the bus to perform other transactions. The bridge temporarily stores or posts the write data until it can be written to the other bus. Read-prefetch buffers take the address from a read access and then read additional data. The bridge then holds that data in the buffer until a read access uses it, or until it is unusable.

The bridge chips must also handle system interrupts. When a system interrupt occurs, one device in the system can inform another device of an event that has just happened. By inserting wait states, the bridge chip intercepts and delays system interrupts while the system is flushing the buffers. The chips handle accesses across the bridge by maintaining the order of writes in both directions.

The bridge chips in Table 1 use these techniques to connect a host or secondary bus to the PCI bus. Some chip sets, such as those from Intel, Opti, and Mentor Arc, contain main-memory and cache controllers for the appropriate µPs. These bridge chips, for desktop and laptop applications, include a separate chip that connects the PCI bus to a secondary ISA or EISA bus.

Bridge chips for the embedded market, such as those from PLX 22Technology and SIS Microelectronics, connect the Intel i960 and AMD 29000 RISC µPs, respectively, to the PCI bus. The PCI bus is finding its way into a variety of embedded applications, including printers, adapter boards, and backplanes for hubs and routers.

Many embedded applications do not contain a BIOS, so the µP must configure the system during power-up. For example, Cyclone Microsystems' i960-based intelligent I/O processor card uses the PLX Technology PCI 9060 bridge chip to connect to an on-card PCI mezzanine bus.

For industrial-control applications, Newbridge Microsystems offers the Universe chip, which bridges the VMEbus to the PCI bus. In 64-bit mode, the PCI bus transfers data more than three times faster than the theoretical limit of the VMEbus. Universe lets the VME industry take advantage of the new spectrum of high-end, low-cost PCI chips and for PCI vendors to take advantage of the large and diverse market for embedded-control applications.

The PCI-to-PCI bridge offerings from DEC and IBM extend the capabilities of the PCI bus, and DEC even includes an integrated PCI interface directly on the DECchip 21066 Alpha AXP µP. The PCI-to-PCI bridge chips expand the PCI bus limits beyond its maximum specified load limit of 10. Each connector on the bus represents two to three loads, so the specification recommends only three connectors per bus. Placing a PCI-to-PCI bridge chip in one of the slots lets you add slots to the base system.

However, according to Sassan Teymouri, engineering director at Adaptec, PCI-to-PCI bridge chips can cause problems with the BIOS. On power-up, the BIOS recognizes a bridge chip only in the primary slot of the PCI bus but not devices in the secondary slots. The BIOS companies are aware of this problem and are working to correct it. [EDN]

Table 1 -- PCI-bridge chips

Vendor	Part no.	Bridge type	No. of chips	Price	Features
Chips & Technologies Inc	F84049 PCI enabler	PCI to VL-bus	One	$10	Converts standard VL-bus signals to PCI bus signals; write buffers boost performance and make flexible timing parameters; supports as many as four PCI bus masters
Digital Equipment Corp	21050	PCI to PCI	One	$28.70 (5000)	One interface connects to the primary PCI bus, and the other connects to a secondary PCI bus; allows concurrent transactions on both interfaces at 132 Mbytes/sec; programmable rotating-arbitration function supports six secondary bus masters; provides read prefetching for memory-read transactions; provides as many as 8 words of write posting for memory-write transactions
IBM Microelectronics	IBM27-82351	PCI-to-PCI bridge	One	$21.50 (1000)	Attaches to the primary PCI bus through one of its two interfaces; second interface provides connection to a secondary PCI bus, which allows multiple expansion slots; operates at 33 MHz and has a 32-bit address and data bus; 160-pin PQFP
Intel Corp	82430NX (Neptune)	Pentium to PCI and PCI to ISA or EISA bus	Two or three	ISA version, $80.30; EISA version, $107.25 (10,000)	Contains an integrated PCI, cache, and main-memory bus controller, integrates SRAM for address tagging; handles 512 kbytes of burst-mode cache SRAM; memory controller handles 2 to 512 Mbytes of main memory; single-chip bridge to ISA bus, two-chip bridge to EISA bus
	82430FX (Triton)	Pentium to PCI and PCI to ISA bus	Four	$41.95 (10,000)	Comprises the Triton system component, two identical Triton data-path units, and the PCI ISA/IDE accelerator; provides an alternative to native signal-processing DSP, which uses 100-Mbyte/sec PCI-to-DRAM data streaming; third-generation chip set aimed at 75, 90, and 100 MHz; contains a PCI busmaster IDE controller; handles 4 to 128 Mbytes of main memory; supports 256 or 512 kbytes of second-level cache
Mentor Arc Inc	Winset-PCI- Pentium	Pentium to PCI and PCI to ISA bus	Three or four	$75 (sampling)	Comprises the 86C257 cache-memory and PCI-control unit, the 86C226 PCI-to-ISA bridge unit, and the 86C229 or 86C239 data-path unit; controls as much as 256 Mbytes of main memory; PCI arbiter handles three PCI masters; power management complies with APM specification for energy (green) PCs; supports 64-kbyte to 1-Mbyte caches
	Winset-PCI-R4x00	Mips R4x00 µPs	Three	$75 (sampling)	Comprises the 64C257 cache-memory, and PCI-control unit; the 64C226 PCI-to- ISA bridge unit; and the 64C229 or 64C239 data-path unit. Write-back cache controller supports 1 Mbyte of cache; memory controller supports 256 Mbytes of main memory; power management complies with the APM specification
Motorola Inc	MPC105	PowerPC to PCI	One	$62 (1000)	µP interface is a 32-bit address bus and configurable 64- or 32-bit data bus; write-through or -back cache controller supports as much as 4 Gbytes of cache memory; memory controller supports 256 Mbytes of RAM and 16 Mbytes of ROM; supports nap, doze, and sleep power-management modes; PCI interface acts as a master and slave device; PCI interface operates from 20 to 33
Newbridge Microsystems	Universe	PCI to VMEbus	One	$160 (1000)	FIFO buffer decouples the VME bus from the PCI bus to handle block transfers; handles little- and big-endian translation in software; FIFO buffer decouples the interrupter from the interrupt handler
Oki Semiconductor	PCI Megacell	i486-like bus to PCI	One	$10,000	Contains a 32-bit PCI and 32-bit i486-like interface; two 4x32-bit address-, data-, and byte-enable-write FIFO buffers; supports master and slave modes; handles burst write-mode operations; PCI bus interface performs byte swapping; programmable address registers respond to accesses on its primary bus if the address falls into the range of those registers
Opti Inc	Viper	Pentium to PCI bus and PCI bus to ISA bus	Three	Desktop version, $30; notebook version, $40 (10,000)	Comprises the 82C556 data-buffer controller, the 82C5557 system controller, and the 82C558N integrated peripherals controller; versions for desktop and notebook computers; notebook version includes power management; works with Pentium-class µPs from AMD (K5) and Cyrix (M1); memory controller handles 256 kbytes to 16 Mbytes of page-mode DRAM; operates from 3 and 5V Pentiums; employs level-1 cache in write-back mode
PLX Technology Inc	PCI 9000 series	i960 to PCI	One (four family members)	9060, $46; 9036, 9060ES, 9060SD, $25	9060 has two DMA controllers with bi-directional FIFO buffers for each channel; FIFO buffers enable high burst rate on the local and PCI buses; eight 32-bit mailbox registers and two 32-bit doorbell registers; generates interrupts from several sources; in direct master mode, the device can generate type 0 and 1 PCI configuration cycles; other devices in family have lower feature set and lower cost
SIS Microelectronics Inc	SPCIB	29K family of RISC µPs to PCI	One	$25 (10,000)	Internal 256-byte dual-port RAM (FIFO) buffers direct DMA and cross-bus transfers; host and PCI bus have separate clocks and operate independently; atomic transfer occurs because the device establishes simultaneous ownership of both buses during operation. Because FIFO is a dual-port RAM, filling and emptying of data can occur simultaneously
Texas Instruments Inc	PCI1050	PCMCIA slot to PCI	One	$12 to $15	Interfaces two PC Card (PCMCIA) slots to the PCI bus; internal signal buffers allow hot insertion and removal of PC Cards; core logic and PCI interface operate from 5V, and the card interfaces can handle any combination of 3.3 and 5V PC Cards; register-compatible with the Intel 82365SL-DF ExCA controller; can be cascaded to handle as many as eight PC Cards
Toshiba America Electronic Components Inc	Mako PCI chip set	Mips R4x00 series of RISC µPs to PCI	Three	$75 (1000)	Comprises an address-control chip and two identical data-path control chips; asynchronous PCI bus operation as fast as 67 MHz; PCI-to-ISA bus controls four PCI bus masters; handles one to eight banks of DRAM with a maximum of 256 Mbytes; controls as much as 1 Mbyte of second-level cache; the µP and cache operate at 3.3V, and the main memory and PCI interfaces operate at 5V