Designing a PC Card is difficult. The PCMCIA bus is fairly complex, and the PCMCIA specification keeps evolving, so your design will probably have to work on a variety of incompatible systems. To complicate matters, you have to cram a lot of memory or peripheral functions into a package the size of a thick credit card. In addition, your card must be software configurable; switches and jumpers are verboten.

Designing a PC Card can be relatively easy, however. ICs with PCMCIA interfaces now implement all the necessary control, addressing, and data-movement functions between your card's circuitry and the PCMCIA bus. In addition, manufacturers of these ICs often provide PCMCIA reference designs that give you all the details of a working PC Card. You can use a reference design as is or customize it, sometimes saving months of design effort.

Chips available for PC Cards include general-purpose PCMCIA interface ICs and dedicated-function ICs (Table 1). General-purpose chips, available from Zilog, National Semiconductor, Oki Semiconductor, and Germany's Dr Neuhaus GmbH, link the PCMCIA bus to on-card memory or peripheral functions. The dedicated-function ICs provide the same PCMCIA interface in chips that perform specific functions. These ICs include flash-memory controllers, fax/modems, LAN controllers, disk controllers, and SCSI controllers.

Available reference designs have varying levels of detail, ranging from application notes to literally everything you need to manufacture a card. They're available for PC Cards implementing flash-memory storage, Ethernet controllers, modems, SCSI connections, and small disk drives. Prices for reference designs vary widely, depending on level of detail and your manufacturing plans. You may have to pay a flat fee, royalties, or a combination.

If you choose not to use a reference design, you'll need to have a good understanding of the PCMCIA bus and the overall PCMCIA specification (Ref 1). This understanding is also helpful, of course, even if you do use a reference design. As a first step, you should learn the basics of the interface to the most basic PC Card, a memory card. This interface is a subset of the interface to the other type of PC Card-an I/O card-and must be part of either type of card. To the memory-card interface, the I/O interface adds I/O read and write strobes, an interrupt request, and a status-change indication.

A memory PC Card can contain a maximum of 64 Mbytes of memory, accessed via 26 address lines. (Existing PCMCIA socket-controller ICs in host computers allow only a 64-kbyte address space, however.) Memory can be of mixed types (flash and SRAM, for example), and a PC Card must allow both 8- and 16-bit memory accesses. A memory card may also need to provide write protection, possibly only to selected areas of memory. Guarding against unwanted write operations is a responsibility of the memory card, not the host system.

In addition to common (working) memory, a memory PC Card-or any PC Card-must contain a small amount of attribute memory. Attribute memory stores a Card Information Structure (CIS), which describes the card's characteristics and system requirements to host-system software (in modules called Card Services and Socket Services). A CIS can range in size from perhaps a dozen bytes to as much as 1 kbyte.

Attribute memory can be masked ROM, EEPROM, or RAM loaded from serial EEPROM. General-purpose PCMCIA interface ICs (Figs 1 and 2) contain on-chip RAM (typically 256 bytes, which is adequate for most applications) and a port to serial EEPROM. Although the PCMCIA spec allows parts of a CIS to be in common memory, using volatile common memory for this purpose involves some risk. If the memory becomes corrupted, a host system can misidentify a card and possibly damage it. Using EEPROM provides the additional capability of altering the CIS if necessary.

I/O PC Cards can have three address spaces instead of a memory card's two, adding I/O space to common-memory space and attribute-memory space. Like a memory card, an I/O card contains attribute memory for a CIS, and each I/O card must duplicate a memory card's memory interface to allow attribute-memory access. However, because most I/O cards have no need for common memory, a typical I/O card has only an I/O address space and an attribute-memory address space.

On power-up or reset, an I/O card must appear to its host system as a memory card. (This requirement enables early PCMCIA systems, designed only for memory-card use, to identify and gracefully reject I/O cards.) However, the card's CIS-loaded from serial EEPROM into a PCMCIA interface chip's attribute memory-identifies the card to the host as an I/O card. After the host reads the CIS and identifies the card as an I/O type, Card Services software reconfigures the card for I/O mode. Part of the reconfiguration from memory card to I/O card involves redefining certain card pins to account for differences between memory and I/O functions.

Configuration of a PC Card occurs by setting bits and fields in function configuration registers (FCRs). Information in the 8-bit FCRs specifies, for example, an I/O or a memory card, the card's I/O address, 8- or 16-bit interface, and level or pulse-mode interrupts. A very basic I/O card may require only one FCR; cards with additional features require more. PCMCIA ICs typically have four FCRs, a number adequate for most applications. Access to the FCRs is through attribute-memory space.

FCRs appear in the latest release of the PCMCIA spec in place of earlier releases' card configuration registers (CCRs). Because earlier releases provided for only one function per card, one set of CCRs was adequate. However, the new release provides for multiple functions per card, and thus requires multiple sets of configuration registers with a new name.

In keeping with the predominance of single-function PC Cards, most PCMCIA interface ICs have one set of FCRs, allowing configuration of one on-card function. National Semiconductor's PCM16C00, the only multifunction PCMCIA interface IC, provides two FCR sets and allows two on-card functions. The chip likewise contains additional required multifunction features, such as the ability to use only one interrupt line for the interrupts from two devices.

Each I/O PC Card must be able to generate either pulsed or level-mode interrupts. The PCMCIA spec allows both, and the host system specifies which type it expects to see. Fortunately, ICs with a PCMCIA interface provide the type of interrupt the host requested.

In addition to sharing an interrupt line, multiple functions on an I/O card require other multifunction capabilities. For example, each function must have separate attribute memory (for an individual CIS) and separate FCRs. Although these features are present in National's PCMCIA interface IC, hardware features are only part of multifunction requirements. Host-resident Card Services software must determine, for example, which function is interrupting and then service that particular interrupt.

Unfortunately, the large installed base of computers with PCMCIA slots does not make use of multifunction-capable Card Services and Socket Services software. As a result, you'll likely have to provide special software to work around this lack if you're designing a multifunction card. Similarly, direct-memory access (DMA) is new to the PCMCIA spec, so if you include DMA in a design, you may also want to include programmed I/O for compatibility with most existing systems.

In general, compatibility will be your greatest concern in designing a PC Card. You'll have to plan for systems that don't adhere to the latest release of the PCMCIA spec (or perhaps to any release), can't provide enough power for your card, have addressing limitations, and more (see box, "Avoiding the "gotchas" of PC Card design").

To avoid many compatibility problems and other technical snags, you can turn to IC manufacturers' PC Card reference designs for help. These tried-and-tested designs give you the benefit of other designers' experience and can sometimes save you months of effort. Reference designs are available for PC Cards implementing flash-memory storage, Ethernet controllers, modems, SCSI connections, and small disk drives.

What you get in a reference design varies greatly from vendor to vendor. At the high end, you get everything you need to manufacture a PC Card. Zilog's reference design for a PCMCIA-to-SCSI adapter card, for example, includes all production files and documentation: Gerber files, netlist, schematic, bill of materials, and object-code software drivers. At the other extreme, a reference design may be more like an evaluation kit.

If you're not ready to commit to an up-front licensing fee for a reference design, evaluation kits offer a compromise approach to speeding the design process. These kits lack manufacturing data, but they contain all the hardware and software for you to become familiar with a reference design. Zilog's Z8601700ZCO PCMCIA-to-SCSI kit, for example, contains a PCMCIA Type II card, all necessary ICs, adapter cable, and software documentation. The kit sells for $220.

To design an Ethernet PC Card, you can use a reference design from National Semiconductor, Standard Microsystems Corp (SMC), or perhaps other companies. National's design uses the NE2000 (St Nic) chip; SMC's design uses the SMC91C92 or SMC91C94. National sells a design license for less than $1000; SMC, however, did not provide pricing information for inclusion in this article.

National's <$1000, one-time price for a reference design is an exception to the usual royalty and/or fee plan. National's kit includes all the hardware for one Ethernet PC Card in addition to software and everything you need to produce the card, such as layout information. All you have to do is determine the network certification you want (Novell, for example), conduct whatever testing is necessary (FCC tests, for example), and possibly customize the software to add value to your product.

For disk-emulating flash-memory PC Cards, you can turn to Cirrus Logic for either a reference design or OEM cards produced from the design. The design is based on Cirrus's new CL-FC1120 PCMCIA flash-memory controller. It yields Type I PC Cards with capacities from 4 to 8 Mbytes or Type II cards with 16 or 20 Mbytes. The controller enables emulation of an ATA disk drive and includes special features, such as wear leveling, to account for flash memory's unique characteristics.

If you don't expect to need enormous quantities of flash-disk cards, buying Cirrus's OEM cards may make more sense than licensing a reference design. The reference design, Cirrus says, is more appropriate if you can provide your own high-volume, low-cost manufacturing. A 4-Mbyte OEM card from Cirrus costs $140 (1000); licensing and royalty fees for reference designs vary with each situation but usually are geared toward large-volume chip purchases. Cirrus also provides reference designs for fax/modem cards and hard-disk cards.

Reference designs aren't the only help available from IC manufacturers. Short of reference designs, there are development kits, application notes, and, of course, helpful application engineers. So, if you haven't yet discovered a reference design that ideally suits your needs, don't despair. Look for an IC that meets your needs, and then learn what help its manufacturer can provide.

Other aspects of PC Card design you should consider include programmable logic, software, components, and packaging. Programmable logic can be an alternative to PCMCIA interface chips, especially with appropriate application notes from PLD and FPGA manufacturers. PCMCIA system software is complicated and bewildering, but, fortunately, reference designs provide some guidance through the maze. Components and packaging are a special issue in PC Card design, because of the very small, thin cards involved. Look for more on packaging issues in the next issue of EDN. EDN

1. PC Card 95 (PCMCIA specification), PCMCIA, Sunnyvale, CA, 1995.

2. Mori, Michael T, and F Dean Welder, The PCMCIA Developer's Guide, Second Edition, Sycard Technology, Sunnyvale, CA, 1995.

3. Legg, Gary, "Buy this book!" (review of The PCMCIA Developer's Guide, First Edition), EDN, May 26, 1994, pg 130.

The PCMCIA Developer's Guide, Second Edition (Ref 2), by Michael T Mori and F Dean Welder, provided a great deal of useful PCMCIA-design information for this article. Mike Mori was also helpful in answering technical questions.