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[ Chapter start ] [ Previous page ] [ Next page ] 4.4 Practical IssuesSystem companies often select an ASIC technology first, which narrows the choice of software design tools. The software then influences the choice of computer. Most computer-aided engineering ( CAE ) software for FPGA design uses some type of security. For workstations this usually means floating licenses (any of n users on a network can use the tools) or node-locked licenses (only n particular computers can use the tools) using the hostid (or host I.D., a serial number unique to each computer) in the boot EPROM (a chip containing start-up instructions). For PCs this is a hardware key, similar to the Viewlogic key illustrated in Figure 4.7 . Some keys use the serial port (requiring extra cables and adapters); most now use the parallel port. There are often conflicts between keys and other hardware/software. For example, for a while some security keys did not work with the serial-port driver on Intel motherboards—users had to buy another serial-port I/O card. Most FPGA vendors offer software on multiple platforms. The performance difference between workstations and PCs is becoming blurred, but the time taken for the place-and-route step for Actel and Xilinx designs seems to remain constant—typically taking tens of minutes to over an hour for a large design—bounded by designers’ tolerances. A great deal of time during FPGA design is spent in schematic entry, editing files, and documentation. This often requires moving between programs and this is difficult on IBM-compatible PC platforms. Currently most large CAD and CAE programs completely take over the PC; for example you cannot always run third-party design entry and the FPGA vendor design systems simultaneously. There are many other factors to be considered in choosing hardware:
4.4.1 FPGAs in UseI once placed an order for a small number of FPGAs for prototyping and received a sales receipt with a scheduled shipping date three months away. Apparently, two customers had recently disrupted the vendor’s product planning by placing large orders. Companies buying parts from suppliers often keep an inventory to cover emergencies such as a defective lot or manufacturing problems. For example, assume that a company keeps two months of inventory to ensure that it has parts in case of unforeseen problems. This risk inventory or safety supply, at a sales volume of 2000 parts per month, is 4000 parts, which, at an ASIC price of $5 per part, costs the company $20,000. FPGAs are normally sold through distributors, and, instead of keeping a risk inventory, a company can order parts as it needs them using a just-in-time ( JIT ) inventory system. This means that the distributors rather than the customer carry inventory (though the distributors wish to minimize inventory as well). The downside is that other customers may change their demands, causing unpredictable supply difficulties. There are no standards for FPGAs equivalent to those in the TTL and PLD worlds; there are no standard pin assignments for VDD or GND, and each FPGA vendor uses different power and signal I/O pin arrangements. Most FPGA packages are intended for surface-mount printed-circuit boards ( PCBs ). However, surface mounting requires more expensive PCB test equipment and vapor soldering rather than bed-of-nails testers and surface-wave soldering. An alternative is to use socketed parts. Several FPGA vendors publish socket-reliability tests in their data books. Using sockets raises its own set of problems. First, it is difficult to find wire-wrap sockets for surface-mount parts. Second, sockets may change the pin configuration. For example, when you use an FPGA in a PLCC package and plug it into a socket that has a PGA footprint, the resulting arrangement of pins is different from the same FPGA in a PGA package. This means you cannot use the same board layout for a prototype PCB (which uses the socketed PLCC part) as for the production PCB (which uses the PGA part). The same problem occurs when you use through-hole mounted parts for prototyping and surface-mount parts for production. To deal with this you can add a small piece to your prototype board that you use as a converter. This can be sawn off on the production boards—saving a board iteration. Pin assignment can also cause a problem if you plan to convert an FPGA design to an MGA or CBIC. In most cases it is desirable to keep the same pin assignment as the FPGA (this is known as pin locking or I/O locking ), so that the same PCB can be used in production for both types of devices. There are often restrictions for custom gate arrays on the number and location of power pads and package pins. Systems designers must consider these problems before designing the FPGA and PCB. [ Chapter start ] [ Previous page ] [ Next page ] | |
