| ??? 09/21/11 01:45 Read: times |
#183817 - What would YOU suggest? Responding to: ???'s previous message |
Andy Peters said:
Richard Erlacher said:
The one advantage of the CPLD over the FPGA, now making the distinction, is that, once programmed, the CPLD doesn't require it be programmed under system power each time the power is turned on. There are some FPGA families which have onboard non-volatile configuration storage with "instant" configuration, meaning that as soon as power is stable the FPGA is configured. Actel's flash parts work this way. There are also OTP FPGAs (QuickLogic, Actel) which are also ready as soon as power is stable. I wasn't aware that any of these relatively new devices are 5-volt-tolerant, hence, would meet Per's need. However, if you know of some, then please let us all know. Cost might also be a problem. However, the old-timers that are 5-volt tolerant would allow him to avoid having to use level shifters with their inherent prop-delays. The real difference between a CPLD and an FPGA is that of resources. A CPLD macrocell has a much wider combinatorial logic resource in front of its (single) register than an FPGA's slice. An FPGA's slice may have one, two or even four registers and a simpler (3-, 4- or 6-input) combinatorial lookup table. FPGAs also have significantly greater routing resources than CPLDs, which is necessary if you need wide muxes and such.
Timing in a CPLD is simpler to analyze because the paths are simpler and delays fairly constant, but that's not really interesting any more since you do your FPGA design, set a clock period constraint, and let the tools have at it and they'll tell you if you win or not. Again, the CPLD architecture more closely addresses Per's stated need. Now, the older FPGA families have features like 5-volt tolerance, among others that might be desirable, and Per can decide whether it's acceptable to him to allow the FPGA I/O's to be in an undefined state for a very brief time. -a I didn't mean to suggest that the old-timers were the only rational solution to Per's problem. I use the old guys because (a) I reuse them all the time, (b) I bought a bunch of them at a very low price, and (c) I have a number of compatible application circuits that I frequently use with these old parts. RE |
