| ??? 11/27/07 19:22 Read: times |
#147468 - If only one could rely on them ... Responding to: ???'s previous message |
Andy Peters said:
I don't wish to drag this out any longer ...
but if you don't wish to read data sheets before deciding which device to use, then I can't help you. I've never had a problem with reading datasheets, but I don't like the misleading marketing information that's often presented on the first page of each datasheet. That casts doubt on the entire document. I like the DLL's in SP-II's but prefer PLL's (not the crap in FPGA's, but the real thing, for which there are real spec's, acquisition range, acquisition time, tracking range, etc.) when I have to synchronize processes. DCM's, DLL's and Altera's PLL's are of little use when, for example, you have to sync up with one of eight data streams, each driven by crystals that have their own idea of what 80 MHz means. You misunderstand the intended use of these clocking resources. They are NOT meant (and can't be used for) things like recovering embedded clocks in data streams or locking to any arbitrary incoming clock. I use them to double my global clock so I can read two bytes from a buffer at the same time, and the frequency synthesis is handy for this Camera Link interface I'm working on. With all of these clock managers, you must understand that you need to tell the tools, at synthesis time, the expected input clock frequency and clock multiplication/division (if using the synthesizer and not just the DLL). For that, they work quite adequately. They'll effectively double or, using two of 'em, quadruple a reference clock and make various quadrature phases available along the way. I don't misunderstand the intended purpose of these resources at all. I do, however, disagree with their presentation as PLL's, when that's really not at all what they are. Actually, PLL's would be a GOOD thing to add to an FPGA that has to process clock-modulated serial data. As for syncing to eight data streams with the same clock frequency and different phases: I know nothing about your application and really, maybe an FPGA is the wrong thing for your app. But maybe you SHOULD look into Virtex-4 FX or Virtex-5 LXT devices. Maybe these newer (Spartan 2 is positively ancient in FPGA terms) devices will be able to do what you need them to do. They may bust your budget and they don't have 5V tolerance. What makes 'em budget busters isn't only the cost of the level shifters, but the >>$100-1000 per each cost that the distributors quote. I've found eval boards (which is the way I most often use these devices) that cost much less than the bare FPGA in sub-1k quantity. Sure, the data sheets are a thousand pages and there's a lot you'll have to learn about before you can get your money's worth out of them.
But if it was easy, everyone would do it, right? What actually happens is that everyone does it, but most do it badly. -a Actually, I read the datasheet thoroughly just to determine whether the device family is going to be of any use at all. After that, I have to determine whether the cost is justified by the utility. I've occasionally found that the cost varies with geography, and that a part costing $35 in Denver can be bought from the same distributor in CA for $4.32 at the same quantity. With distributors playing games like that, with low quality PCB's made here in the U.S. and with the low-cost high-quality manufacturing and assembly available in Asia, I find the whole business is a crapshoot. RE |
| Topic | Author | Date |
| Tri-state busses in FPGAs | 01/01/70 00:00 | |
| Tristate Buffers (TBUFs) have been phased out | 01/01/70 00:00 | |
| Thank you | 01/01/70 00:00 | |
| Closing the loop | 01/01/70 00:00 | |
| siumulate? | 01/01/70 00:00 | |
| I didn't simulate it (yet) | 01/01/70 00:00 | |
| hmmm | 01/01/70 00:00 | |
| So ... what about a BIG multi-party bus? | 01/01/70 00:00 | |
| delay | 01/01/70 00:00 | |
| nevertheless ... | 01/01/70 00:00 | |
| re: nevertheless | 01/01/70 00:00 | |
| What disappoints me is the advertising vs reality | 01/01/70 00:00 | |
| advertising | 01/01/70 00:00 | |
| advertising, badvertising ... lies! | 01/01/70 00:00 | |
| oy | 01/01/70 00:00 | |
If only one could rely on them ... | 01/01/70 00:00 | |
| largely, it's because it's not an option | 01/01/70 00:00 | |
| Zackly | 01/01/70 00:00 | |
| If you have internal tristate resources ... | 01/01/70 00:00 | |
| I have new worries now | 01/01/70 00:00 | |
| tristates in FPGAs | 01/01/70 00:00 |