| ??? 01/18/05 08:28 Modified: 01/18/05 08:29 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#85164 - Great idea, but 2 things Responding to: ???'s previous message |
Jez Smith said:
In your original post all your problem can be overcome with a cpld either a xc9500 type device from www.xilinx.com or a similar from www.altera.com because witha cpld you avoid having to have millions of packages and the propagation delay is controlable.
If I was in a good mood I might even write you the vhdl to perform the decoding for you.I dunno the caffiene hasnt kicked in yet. Jez Smith said:
you should also remember that with cmos fanout is virtualy infinite.However the loading presented by 32 dil packaged devices would be conciderable,not to mention the routing and the decoupling so I do think that a cpld is your best bet. As Jez pointed out, the use of a CPLD is probably going to be a really attractive option here, in terms of component count, board resources, and performance. I have a similar, albeit smaller situation going on right now in one of my designs, and I am designing in a Xilinx XC9500-series device as a result. As I have found out though, there are a couple of issues worth considering before choosing this device: 1. What kind of loads are you driving? Some 74xx573 chips are going to be able to sink more currrent than the outputs of the CPLD, even more than the generous XC9500 outputs. This consideration is a non-issue if you are just using the CPLD for address deccoding. But if you are also using it to serve the function of the latches, it is a relevant concern. 2. Dou you really want/need an XC9500-series device? There are a couple of excellent reasons to choose them. First, they are one of the few devices that offer true 5V I/O these days (not just 5V-tolerance). Secondly, certain members of the XC9500-family are available in PLCC packages, which is about as prototype-friendly as you are likely to find among CPLDs. While these benefits are sometimes highly desirable, it is worth mentioning that these devices are quite power hungry, rather expensive, not overly fast, and not particularly dense; you can get lower power dissipation, lower cost, better performance, and more functionality from one of the newer families of devices. Good luck. --Sasha Jevtic |
