On 05/14/2012 07:05 PM, Jim Brain wrote: > On 5/14/2012 11:22 AM, Gerrit Heitsch wrote: > >> Also, almost all 2364 to 27xxx adapters have the same design flaw, >> they tie _CS for the EPROM to GND and use _OE to control it. This >> works obviously, but it keeps the EPROM active all the time, eating >> power, since _OE only controls the output drivers. Unless you want to >> use very slow EPROMs and need the extra few ns a grounded _CS will >> give you, the proper way is to tie _CS and _OE of the EPROM together > It might be proper, and I considered it on the 2364 and 23XX adapter, > but I chose not to implement for a few reasons: > > * I didn't think the extra power requirement would be an issue. With a CMOS-EPROM it's not much, but there are still a few NMOS-EPROMs around that could get used and with them it makes a difference, at least in heat output. > * I was worried that the delay of enabling the ROM and it's output > drivers would cause timing issues. On a 250nS 27C64, it can take up > to 350nS from !CE to data valid. That's compared to 120nS for !OE to > data valid. Where did you get the 350ns for a 250ns EPROM? I'm looking at the datasheet for a 27C64 from NS and that one states that for a 250ns type, tCE (_CE to output delay) is 250ns and 350ns for a 27C64-350. In that datasheet the times are supplied in a table and can be misread. Since you mentioned 120ns (which is tOE for the 27C64-350, the 27C64-250 has a tOE of 70ns), you might want to take another look at that datasheet you used. > If folks are sure the two can be tied together, please let me know. I have done it and didn't get any complaints from the circuits I used that in yet, namely 1541 and C64. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2012-05-14 18:00:06
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