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To follow Calamity Jane, check for cold solder joints. They are notorious for working an amount of time and then failing without explanation. Emmett Redd Ph.D. mailto: Associate Professor (417)836-5221 Department of Physics, Astronomy, and Material Science Southwest Missouri State University Fax (417)836-6226 901 SOUTH NATIONAL Dept (417)836-5131 SPRINGFIELD, MO 65804 USA > -----Original Message----- > From: Brian Moerdyk [mailto:] > Sent: Monday, September 01, 2003 6:33 PM > To: > Subject: Re: [m68HC11] Re: 27c256 failure > > To answer John's question: > > Yes, the window of the EPROM is covered with black electrical tape. > I've found this to be a particularly useful way to do it, since the > electrical > tape doesn't stick "too hard" to be easily removed, and tends not to gum > up the > window either. > > To answer Calamity Jane: > > Yes, these are EPROM that worked in this circuit until very recently. > None of the hardware changed from the working to non-working scenario. > In fact, very little of the code even changed between times when it did > and did > not work. What makes me believe it is the chip is the fact that I can > program > the identical code into a more recent EPROM, and everything works fine. > All my CS lines are tied correctly, as are the unused interrupt pins on > the > HC11. > The decode logic for a 32k ROM is very simple - basically an inverter on > AD15 > for -CS, and a NAND of E and R/W for the -OE signal. The delay times, > according to the data sheets for these part should be less than 10ns. > > The 27c256 I am using are 120ns parts, but my master clock is only 4MHz, > which > means E is 1MHz. A 120ns part should be plenty fast to meet the timing > criteria. > > I believe I have plenty of bypass caps on the board, but I'll try adding > some > more. > > Thanks to all! > > --- WinnonaD <> wrote: > > It sounds like "it's running on FFs". > > > > Did these chips work before and don't work now (in > > this same board)? > > > > If it's the first time around in this board, these are > > basics, but good to check as they are the most likely > > causes: > > > > 1- Are all the unused CS lines tied firmly to +5v or > > ground? > > > > ROMs can vary in the loading and "float point" of > > various inputs. > > > > 2- Does the address-decode leave enough access time > > and is it free of noise or extraneous swithing? > > > > Some ROMs can be way faster than average and respond > > to extraneous switching; some are slow and need the > > FULL specified access time. Some are more sensitive to > > noise. > > > > 3 - Ensure you have a Vcc/Gnd bypass cap. > > > > Usually if some chips work and others don't, there's > > something borderline in the board circuitry. > > > > Calamity Jane > > > > __________________________________ > > __________________________________ > > ------------------------ Yahoo! Groups Sponsor > > To unsubscribe from this group, send an email to:

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