On 06/04/2013 11:01 PM, Jim Brain wrote: > On 6/4/2013 12:57 PM, Gerrit Heitsch wrote: >> On 06/04/2013 07:23 PM, Jim Brain wrote: >>> On 6/4/2013 11:59 AM, Gerrit Heitsch wrote: >>>> On 06/04/2013 04:14 AM, Jim Brain wrote: >>>>> As I look at it, I can;t help but wonder if some juggling of the >>>>> address >>>>> lines and such would simplify things and remove the requirement for an >>>>> inverter altogether, but I am not seeing an easy way to do it. >>>>> >>>>> http://jbrain.com/vicug/gallery/vicmidi/VIC_MIDI_Schematic >>>> >>>> You have a few unused inputs on the LS688. Can't you use them to >>>> include PHI2 into the _SEL signal already? That would mean you could >>>> free up some inputs on the other logic simplify the circuit there. >>> My concern there is that it delays PHI2 all over, and it's typically the >>> last signal to switch. I felt putting it through logic would create >>> timing hazards. >> >> That shouldn't be much of an issue with the current logic chips and >> become less so if you can reduce the logic further. > I'm struggling with that. ahct138 is 12nS worst case, and hct688 is > 34nS worst case (@25C) That's over 46nS of delay, not including the > ahct14 delay of 8ns for ahct14. 54ns is quite a bit. Doesn't the VIC-I > use the other half of the cycle? Yes, but a halfcycle is 500ns long. You could also ditch the LS688 and just use the LS138 even if that means incomplete (but faster) decoding. Or you try a faster logic family. Like 74Fxx or 74ALSxx. >>>> Then you should read the datasheet for the Flash since for most RAMs, >>>> _WE overrides _OE, meaning you can tie _CE and _OE together. If that's >>>> the same for the flash, you can free up an inverter for use elsewhere. >>> That. I did not know. I will check that tonight. >> >> Checked the datasheet for a µPD43256 from NEC, as soon as _WE goes >> low, _OE becomes 'x' (don't care) > 39sf010 datasheet specified !OE must be Vih (High) during write cycle. Annoying... Someone wanted to save a few transistors. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2013-06-05 18:00:03
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