Hi For around $4 or so you can get a CPLD and socket that will replace all of the logic chips. They will run on +5 and go to >100 MHz. My guess is that they would be a bit more repeatable. The downside is it's another part to program.
Bob Sent from my iPhone On Jul 9, 2010, at 10:02 PM, "Richard H McCorkle" <mccor...@ptialaska.net> wrote: > Ed, > This should work just as well as the original without compromizing the > maximum clock rate. > > Richard > > >> Richard, >> >> For the original circuit board, what about the 74AC175 SOIC soldered to >> an SOIC to DIP adapter? These adapters are available cheap on the >> auction site and there should be very little difference between this and >> the DIP version (the pinouts for both versions are the same). Would this >> compromise the higher frequency versions of the PICTIC II? >> >> Ed >> >> Richard H McCorkle wrote: >>> Time-Nuts, >>> >>> When the PICTIC was first developed a number of front-end designs were >>> evaluated with the 74AC175 providing the best characteristics, size, >>> and speed in a DIP format. When the PICTIC was released in 2008 the >>> 74AC175PC was in full production, but by the time the PICTIC II with >>> the diode interpolator was released in 2010 the 74AC175PC was in >>> lifetime buy status and there were no other manufacturers supplying >>> the device. On 6/01/2010 when I checked there were 1500 available at >>> Mouser and 1500 at Digikey with 10K available at the factory for >>> lifetime buy so I was hopeful the remaining stock would satisfy the >>> needs for PICTIC II construction for a while. By 07/05/2010 all stock >>> at the factory and USA distributors was exhausted. There are 3500 left >>> at Arrow Germany according to the Fairchild site, but that doesn’t >>> help us much here in the USA. The rapid depletion of the stock makes >>> it clear the 74AC175PC was in use commercially and the decision to >>> stop production not well thought out based on the high demand for >>> the remaining stock. >>> The SOIC-16 version 74AC175SC is still available and can be used >>> but will require a little creative imagination to attach it to the >>> existing board. To retain the characteristics of the original design >>> the 74AC175PC can be replaced with the SOIC version or by using two >>> 74AC74 packages although either of these options will require a >>> different circuit board layout to do it right. The dual 74AC74 >>> synchronizers will probably not operate up to the same maximum clock >>> speeds as the 74AC175 design due to higher distributed capacitance >>> in the clock lines to the four flip-flops, which is one of the >>> reasons the 74AC175 was originally chosen. >>> The diode interpolator uses a 10ma charge current that the logic >>> needs to sink to ground, and has a 0.7-2.7v span across the sample >>> cap and up to 3.4v at the anode of the current source shunt so the >>> logic high must reach 4.1v to keep the shunt diode fully turned >>> off at the high end of the charge cycle. The 74HC175 is slower >>> and only sinks 5.8ma so it can’t be used unless the clock rate is >>> below 30 MHz, the interpolator current is reduced to 5ma, and >>> smaller value sample caps are used. The requirement for the 4.1v >>> logic high to keep the diodes turned off precludes the use of >>> 74S175 or 74F175 bipolar logic as they only guarantee a logic >>> high of 2.5v. >>> Due to the rapid depletion of the available stock the 74AC175PC >>> is now unobtanium, so for a DIP package solution to simplify >>> construction the 74AC74 option appears to be the best compromise. >>> I have posted two revisions of the PICTIC II board on the WIKI, >>> one for use with the SOIC and one that uses two 74AC74 packages >>> to replace the 74AC175. If you have a 74AC175PC already the >>> original version of the board may be preferred, as it has been >>> tested to 80 MHz. I have not had a chance to test the new board >>> designs yet using the SOIC or 74AC74 synchronizers, but the >>> changes do not affect the code or other portions of the design >>> so they should provide similar results to at least 50 MHz and >>> will probably run faster than this. >>> The rapid depletion in 74AC175PC stock was a surprise to me >>> and I am sorry for any inconvenience using this device in the >>> PICTIC II may have caused. If anyone on the list stocked up on >>> 74AC175PC devices before they became unobtanium perhaps they >>> would be willing to sell some to those that have ordered boards >>> from Stanley. For future users one of the revised boards using >>> the SOIC or dual 74AC74 synchronizers on the K04BB WIKI can be >>> used instead to get around the problem. >>> >>> Richard >>> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.