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 doesnt >> 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 cant 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 -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
