[time-nuts] PICTIC II Parts at Mouser

Ed Palmer ed_palmer at sasktel.net
Fri Jul 9 21:52:45 EDT 2010


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
>   




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