[time-nuts] PICTIC II Parts at Mouser

Richard H McCorkle mccorkle at ptialaska.net
Sat Jul 10 02:02:16 UTC 2010


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
>>
>
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