[time-nuts] PICTIC II Parts at Mouser

Bob Camp lists at rtty.us
Fri Jul 9 22:51:41 EDT 2010


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.


Sent from my iPhone

On Jul 9, 2010, at 10:02 PM, "Richard H McCorkle" <mccorkle at 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
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