[time-nuts] PICTIC II Parts at Mouser

Richard H McCorkle mccorkle at ptialaska.net
Fri Jul 9 21:21:54 EDT 2010


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


> Hi
>
> The 74F175 is in stock at Mouser. I'll let Rich decide if it's "good enough".
>
>
> Bob
>
> Sent from my iPhone
>
> On Jul 9, 2010, at 5:16 PM, Stanley Reynolds <stanley_reynolds at yahoo.com> wrote:
>
>> I was thinking of using a 74F175 not as fast uses more power but I have it.
>>
>> Stanley
>>
>> <snip>
>>
>> 7) The 74AC175 is un-obtanium in a PDIP package the project shows a 74ACT SOIC
>> version, you'll have to kludge it on to the board Logic levels on the clock may
>> impact accuracy (but probably won't).
>>
>>
>> <snip>
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