[time-nuts] PICTIC II Parts at Mouser

[Ed Palmer]

I see that the 74AC175 in the Mouser project has now morphed to a 
75AC*T*175. Was that substitution intentional? I think it should be 
okay, just wanted to check. (What? Paranoid? Who said that?!)

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
>
>
>   
>> 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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>
>
>
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