11/23 clock issue

Tothwolf tothwolf at concentric.net
Sun Feb 8 19:21:46 CST 2015

On Sat, 7 Feb 2015, Brent Hilpert wrote:
> On 2015-Feb-07, at 8:47 AM, Noel Chiappa wrote:
>>> From: Tothwolf
>>> Before I forget again, did you check for +5V on pin 1 (enable) when 
>>> you were testing your existing oscillator?
>> Wow. Never thought to try that. Then again, I don't look for +5V on 
>> your average 74xxx when I'm debugging, either! :-) I mean, it's a 
>> trace, the solder on the pin looks good, that's as far as I go, 
>> usually! And there's nothing shown as connected to that pin on the 
>> circuit diagram.
>> So I looked, and... it's at ground (or floating). The only pin that has 
>> anything is 14, at +5V (expected). But I looked online for some 
>> datasheets for similar oscillators, and some of them say 'pin 1 - N/C'. 
>> Are yours tri-state? (That's the enable pin on the tri-state ones.)
>> I suppose even if yours are tri-state, I can still use them; a quick 
>> ohmmeter check shows that pin 1 isn't connected to either power or 
>> ground, so I can probably tie it high (via a resistor, which in 
>> addition to being normal practise, will prevent a major disaster in 
>> case I'm confused - a state I'm often in :-).
> You shouldn't need to do anything regarding pin 1, (this) datasheet 
> indicates they (the FOX F5C-2 series) have an internal pull-up R on pin 
> 1, so the output should be active by default.
> http://www.datasheets360.com/pdf/8636385546948709069

I checked 4 of the F5C-2 parts at random, and measured ~95k-96k between 
pin 1 and pin 14 (vcc) and infinite resistance (or at least off the scale 
on my Fluke DMM) between pin 1 and pin 7 (gnd). I guess that means they do 
indeed have some sort of internal pull-up resistor, but it certainly isn't 
much of one. OTOH, that makes sense since a larger value pull-up would 
limit current draw if you ground pin 1 to disable the oscillator.

> As your fault description sounds like it could be stuck in tri-state, 
> you could try an experiment with your existing osc. and pull pin 1 high 
> through an R to see if it activates, on the small probability the 
> internal fault is loss of that internal R.

This does make me wonder even more what may have gone wrong in Noel's 
oscillator. I am familiar with some of these type of oscillators (and 
crystals) growing tin whiskers inside their cans, and I'm really starting 
to wonder if that's what caused that oscillator to fail.

> I have one 13.824 unit I was going to offer but Tothwolf has lots of 
> them, so all the better there. They are a baud-rate generator frequency 
> for the standard (300..9600..19200...) baud rate series as well as the 
> 14400..57600.. series. Could probably be found in some 90s-era modems.

That makes sense. I'm almost 100% certain these parts did indeed come from 
an auction of a company that designed ASIC parts for modems (POTS, leased 
line, and ISDN) that closed one of their R&D labs in the 90s. I know for 
sure I still have some prototype 9600 and maybe even some 14400 modems 
that came along with the parts cabinets I bought.

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