11/23 clock issue
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.
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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