11/23 clock issue

[Brent Hilpert]

On 2015-Feb-05, at 5:41 PM, Noel Chiappa wrote:

>> From: Brent Hilpert
> 
>> Well, try removing W1 to isolate the output of the xtal oscillator E15
>> from inverter input pin E4.4.
> 
> Thanks! Too late to try it today, but first thing tomorrow..
> 
> It seems like every time you answer one of my queries, I smack my forehead
> and say 'That's an excellent idea! Why didn't _I_ think of that!?!?' :-)
> 
>> If you're not familiar with them, E15 is an integrated xtal oscillator,
>> not just a crystal - the crystal and oscillator logic are both in that
>> can
> 
> I'm not familiar, so thanks for the tip. Wonder where I'll find one if that's
> what's bad... :-)
> 
>> Could also (with W1 removed) pull E4.4 hi/lo while watching the
>> inverter output E4.16 to see if it responds correctly with the
>> inversion.
> 
> Since it's a tri-state, I may need to hit the enable somehow. Oh, wait, I
> see, it's hard-wired on via a pull-down. (I wonder why they needed a
> pull-down? And I wonder what the test point is there for?) And I guess I need
> like a 5K series pull-up resistor to +5, right? (Ground I can just tie
> directly, right?)
> 
> Anyway, thanks again for the help - very, very useful.
> 
> 	Noel

TP4 looks to be an 'input' test point, rather than an output. The pull-down resistor value (150 or 180 ohms) has been selected such that it is low enough to allow enough current to flow through the tri-state control input E4.1 to pull it low normally, but high enough to allow one to connect TP4 high, to enter tri-state, without unduly high current flow. What the point of going to tri-state is, is not clear, considering that W1 is there for an external clock.
A conjecture is there may have been some external test fixture that tri-stated it for some sort of synchronous single-step clocking.

In testing input E4.4, you could just send it directly to +5 and GND, however it would be a little safer to use a 100-or-so ohm resistor in series for both directions, to limit current in the event the input is shorted internally.

Could also check the V reading on the open E4.4 input.