This may be the time to study it some more. You can do quite bizarre things with CMOS logic if you fully understand what's going on in there. Like using them for analog functions. That always generates scoffing during design reviews. Until the BOM cost totals are presented.
Wonder who it was that reworked the 'HC405x device designs back in
2000 ?:-)
Most data sheets lie ;-)
Go ahead and dump backwards thru an output pin into VDD... at your own peril... just don't come crying to me ;-)
...Jim Thompson
Oh. Coffee works better than beer in these situations. And schottkies may work better than PN diodes.
The thing is, we were driving down Valencia street and we
*saw*a*parking*space*!! Of course, we nabbed it instantly, and then looked around to see what was nearby. It was Frjtz, a Belgian Fries place, with pretty good twice-cooked fries and some interesting beers on tap.That's how you pick restaurants in San Francisco.
John
You? Whose? What was their latch threshold current before?
As I said, I've never seen it happen in HC. I rarely use the esd diodes for clamping, and never allow more than a mA or so on parts spec'd for 50. No smoke so far.
Sometimes this just happens, as when two boxes are interconnected and one power cycles.
Recently, one of my better customers started blowing up Tiny Logic tristate buffers that can gate a clock out to a connector on one of our VME modules. They never enable the output function, and only apply
+3 dBm RF to the connector as an input, so we can't imagine how the chips fry. You can tell the dead ones by the tiny bubble of charred conformal coating on top the SOT-23 package. We tried every sort of abuse we can imagine and can't zap them here. After a zillion meetings and conference calls, we decided to remove the chip.John
Moto/OnSemi. I don't know the latch current numbers... that's a _measured_ thing. I just added some output features which make back-flow hi-Z.
That's only 0.9V P-P at 50 Ohms. Is there any DC present?
...Jim Thompson
Shouldn't matter much since the drop is the same for both diode halves.
Ah, did you try a Chimay? Where smoke wafts out of the bottle after popping the cork?
Now just imagine the SW consultant working on the current project at a client here, trying to park his Dually crew cab with horse trailer.
We love Chimay, the red label in the huge bottle with the champagne cork. But they had these other interesting things on tap.
He brings a horse to work? Can it code?
John
Why does he consume 186.4 watts? Best regards, Spehro Pefhany
Probably not in C. Out here it would not be unusual if someone shows up on horseback:
They claim not. I tried up to 20 volts p-p from a function generator, power cycled that, everything I can think of, and can't kill it. Our manual says this is a "ttl" i/o, but still a 3 dBm sinewave shouldn't hurt anything.
It's a Fairchild NC7SZ125.
They also only fail at the customer's system test step, never in the field. That's good, since it's a test system for B52 radars, deployed all over the world.
I was thinking maybe a really charged-up coax, but they say that's not happening. Another mystery.
John
Some of those watts come back out. But those watts are quite smelly.
Ask them what kind of tile they have on the floor there and what type of shoes the guys are usually wearing.
of
Yep. It's that time of year. Walking thru Sam's Club yesterday. I kept getting zapped until I maintained a constant grip on the cart :-(
...Jim Thompson
of
Despite the fact that almost everybody has personal experiences like that I often find it tough to convince engineers that it poses a problem. "But there is a plastic bezel in front..." Until I hook a scope to an input, set it to normal trigger, walk a few steps and swipe my hand across. "Whoa!" phssst ... poof. This week I was lucky, they immediately agreed. I had put the scope plot into the report ...
I'd use an R and a BC847BPN configured as a pair of diodes a-la Joerg, but thats just me. if the logic family spec'd the acceptable clamp diode current at > 2.5mA, a 330 and a single PN diode would do fine, leaving me one spare 7V diode/50V diode/8V zener/60V zener/transistor.
you guys inspired me, so I grabbed a Speights Old Dark.
Cheers Terry
I fixed a problem for a customer in China last year, using an on-semi HC405x (2 IIRC). they fed 5V thru 100R into the 3V3 powerd HC405x. net result was Vout = 1.4V for a logic zero. no latchup of course, only 10mA (thanks Jim), but the rest of the circuitry stopped working.
their head of engineering reckoned the chip was faulty; his solution:
- remove "faulty" chip
- cut track
- replace faulty chip
- tack schmitt trigger ontop of another chip
- jumper wires to "buffer" the not-so-zero to get a "proper" zero
my solution:
- change 100R to 1k
- add 1k5 shunt resistor at HC405x input
voila, everything worked again.
their schematics didnt show the chip Vcc connections at all - hidden pins and power net names (Grrr). I ended up using a multimeter and a blank board to figure out what chip ran from what supply voltage. they had repeated this mistake dozens and dozens of times.
they had a 600MHz coax datalink using an agilent ser4ialiser/deserialiser chipset. This chip had 21 digital inputs, ALL of which had this exact problem. oddly enough, the chip ran real hot.
I think it's sinking in now. Vout >> Vdd = Bad Charging portion of circuit omitted for clarity I guess.
The diode conducts when Vcap exceeds the Vdd rail. That way the gate doesn't do anything goofy..
D from BC British Columbia Canada.
John, I imagine your output circuit is something like this, to source terminate a 50-ohm coax?
7sz125 39 --|>o---/\\/\\----(o) BNCWhat if you did something like this instead,
,--|>|-- +V 7sz125 20 | 20 --|>o---/\\/\\--+----/\\/\\--(o) | '--|
Are you saying these buffers are off a bidirectional port on your module and the customer is applying a 3dBm clock to it? How is this terminated for input mode drive? Some generators are very particular about termination and can do all kinds of oscillations and non-linear things when driven into Hi-Z, to include putting DC on the line as well as damaging peak voltages. This may be especially true of a video grade signal generator with lots of programmability in amplitude and DC offset. This would also explain why the failures are not occurring in the field. Did the numbskulls even non-invasively scope out their input waveform, or maybe your circuit blows so fast this is not possible. Obviously you need to hold them by the hand and walk them through it. Pathetic!
Nope, no resistor. The intent was to allow anyone to check the oscillator frequency, or to allow one module to be the master that others phase-lock to. I figured that if they needed fanout or cable drivers, they could be provided somehow. If another module or two is next door, just a little jumper will work.
Actually, its...
SMB | | | | C L | | | | | | gnd gnd
where the LC is resonant at 10 MHz and the comparator works against ground. This way, any input waveform, even mangled by cable reflections, can drive the PLL. I never expected them to apply a sinawave to it, but we still can't figure out why a 3 dBm sine would kill the tristate driver.
The board itself is a 4-channel ARB with fairly complex control memory-- conditional jumps, loops, flags, analog and digital outputs. As usual, it's the simple parts we have the most trouble with... the complex stuff just works.
Next rev, I guess I'll do a series RC to the connector (just in case anybody applies DC) and schottky clamp the gate. But I suspect this customer is going to insist that we still leave it off the board.
John
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