Could be, but I don't remember Virtex doing real 5V I/O. I was using a SpartanXL part for the 5V stuff at the time. I was more concerned about the low end voltages on the Virtex (I used Virtex and the /E version for the 1.5V I/O and pushed that down to 1.2V).
You're probably right about that part.
s
=A0 =A0...Jim Thompson
"The Journey is the reward"
eff.com
No, it is like Jim described. [Yo wouldn't subject the gate oxide to stress.] You depend on the fet to snap back. That is strictly ESD. I you tied a hard source to the input and forced the protection device to flow current big time, I'm sure the device would be very leaky after such treatment. The absmax in the datasheet is what protects the companies arse. BTW, damaged inputs are very easy to detect in the rel lab via liquid crystal or emission scope. The FA engineer would curve trace the input and immediately sense that you fried it, then do the other tests to prove it.
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Actually I figure on tying the dummy device to +3.3v. That clamps the 'real' inputs to
the
rI like the idea but don't have the pins, not even two to spare. That's why the i2c port expander.
-- Cheers, James Arthur
Do you think this app note is misleading? (page 5)
It's not very clearly written.. I don't see why the clamp voltage of the "thick field" transistor would vary with the Vdd, for example.
Best regards, Spehro Pefhany
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The data sheet doesn't say anything. Snarl. I grabbed an app note for the family, and it says stuff like "the outputs are high impedance when Vdd=3D0." That being true means "no input diode to Vdd", at least AFAICT.
One other option is to put caps on the lines...naah, that makes the feedback too slow.
So far it looks like a dummy 'hc244 tied to +3.3v. One part, done.
-- Cheers, James Arthur
Not a bad solution. 10 clamps for
rote:
That gate-to-drain n-FET with a high threshold is kind of like a sloppy zener. If that were the protection _and_ it clamped to a safe voltage, then I wouldn't need any external clamps at all.
That's what I was hoping for. I mean, if it _didn't_ clamp to a safe voltage, what good is it?
But, absent info to the contrary, I'll clamp myself to be sure.
Me neither.
-- Cheers, James Arthur
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t 16 clamps if I tri-state the drivers and use the outputs as clamps too. I need 13 clamps, so that gets it done with 3 clamps to spare.-- Cheers, James Arthur
I don't understand. You're reading back the state of relays but don't have the pins to read them back? You suggested using the '244s for their clamps. What pins are you missing?
You could also use the '244s as port expanders on port expanders ;-), using a couple of bits to drive the enables.
We'll have to call you "Jed".
Best regards, Spehro Pefhany
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amps.
Problem: Existing system. I need to add read back for 16 digital drivers, 13 of which may have inductive loads.
Resources: I have one i/o line not already committed. OTOH, I have an i2c bus already serving other devices.
Solution: I'm tacking a port expander chip onto the i2c bus. That gives me 16 extra i/o, uses no processor pins, and preserves the one I have in reserve.
So, the question is of protecting the port-expander chip (PCA9555, NXP, TI, OnSemi), should it see a little glitchlet.
-- Cheers, James Arthur
Well, better Jed than Ellie-May.
-- Cheers, James Arthur
I think you need to show us a schematic of this interface. You're about to commit a faux pas :-) ...Jim Thompson
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It's from those inductive loads I mentioned. I'm knocking the 24v source to 3.3v with a resistive divider, hi-z, which then feeds the i/ o expander + clamps.
22k, or maybe 47k.-- Cheers, James Arthur
OK. Much different from passing 5V logic outputs to 3V logic inputs (but 5V tolerant). ...Jim Thompson
snipped-for-privacy@yahoo.com:
If speed is not an issue and you really don't have 3 spare pins on the MCU, you coud use an 8 lines I/O expander and, with that, control and read a bunch of well known and cheap HC161.
You can daisy chain them but, if you don't, you have 9 inputs per chip.
F. Bertolazzi:
Better 165.
the
This is where language gets in the way. You want to "add read back", implying that these drivers (forcing signals) already exist. So why can't you used '244s in series with the read-back, rather than funky "shunt" (that will forever confuse anyone who looks at your schematic)?
BTW, at eight for a buck (perhaps eight or nine cents from a real disty), three of these get you there, too (I think).
...or a Schottky version (two of these and a discrete) for a bit more:
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