Input protection for 3.3V FPGA in a TTL world... (2023 Update)

For an input to the FPGA, you could use just a series resistor and let any overvoltage or spikes hit the ESD diodes. Debounce as needed.

10K or something. Paranoids could add a cap to ground, 1 nf or so.

Topic? We have a topic?

The MAX 10 datasheet has very specific input overvoltage specs, for instance 4.27V for <4.3% of the time over 11.4 years, equally bizarrely they specifiy the supply voltage to three decimal places.

I think Intel really do not want the ESD diodes to be touched?

piglet

What are they for?

There's this that does what you want, maybe too much.

If you click on the schematic link, you get some MOSFET level shifters: Since you're too busy to tell anyone whether you're talking about 8 I/O or 500 I/O, tough shit if this doesn't work for you.

Ah, I didn't think to give an IO count. It is around 50 in total, with

20 or so actually going to the outside world.

Thanks!

John :-#)#

Not an entirely safe approach. When you let ESD diode dump current into the substrate it can come out in odd places, and the FPGA could start behaving strangely. I've never tried it with an FPGA but I did get my fingers burnt with a less complex integrated circuit.

Depending on how many 'switches' there are, the venerable MC1489 tolerates lots of input volts and is a multiple-sourced jellybean. I"m wary of diode-to-Vdd clamps, though, since that just puts trash currents into your 3.3V regulator; the chip can survive, but will regulation suffer? No need for a series resistor, that's built-in, and '1489 input clamping is to GND.

Actually the diode clamps are tied to the 5VDC Vcc, and there will be a

5.6V Transorb as well to take care of surges before the regulator is affected.

My I/O are switches on the playfield and coin acceptor and self test switch, all are the most at risk of static shock from the player. Considering that there are around 20 I/O to protect I don't think a group of 1489s is going to fit as well as some TVS arrays and resistors that will blow open if any real current starts to flow.

It does raise the problem of ground or power loops though if there is a surge on one of the lines. I'll talk that over with my cohort - we have options for ground and 5VDC tie points.

Thanks,

John :-#)#

On a sunny day (Thu, 14 Oct 2021 23:05:16 -0700) it happened John Robertson snipped-for-privacy@flippers.com wrote in <VPKdnVOaSuaCh snipped-for-privacy@giganews.com:

I had such a user interface problem long ago and used opto couplers.