Electronics > FPGA

ice40 HX, A 5v tolerant FPGA ??

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woofy:
Well this is curious. I'm interfacing a PS2 keyboard to an ice40 HX8k and I needed to protect the 3.3v inputs from the 5v PS2 signals. The PS2 appears to have 2k2 pull-ups as I get about 2.2mA shorting clk or data to ground. So I thought I'd add additional 1k8's in series with the signals and let the ESD diodes do the clamping for me. Imagine my surprise when I looked on the 'scope and still saw 5v signals on the FPGA inputs! It works too, as I can echo the signals out on other pins. It seems as though there are no input protection diodes, or maybe there are and they clamp to 5v. The HX data sheet is clear, 3.6v absolute Max. I did a little poking around on the web and found the ice40 was acquired by Lattice from Silicon Blue, and Silicon Blue's previous ice65 ~was~ 5v tolerant.
So I'm thinking maybe the ice40 is 5v tolerant, at least the HX8k.
Does anyone know for sure?

Maybe I'll see the magic smoke at some point, but hey, its a hobby project.

 

asmi:
You are free to try of course, but I'm 99% certain you will blow an IO buffer, or maybe the entire IO bank (depending on how it's wired internally). The fabric will probably survive because it's typically in a separate voltage domain than IO blocks, but who cares about it if you can't do any IO?

woofy:
Well, I've done a few more tests, feeding a higher voltage onto the HX8k pins via a 47k resistor.
At 5v input there is no current draw. The voltage is the same both sides of the resistor.
At 6v it is still almost 6v on the pin and its pulling 550nA.
At 6.5v it has 6.35v on the pin
7v gives 6.38v on the pin.
8v, 6.39v
9v, 6.39v
10v, 6.4v
20v, 6.41v

This is the same across every random pin I checked.
It seems the HX8k has an avalanche FET clamp rather than the expected diode to VCCIO, clamping at 6.4v.
The only reason for this that makes any sense is that this is a 5v tolerant device, designed that way by Silicon Blue.

I think for Lattice this is an unwanted feature they don't include in their other devices and don't want to cloud the range by having mixed tolerance devices.


nonarkitten:
I just wanted to say thanks for this discovery. We'll test this out some more, but if the original generation of iCE40's are in fact still 5V tolerant, then that's just amazing news.

SiliconWizard:
How much current does it draw @20V?

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