[Oberon] [EXT] Re: FPGA - Voltage Convertor

Skulski, Wojciech skulski at pas.rochester.edu
Fri Nov 27 19:56:56 CET 2020

I thought that the "convertor" means "converter", which means power supply. Sorry for that.

The official name for signal level translators is "translator" or "level translator". 

Be careful with those. Recently I used TXS0104 which is similar to the TXB0108. It worked with one particular circuit, and it did not work with another similar circuit which looked almost the same. We never understood the difference between the two use cases. It just refused to work in one case, though it did work in a seemingly identical other case. No amount of investigation yielded any clue. We ended up with some workarounds to avoid using it altogether.


From: Oberon [oberon-bounces at lists.inf.ethz.ch] on behalf of Paul Reed [paulreed at paddedcell.com]
Sent: Friday, November 27, 2020 12:17 PM
To: ETH Oberon and related systems
Subject: [EXT] Re: [Oberon] FPGA - Voltage Convertor

Hi Tomas,

> Just discovered this convertor, it is Arduino breed.
> Wondering how could be used with FPGA, what should connect where?

It seems (without having more information to confirm) that this is a
MOSFET-based bidirectional level convertor like this one from Adafruit:


It is designed for level-converting open-collector-style busses like
I2C, but can, with limitations, be used for a variety of applications.

The Adafruit page refers to the application notes from Philips/NXP which
explain its operation, which I think is quite elegant and they are well
worth a read.

I have used this type of converter on PS/2 signals (pulled up to 5V)
which need to be fed into and from a 3.3V or a 2.5V I/O bank on an FPGA.

In that case with this board, I'd probably connect HV to 5V and LV to
3.3V/2.5V, and then the corresponding LV1..LV4 signals to the FPGA, and
the HV1..HV4 to the corresponding PS/2 clock and data lines. But I'd
want a schematic of the board to be sure.

Note that the circuit is so simple it doesn't even need a ground (0V)
connection. That's because its bidirectional function is really: if
grounded on one side, bring the other side to ground.

This is different from the series resistor level-convertor on the
Digilent Spartan 3 PS/2 connector, which relies on the clamp diodes
inside the FPGA to keep the low-voltage level on the FPGA side. Not all
FPGAs are the same here, hence the MOSFET approach is safer (and the
MOSFET will probably blow up before the FPGA, providing some crude

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