[Oberon] FPGA - Voltage Convertor

[Paul Reed]

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:

https://www.adafruit.com/product/757

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 
protection).

HTH,
Paul