how to make 5V tolerant pins out of 3.3V tolerant pins?


I need to interface my BlackFin BF531 to 5V bus, but the BF531 pins are not 5V tolerant (they are 3.3V digital inputs and outputs).

I don't want to use any additional logic which would allow this, I would like to use some resistors and diodes if possible.

If the pins would be only inputs, I would use resistor divider. I could also use pull-up diode on every pin, which would cause to limit input voltage to 3.3V + Uf of diode. Or I could just use a 3.3V zener diode on those lines which would cut down the voltage to 3.3V as well.

But I'm worried what will it do with the bus as this device might be connected to that bus with other devices, which 5V logic and I wouldn't like to draw current from other devices by limiting the inputs by the diodes... And also more of these devices (which I'm trying to develop) might be connected to that 5V bus at the same time.

So how should I do it the proper way?

Regards, Jookie

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A one way around is to use a bi-directional level shifter, perhaps using one GP MOSFET (like 2N7002) and one resistor. See schematic and theory of operation in NXP's an97055, Figure 2.

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(Although it's I2C related, the level shifter can be applied for every line of your BF531)

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