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5V to 3.3V UART conversion

Hey,

I am trying to link the 5V serial port on my ADuC831 (8051) to the 3.3V serial port on my Analog Devices DSP board. Transmission is one way only (5V -> 3.3V). I have no idea what to do to drop the voltage.

I read somewhere that it should simply be a case of adding a series resistor, the value of which could be calculated by finding out the max current from the data sheets of both boards. However I haven't been able to find any current values on the data sheets.

Should I use a resistor, transistor, a diode maybe??

Greatful for all suggestions!

Thanks

Ronan

Reply to
omalley.ronan
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Assuming the DSP inputs aren't 5V tolerant:

Two resistors in a voltage divider. One resistor and a zener. A level translating IC - a 3.3V IC with 5V tolerant inputs.

Comms the other way should be no problem.

Paul Burke

Reply to
Paul Burke

how would i determine the resistor value if i choose the zener aproach? will any resistor do?

Reply to
omalley.ronan

Depends how fast the edges need to be. The capacitance of the input plus the zener will combine with the resistor to give you a time constant (CR). The resistor needs to be small enough to make the rise time within whatever the data sheet recommends. 1k or thereabouts should be OK, at a guess.

Paul Burke

Reply to
Paul Burke

It's much easier with a transistor, using 3V on the collector and bias resistor on the base. This gives you a better drive current, especially for long links.

Reply to
linnix

Hi

I've used the resistor divider approach successfully, simple and v cheap.

Alan

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