5V to 3.3V UART conversion

Feb 22, 2006 5 Replies
O

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



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P

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

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O

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

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P

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

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L

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.

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E

Hi

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

Alan

formatting link

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