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Microcontroller Programming » Serial Tx, Rx voltage level translation

March 30, 2013
by dvdsnyd
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Hi all,

I want to run the uC off of 3.6 volts. I have confirmed that the NK works at this voltage. The reason I am doing this is due to some devices that aren't 5 volt tolerant. I want to be able to be able to program using the serial cable. The Tx and Rx lines for the cable are both running at 5 volts correct? If this is the case, I can make a simple logic level translator using 2 2N7000 mosfets and some resistors that will switch the voltages and allow my computer to communicate with the uC.

The main reason I am asking this is I thought I found somewhere that the TX and RX data lines from the computer run at different voltages.

Thanks for all your help David

March 30, 2013
by Noter
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The TX and RX data lines from the PL2303 USB to serial adapter already operate at 3.3v. You can verify this with an ohm meter on the TX (RX on the uC) because it remains high when idle.

March 30, 2013
by Noter
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Oops, better use a volt meter instead of an ohm meter.

March 30, 2013
by Ralphxyz
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dvdsnyd, you could use sask55's optoisolation circuit to make sure.

Ralph

March 30, 2013
by dvdsnyd
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Thanks for your responses!

SO, I only have to worry about translating the RX line(TX on the uC, since the USB serial adapter is going to be expecting a 5V logic signal? I checked the adapter, and the TX line was running at 3.6V, which is what I am expecting to run my altimeter on.

Thanks again! Dave

March 30, 2013
by Noter
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No, you don't have to do anything since the serial already operates at 3.3v. Just hook it up as usual and it will work fine.

March 30, 2013
by Noter
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Although the USB serial adapter operates at 3.3v, it will tolerate 5v and that is why it works as is on a 5v nerdkit. The 5v nerdkit only needs to see about 2.6v on a pin to consider it high so 3.3v (or 3.6v) from the serial adapter is enough for it to work.

March 30, 2013
by dvdsnyd
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Thanks Noter,

I wish I would have known this sooner, it simplifies things!

David

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