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Topic: Connect Rx/Tx of Arduino Pro Mini 5v to 3.3v UART? (Read 29800 times) previous topic - next topic

ldfka

Hello, i can't find a schematics proven to work with a device which is *not* 5v tolerant. Can someone tell me how i should do that?

Many thanks!

Grumpy_Mike

Transmit from 3v3 device to 5v device just connect up.

Transmit from 5v device to 3v3 device, through two resistors forming a potential divider, 1K8 in the top and 3K3 in the bottom.

ldfka

Thanks for your answer, did you already give it a try with an arduino promini or with another arduino? Can it be kept like this in the long term?
Will it affect transmission speeds?

So that's how you would do it:

5v Pro mini        3.3v serial              
GND ---------------- GND
Tx ---1K8---3K3--- Rx
RX ------------------ Tx

Can you confirm? it would mean that the arduino pro mini understands the 3.3v logic on Rx!?

What is strange is that i different way to do that, some use a 1N4148 and a 330R on both Rx and Tx, but nothing really proven to work.

Many thanks

Grumpy_Mike

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Can it be kept like this in the long term?

Yes.
Quote
Will it affect transmission speeds?

No, there are no capacitors in the circuit that is the only thing that would affect the transmission speed.

Quote
Tx ---1K8---3K3--- Rx


No

TX --- 1K8 --- Rx & 3K3 --- ground

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it would mean that the arduino pro mini understands the 3.3v logic on Rx!?

There is sufficient voltage on the 3V3 output for the 5V input to recognise it as a logic one.

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What is strange is that i different way to do that,

In electronics there are often many ways to solve a problem, this is none of them. The advantages of simplicity are balanced against the fact that the noise margin is worse and the impedance is higher. However it will work. Do you want to do it cheap or right?

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did you already give it a try with an arduino promini or with another arduino

I have used this technique for converting logic levels but not used it in this specific configuration. This should not be a concern of yours.

ldfka

I really appreciate your answer, i'm not quite familiar with electronic, and your answer really help me.

Just to make sure, the layout should be the following:

5v Pro mini        3.3v serial              
GND ------3K3------ GND
Tx --------1K8------ Rx
RX ------------------ Tx

Sorry if its obvious i don't want to fry the boards.

Regards

Grumpy_Mike

No read again my previous answer.

Read about a potential divider
http://en.wikipedia.org/wiki/Voltage_divider

don't put resistors in the grounds EVER

ldfka

Thanks, i think i got it!

5v Pro mini        3.3v serial              
GND ---------------- GND
RX ------------------ Tx
Tx ---1K8----3K3-- GND
               |
               --------- Rx

right?


willich

i don't know what's bump. But your circuit is what he was talking bout ;)


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