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I am trying to send a floating point number from a python script to an Arduino. I am not sure how to do this, especially in a pythonic way.

A little bit of research brought me to this very similar question: How to send numbers to Arduino UNO via Python 3 and the module serial I vaguely understand why this would work for an int but don't know how to modify it.

1) In this question it makes sense to convert to a char, send, and then convert back to a int by casting on the Arduino, I can't think how to do that for a float.

2) Why do they convert to a Char anyway?

3) What code do I run on the arduino to get this back into a float?

4) Can I use bitwise operators on floating point numbers?

I'm new to python and also to low level programming. I am also aware that python and low level don't mix to well. I am using Linux but ideally I need a cross platform solution. Although I would prefer to get it working then worry about cross platform later.

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1 Answer 1

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In this question it makes sense to convert to a char, send, and then convert back to a int by casting on the Arduino, I can't think how to do that for a float.

Instead of using a format specifier of B for an unsigned byte, use f for a single-precision float. AVR-GCC doesn't support double-precision floats, so don't bother with d.

Why do they convert to a Char anyway?

You don't. You convert it to bytes (hence the b prefix on the representation of the value) so that the byte-oriented serial protocol can transmit it. On Python 2.x str is already bytes, so no prefix is required there.

What code do I run on the arduino to get this back into a float?

Use Serial.readBytes() to dump the bytes from the serial connection into a float variable.

float f;
 ...
if (Serial.readBytes((char*)&f, sizeof(f)) != sizeof(f)) {
   ...

Can I use bitwise operators on floating point numbers?

... Well... no... because they may not necessarily remain valid floating point numbers afterwards.

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Thanks for such a quick answer, this is exactly what I want, but this line doesn't work: I'm not really sure what I am doing wrong here, err msg: ser.write(struct.pack(@f,volt_out)) ^ SyntaxError: invalid syntax –  user3157094 Nov 20 '14 at 21:48
    
The format must be specified as a string. struct.pack('...', ...) –  Ignacio Vazquez-Abrams Nov 20 '14 at 21:50
    
Thanks, it works :) I changed the arduino side code while(!Serial.available()); float f; Serial.readBytes((char*)&f, sizeof(f)); the if statement seemed a little redundant in my setup –  user3157094 Nov 20 '14 at 22:45

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