How do I Serial write unsigned bytes?

Core Library Questions

chronoless

How do I Serial write unsigned bytes?

in Core Library Questions • 3 years ago

I would like to be able to write an array of 80 unsigned bytes (0-255) through the Serial port (Xbee).

However it only accept signed bytes. Is there is way around this?

I need to send the array at once instead of calling write 80 times as there is a small delay in between write calls.

Replies(6)

PhiLho

Re: How do I Serial write unsigned bytes?

3 years ago

Signed or unsigned is just an interpretation of the bytes by one application. The only difference is to see the high bit as a sign bit or just as another bit.

chronoless

Re: How do I Serial write unsigned bytes?

3 years ago

The problem is that the interpretation of the receiving end is fixed. So I will have to format the byte from the sending end.
I've been trying to figure out how to convert an int (values 0-255) into a byte that, when sent through the Serial port and Xbee, my arduino interprets as the same value, but I haven't been succesful. Could somebody maybe help me out ?

Jack Kern

Re: How do I Serial write unsigned bytes?

3 years ago

One way to handle the problem is to use shorts or ints and cast to byte before transfer. Fortunately with bytes you don't have to worry about all the endian-ness issues as well.

Also, arduino's serial connection can be funky and unreliable. I've discovered it's usually best to create some kind of protocol no matter how simple to send information, like a specific two bytes to begin the message, a byte for the type of data, and then the data followed by another marker. This way, you can tell your Arduino sketch to ignore any messages it receives starting up so the app doesn't get in a strange state. If you're feeling ambitious I know you can call some native CRC functionality from the arduino libs too, if you really want to be certain what you send is what you receive. Last but not least if you're using both the hardware and software serial connections, those can be very picky working together, best bet is to scale down the baud rate to something really manageable for the chip if you're doing that.

Hope that helps a bit!

Jack

http://www.jackkern.com
http://www.twitter.com/ElectricJack

chronoless

Re: How do I Serial write unsigned bytes?

3 years ago

Thanks Jack!
I have a protocol and it works pretty ok. I am able to send data through max/msp without a problem. But I prefer to use Processing for my next project. So the protocol and all the hardware works well with max/msp but not with Processing.
My problem is that I can't cast an int above 127 into a byte because it's a signed byte. What I would like to know is how to cast an int with a value of e.g. 160 into a byte that when sent by processing is interpreted by an arduino as 160.

chronoless

Re: How do I Serial write unsigned bytes?

3 years ago

Fixed it. The trick was to do this:

if(value1 < 128)
    {
    value2 = (byte)value1;
    } else {
    value2 = byte(value1 - 256);
    }

Jack Kern

Re: Re: How do I Serial write unsigned bytes?

3 years ago

Ah that makes sense. Thanks for posting your solution! Sounds like a helper function is in order;

byte unsignedByte( int val ) { return (byte)( val > 127 ? val - 256 : val ); }

Might be worth posting something in the wiki...

Anyways, good luck with your project!

Jack

http://www.jackkern.com
http://www.twitter.com/ElectricJack

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