Working with serial input

Using Arduino Programming Questions
#1

First of all, hello to everyone.

I'm quite new to Arduino. startet with building a Wiicopter based on the Arduino Pro Mini. But now i want to get a bit deeper into Arduino usage. I played around a bit with the code examples and for now i think I understand the most of it.

So my new project should Display the Livedata of my RC-model Datalogger (in the future by wireless transmission).

So here is my question:

i get a Dataaset of 24 Byte on the serial input. Now I have to divide the dataset into the different information. to start I wanted to store the information in an array of 24 Bytes. So i should get all the information stored in 1 Byte each. But there are also some information sored together in 1 Byte

here an Example:

Byte 1 : bit 1-4 dataset type ; bit 5-8 datasetnumber

any help is welcome.

thanks Uwe

#2

You can use bit-fields in a structure.

struct logData {
  unsigned int dataSetType : 4;
  unsigned int dataSetNumber: 4;
};
#3

The conventional way to deal with this sort of thing in C is via shifting and masking bits. Maybe something like this:

// Byte 1 : bit 0-3 dataset type ; bit 4-7 datasetnumber
#define DATASET_TYPE_MASK 0x0f
#define DATASET_NUM_MASK 0xf0

#define DATASET_TYPE_ZEBRA 0x01
#define DATASET_TYPE_PONTOON 0x02

my_code()
{
  uint8_t buf[10];
  int type, num = 5;

  // Encode type=pontoon and num
  buf[6] = DATASET_TYPE_PONTOON | ((num << 4) & DATASET_NUM_MASK);

  // decode position 9
  type = buf[9] & DATASET_TYPE_MASK;
  num = (buf[9] & DATASET_NUM_MASK) >> 4;
}
#4

Thanks for your fast replies,

the first example looks good for me. The second one, I'm sorry, but my Programming skills aren't that advanced at the moment :disappointed_relieved:. Furthermore I think the first method would be better as nearly the whole mentioned 24 byte are used bitwise to send some specific data. So I suppose i could define the whole datastructure in the structure.

as I've mentioned my programming skills are still developing. Is there a good reference I can use to learn more about the structure and bit-fields. I did'nt find much as I searched for it now.

Thanks

Uwe

#5

Is there a good reference I can use to learn more about the structure and bit-fields.

Google "C bit-fields" claims 24 million hits. I don't expect they're all good, but a quick glance suggests the top five are helpful.

#6

Bit fields may look simple to use, but there is a good deal of trickiness to hurdle over yet.

First, the operation of bit-fields is compiler- and platform-dependent, so you will want to look specifically at AVR-GCC bit fields. Here's an interesting thread:

http://old.nabble.com/Bit-wise-structure-and-unions-td7477213.html

AWOL did not mention how you will convert the int you read from the serial port into the bit-field struct so you can extract the data. That will look just as scary as what I posted.

#7

AWOL did not mention how you will convert the int you read from the serial port into the bit-field struct so you can extract the data.

A simple cast is "scary"?

#8

It sounds like what you want is a combination of bit fields and a union:

union convertor
{
    struct
    {
      byte type: 4;
      byte number: 4;
    } pieces;
    byte whole;
} buff[24];

union convertor buff2[36]; // alternate way of declaring later in code

Once you have this, you ‘pour’ the bytes into buff, and get them out the other side as bit fields pieces:

for (i=0;i<24;++i)
{
  buff[i].whole= {get data in somehow} ;
  int y=buff[i].pieces.type; // the type of each entry
  int z=buff[i].pieces.number; // the number of each entry
}

Of course, if the bitfield repeats the same for all 24 bytes, then you don’t need the array - one temporary value will do:

union convertor data;
data.whole= {get data in somehow} ;
int y=data.pieces.type; // the type of each entry
int z=data.pieces.number; // the number of each entry

But, if the data layout changes throughout the 24 bits, then you’ll need to tweak the union for all the bytes.

#9

Hello,

thanks for the third optionj David.

As told yesterday, I'm not a pro in programming, and so startet some deeper investigation yesterday evening. I tried to understand the example with the masking and shifting, and basically i understood what should happen. But I still do not understand gardeners example. I think I'll have to get into all this a bit deeper.

So I'm shure the next questions will come, but for now I have a lot to read and to learn.

Thanks Uwe

#10

Glad to help Uwe.

Because I had (obviously) too much time on my hands, I wrote it up for my blog, including some different examples - perhaps it will explain it more clearly for you:

http://www.utopiamechanicus.com/125/data-splitting-union-and-struct-c/

And as already shown here, there are many ways to do what you want - unions just happen to be one way.

#11