Variables - Convert 16 bit into 2x 8 bit and vice versa

Projects Programming
1

Hi there,

I am having trouble finding how to convert a 16bit variable into two 8 bit variables. This is to store on a EEPROM.

The same is varil to read several 8bit variables and convert them back into 16 and 32bit words.

Thanks in Advance for any help

2

You can mask off the bytes you want to use, or you can maybe look into a Union.

int Data = 12345;
byte Hi = Data & 0xFF00;
byte Lo = Data & 0x00FF;
. . .
// print bytes

union myData {
  int Data;
  byte Hi;
  byte Lo;
} myBytes;

. . .

myBytes.Data = 12345;
Serial.print(myBytes.Hi);
Serial.print(" ");
Serial.println(myBytes.Lo);
3

HazardsMind:
You can mask off the bytes you want to use, ...
[/url].

How would I do this?

Regards

4

There are also the highByte and lowByte functions. Look in the reference.

5

I think you missed a shift in your first Hi calculation.

6

The above is good to write a single variable, but I have up to 32K of memory.

What I need is some function that will take a 16bit variable and output 2 8bit variables, or vice versa. Is there anything similar to this?

7

**I left out a very important part that allows you to do what you want, and that part is a struct. If you tried my first code, you would have seen that both Hi and Lo output the same thing. Well with a struct, it will now actually split the memory of the int into Hi and Lo.

Use an array

union myData 
{
  int Data;
  struct bytes
  {
    byte Lo; // <- notice the order, Lo is first. The explanation to this is because memory is read as Lo to Hi, not Hi to Lo
    byte Hi;
  }B;
}myBytes[20];

void setup()
{
  Serial.begin(115200);
  for(byte i = 0; i < 20; i++)
    myBytes[i].Data = random(10000,32000);

  for(byte j = 0; j < 20; j++)
  {
    Serial.print(myBytes[j].Data,HEX);
    Serial.print(" ");
    Serial.print(myBytes[j].B.Hi,HEX);
    Serial.print(" ");
    Serial.println(myBytes[j].B.Lo,HEX);
  } 
}

void loop()
{

}

I would also like to point out a new milestone, that this is the very first time I have ever had a reason to use a Union.
Sad, I know.

8

I saw a few functions that used the above method. Makes sense.

I found that for little data arduino has a built in function already.

For example:
tft.print((lowByte(temp)));
tft.print(" ");
tft.print(highByte(temp)));

9

Here is an example which uses nice property of shift.

Now please explain how would using union improve the code or why is it necessary ?

The Data is unchanged , so this flow

byte bByte = Data >> 8;
.... save hi byte
bByte = Data & 0x00FF;
.... save Lo byte

reuses same bByte variable anyway.

Cheers Vaclav

int Data = 12345;
byte Hi = Data >> 8;
byte Lo = Data & 0x00FF;
Serial.println(Data,HEX);
Serial.println(Hi,HEX);
Serial.println(Lo,HEX);
Serial.println(Data,HEX);
10

You don't need to mask the low order byte. This works.

byte Lo = Data;

Pete

11

Now please explain how would using union improve the code or why is it necessary ?

I said it was another option and it can handle multiple types. Good for future practices.

Yes, it was mistake to miss the simplicity as right shift, and yes, I was looking for a reason to finally use a union.

12

Now please explain how would using union improve the code or why is it necessary ?

You can use the union method to convert a float into four bytes and save it. I doubt this would work using the alternatives.

13

el_supremo:
You don't need to mask the low order byte. This works.

byte Lo = Data;

Pete

Even better, nice and smart way showing you know your stuff.
Still not fully convinced that one needs union to convert say 32 bits to bytes.
Actually I am building 40 bits word from bytes byte by byte anyway.
Cheers
Vaclav