Convert from 16bit INT to 8bit CHARs

[lotharamious]

Hey guys,

I am trying to send data to a kestrel autopilot using an ATmega16. Basically, you feed the autopilot serial data written in arbitrary packets that are well-documented. However, there is a checksum value (of type INT) that needs to be sent (as 2 CHARs) in everyone of these packets. My question is how to break an INT into 2 separate CHARs.

For example... say I want to send an INT of value 1451 but needs to be of type CHAR. The hex value of 1451 is 0x05AB. This can be split to 8bit CHAR hex values 0x05 and 0xAB. Yes? Then I could send each of these individually.

I hope this makes sense, and thank you for your help in advance,

 

[lotharamious]

sorry... just a quick note. I'm using embedded C to program.

 

[alpha88]

I would use bitwise ands and right shifts to seperate the halves.

I'm not sure if this code will work, but the solution probably has a similar method. (You'll need to make sure the 2s compliment doesn't mess you up)

char left = theint >> 8;
char right = theint & 0x00FF);

 

[Cooler]

You could do a union. That seems to be the cleanest way.

 

[bsobel]

Originally posted by: alpha88
I would use bitwise ands and right shifts to seperate the halves.

I'm not sure if this code will work, but the solution probably has a similar method. (You'll need to make sure the 2s compliment doesn't mess you up)

char left = theint >> 8;
char right = theint & 0x00FF);

He will need to cast, but short of that, the solution you showed is fine.

 

[EagleKeeper]

Originally posted by: Cooler
You could do a union. That seems to be the cleanest way.

union {
int iCheckSum;
unsigned char CheckSumBytes[2];
};

iCheckSum= 0x05AB;
printf ("Byte 0 0x%2X Byte 1 x%2X \n", CheckSumBytes[0], CheckSumBytes[1]);



Output will show
Byte 0 0x05 Byte 1 0xAB

 

[Cooler]

Originally posted by: EagleKeeper

Originally posted by: Cooler
You could do a union. That seems to be the cleanest way.

union {
int iCheckSum;
unsigned char CheckSumBytes[2];
};

iCheckSum= 0x05AB;
printf ("Byte 0 0x%2X Byte 1 x%2X \n", CheckSumBytes[0], CheckSumBytes[1]);



Output will show
Byte 0 0x05 Byte 1 0xAB

I could not have coded it better myself.

 

[EagleKeeper]

Originally posted by: Cooler
I could not have coded it better myself.

Great minds think alike

 

[lotharamious]

I'm not sure that the printf will store the Byte 0 and Byte 1 values to the array. Will it do this? Also, how do I make sure that the printf manipulates the correct int value?

 

[HigherGround]

Originally posted by: EagleKeeper

Originally posted by: Cooler
I could not have coded it better myself.

Great minds think alike

that solution will bomb across cross-endian platforms.
OP: stick to alpha88's suggestion.

 

[lotharamious]

I'm gonna try both and see what happens. I'll let you know what works and what doesn't.

 

[EagleKeeper]

The printf is only as an example to let you see what has been stored in the array of bytes.

If the data is displayed backward, then you need to adjust the storage logic.
Instead of the left byte going into the even slot, it goes into the odd slot and the right byte goes into the even slot.
This is based on the way chip makers have implimented the micro code.

 

[bsobel]

If the data is displayed backward, then you need to adjust the storage logic.
Instead of the left byte going into the even slot, it goes into the odd slot and the right byte goes into the even slot.
This is based on the way chip makers have implimented the micro code.

With no disrespect, the earlier code is better since it makes no endian presumptions. No code with the exception of network code should be endian specific.

 

[Cooler]

Originally posted by: bsobel

If the data is displayed backward, then you need to adjust the storage logic.
Instead of the left byte going into the even slot, it goes into the odd slot and the right byte goes into the even slot.
This is based on the way chip makers have implimented the micro code.

With no disrespect, the earlier code is better since it makes no endian presumptions. No code with the exception of network code should be endian specific.

Yes the other one is a more general solution that will work on all platforms. Union Is just quick easy method that works depending on you CPU. It sliped my mind or i would not have sugested it.