Em 30/5/2011 20:31, Harold Hallikainen escreveu:
>> Em 30/5/2011 17:51, IV3YNB op. Matteo escreveu:
>>> Hi all !!!
>>>
>>> I'm an hobbyst and I am struggling to find the right example or the
>>> right
>>> application note to read, useful to solve my trouble:
>>>
>>> I have to control a DDS chip and I need to set his frequency: I have to
>>> split a
>>> 32 bits word into 4 bytes to manage the frequency into the DDS chip and
>>> to save
>>> in the memory of the PIC (I will use a 16F877 or similar).
>>>
>>>
>>> May somebody help me? I am not asking for the exact assy code; I am
>>> asking for
>>> the right document to read and understand.
>>>
>>> Thank you in advance for your help
>>> Matteo - IV3YNB
>>>
>>>
>>>  Matteo Copetti -- IV3YNB -- JN65vp
>>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>> http://www.iv3ynb.altervista.org
>>> http://iv3ynb.splinder.it
>>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>
>> Example in C:
>>
>> long var;
>> char byte0, byte1, byte2, byte3;
>> ...
>> var    =3D VALUE;
>> byte0 =3D (char)var;
>> byte1 =3D (char)(var >> 8 );
>> byte2 =3D (char)(var >> 16 );
>> byte3 =3D (char)(var >> 24 );
>>
>>
>>
> The way I like to do it in C (and I like types to tell me how many bits
> there are, so I use stuff like uint8_t), is like this:
>
> union{
>  uint8_t byte[4];
>  unint32_t long;
> }value;
> value.long=3D something.
> most significant byte =3D value.byte[3];
> least significant byte =3D value.byte[0];
> since PIC is little endian.


The shift version is portable and doesn't depend on endianess.
Besides, a good compiler will generate the same code for both versions.

Isaac

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