Swapping a byte's nibbles

Code C - [expand]
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unsigned char SwapNibbles (unsigned char x)
{
  return ((x >> 4) & 0x0F) | ((x << 4) & 0xF0);
}

Code C - [expand]
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unsigned char ch1, ch2;
ch1 = SwapNibbles(0xA3);  //ch1=0x3A
ch2 = SwapNibbles(ch1);   //ch2=0xA3