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#include
#define u8     unsigned char
#define u16     unsigned int

unsigned char CalculateChecksum(const u8 *pData, u16 Length)
{
   u8 byChecksum = 0xff;
   u16 i;
   for (i = 0; i       byChecksum ^= *(pData+i);
   return byChecksum;
}

void main()
{
unsigned char checksum;
unsigned char aa[]={0x04, 0x00, 0x1E, 0x20};
checksum = CalculateChecksum(aa,4);
printf("%x",checksum );

}