*ptr++ = (uint8_t)((crc) >> 8);
    *ptr = (uint8_t)crc;

    *ptr++ = (uint8_t)crc;
    *ptr = (uint8_t)((crc) >> 8);

    *ptr++ = (uint8_t) ~((crc) >> 8);
    *ptr  = (uint8_t) ~crc;

#include <util/crc16.h>

/* Типа пример!

Force Multiple Coils (FC=15)

Request

This command is writing the contents of a series of 10 discrete coils from #20 to #29
to the slave device with address 17.

11 0F 0013 000A 02 CD01 BF0B

11: The Slave Address (17 = 11 hex)
0F: The Function Code (Force Multiple Coil, 15 = 0F hex)
0013: The Data Address of the first coil. (coil# 20 - 1 = 19 = 13 hex)
000A: The number of coils to written (10 = 0A hex)
02: The number of data bytes to follow (10 Coils / 8 bits per byte = 2 bytes)
CD: Coils 20 - 27 (1100 1101)  
01: Coils 27 - 29 (0000 0001)  
BF0B: The CRC (cyclic redundancy check) for error checking.
*/

unsigned char buff[11] = {
  0x11, 0x0F, 0x00, 0x13, 0x00, 0x0A, 0x02, 0xCD, 0x01, 0xBF, 0x0B
};

volatile unsigned short CRC = 0xFFFF;
volatile unsigned short CRC1;

int main(void)
{
  unsigned char i;

  for (i = 0; i < 9; i++)
  {
    CRC = _crc16_update(CRC, buff[i]);
  }
  
  CRC1 = CRC;

  asm("nop");    // <-------- вот здесь CRC1 == 0x0BBF

  CRC = _crc16_update(CRC, buff[9]);
  CRC = _crc16_update(CRC, buff[10]);

  asm("nop");   // <--------- вот здесь CRC == 0

  while (1);
  return 0;
}