2.1 Bitwise

With the bitwise method, the receiver copies the structure of the CRC generator in hard- or software. An algorithm to calculate this could look like this: 1) Initialise CRC Register to low nibble of status register (reversed

(s s 1 s 2 s 3 ‘0000))

2) Compare each (transmitted and received) bit with bit 7 3) If the same: shift CRC register, bit0='0' else: shift CRC register and then invert bit4 and bit5, bit0='1' (see figure 1) 4) receive new bit and go to 2) 5) The CRC value retrieved from the SHTxx must be reversed (bit 0 = bit 7, bit 1=bit 6 … bit 7 = bit 0) and can then be compared to the final CRC value.

(2) XOR XOR

Input bit

XOR

1 2 3 4 5 6 7

Figure 1 Internal structure of the SHTxx CRC-8 generator

2.1.1 Example for bitwise Example 2: RH Measurement (as example in datasheet) Example 1: readout of status register containing 0x40

Inputbit’s
bit7 … bit0 Inputbit’s 0xdecComment
bit7 … bit0 0xdecComment 0000’0000 Start value see below
(1) 0000’0000 Start value see below
(1) 0000’0000 0001
st st nd bit of command 0000’0000 0001
nd bit of command 0000’0000 0002 bit of command 0000’0000 0002 bit of command 0000’0000 000… 0000’0000 000… 0000’0000 000 0000’0000 000 0000’0000 000 0000’0000 000 1 0011’0001 CRC EXOR polynom 1 0011’0001 CRC EXOR polynom 0110’0010 1 0101’0011 11111’0101 F5245CRC after command 11001’0111 97151CRC after command 1101’1011 1
st byte (MSB) of measurement 0001’1111 1
st bit (MSB) of status register 1000’0111 1 0000’1111 0011’1111 0001’1110 0111’1110 0011’1100 1 1100’1101 0111’1000 1010’1011 1111’0000 0110’0111 1101’0001 11111’1111 FF255CRC value 01001’0011 93147Final CRC value 1100’1111 2
nd byte (LSB) of measurement 1010’1111 1 0101’1110 1 1000’1101 0010’1011 0101’0110 1010’1100 10101’1000 5888Final CRC value
(1) (2) Low nibble only, whole byte reversed ( Statusregister = [s 7 s 6 s 5 s 4 ‘s 3 s 2 s 1 s ] -> Startvalue = [s s 1 s 2 s 3 ‘0000] ) This is different to other CRC implementations www.sensirion.comRev 1.07 2/4