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5.2.5 CRC check field
The field allows the error check by master and slave devices. Due to electrical noise
and other interferences, a group of data may be changed while transmitting from
one location to the other. Error Check ensures master or slave devices do not
respond to the distorted data during the transmission, which enhances the system
security and efficiency. Error Check uses 16-bit Cyclic Redundancy Check (CRC 16).
Every message includes an error checking field which is based on the Cyclical
Redundancy Check (CRC) method. The CRC field checks the contents of the entire
message.
The CRC field is two bytes long. The CRC value is calculated by the transmitting
device, and is appended to the message. The receiving device recalculates the CRC
value during reception of the message, and compares the calculated value to the
actual value it received in the CRC field.
CRC check (lo): The bottom (least significant) eight bits of a 16-bit number
representing the error check value.
CRC check (hi): The top (most significant) eight bits of a 16-bit number representing
the error check value
CRC16 calculation:
CRC calculation is first started by preloading the whole 16- bit register to 1’s. The
process begins by applying successive 8-bit bytes of the message to the current
contents of the register. Only the eight bits of data in each character are used for
generating the CRC. Start and stop bits, and the parity bit, do not apply to the CRC.
When generating the CRC, each 8-bit character is exclusive ORed with the register
contents. The result is shifted towards the least significant bit (LSB), with a zero filled
into the most significant bit (MSB) position. The LSB is extracted and examined, if the
LSB equals to 1, the register is exclusive ORed with a preset, fixed value; if the LSB
equals to 0, and no action will be taken.
This process is repeated until eight shifts have been performed. After the last
(eighth) shift, the next 8-bit byte is ex clusive ORed with the register’s current value,
and the process repeats for eight more shifts as described above. The final contents
of the register, after all the bytes of the message have been applied, is the CRC value.