SeeedStudio ORCH S4 4-in-1 Weather Sensor (Temperature/Humidity/Pressure/Light) : User Guide : Page 14

User Guide

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0xBE, 0x7E, 0x7F, 0xBF, 0x7D, 0xBD, 0xBC, 0x7C, 0xB4, 0x74, 0x75, 0xB5,

0x77, 0xB7, 0xB6, 0x76, 0x72, 0xB2, 0xB3, 0x73, 0xB1, 0x71, 0x70, 0xB0,

0x50, 0x90, 0x91, 0x51, 0x93, 0x53, 0x52, 0x92, 0x96, 0x56, 0x57, 0x97,

0x55, 0x95, 0x94, 0x54, 0x9C, 0x5C, 0x5D, 0x9D, 0x5F, 0x9F, 0x9E, 0x5E,

0x5A, 0x9A, 0x9B, 0x5B, 0x99, 0x59, 0x58, 0x98, 0x88, 0x48, 0x49, 0x89,

0x4B, 0x8B, 0x8A, 0x4A, 0x4E, 0x8E, 0x8F, 0x4F, 0x8D, 0x4D, 0x4C, 0x8C,

0x44, 0x84, 0x85, 0x45, 0x87, 0x47, 0x46, 0x86, 0x82, 0x42, 0x43, 0x83,

0x41, 0x81, 0x80, 0x40

};

unsigned short usCRC16( unsigned char * pucFrame, unsigned short usLen )

{

unsigned char ucCRCHi = 0xFF;

unsigned char ucCRCLo = 0xFF;

int iIndex;

while( usLen-- )

{

iIndex = ucCRCLo ^ *( pucFrame++ );

ucCRCLo = ( UCHAR )( ucCRCHi ^ aucCRCHi[iIndex] );

ucCRCHi = aucCRCLo[iIndex];

}

return ( unsigned short )( ucCRCHi << 8 | ucCRCLo );

}

The CRC generated by this function has exchanged high and low bytes and can be sent directly into a

message.

For example: The CRC16 of a certain frame is calculated by this function to be equal to 0x4112, then the

message is placed as follows:

AA

03

RRRR

NNNN

CC

1 Byte

2 Bytes

1 Byte

Slave address,

range 0-247

Function Code is

3

Start register

address, high

byte first

The number of

registers to read,

high byte first

CRC check, low

byte 0x41

CRC check,

high byte

0x12