STGAP1S datasheet(45/67 Pages) STMICROELECTRONICS

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STGAP1S

SPI interface

CRC protection

All the command and data bytes have to be followed by a CRC code. If the CRC_SPI bit is

set high, this code is used to check the data byte is correct, otherwise the CRC byte is

ignored. In this case the CRC byte must be transmitted by the host, but its value is

unimportant.

A failure on the CRC check causes the respective data byte is ignored and the SPI_ERR

flag is set high.

The polynomial generator of the CRC code is X8 + X2 + X + 1 corresponding to the block

diagram in Figure 23.

Figure 23. Block diagram of the CRC generator

The host must transmit to the device the inverted CRC code computed using the following

procedure:



Initialize CRC to all 1



Start the calculation from the most significant bit of the message



Invert the CRC result

In case of a WriteReg command, the CRC of the data byte (i.e.: the new register value) must

be calculated initializing the computation system to the CRC of the command byte (i.e.: the

CRC is calculated on a 16-bit message composed by the command + data byte). This way

a data byte cannot be accepted as a command byte and vice-versa. Some examples are

listed in Table 11.

The device always transmits a response byte followed by a CRC computed using the same

polynomial generator (X8 + X2 + X + 1). The CRC byte transmitted by the device is not

inverted.

If no response is required, the word returned by the device has no meaning and it should be

discarded. Some examples are listed in Table 12.

Message

(from MSb to LSb)

Table 11. CRC byte examples (from host to device)

Command

Command byte

Command CRC

Data byte

Data CRC

StopConfig

0x3A

0xAA

N.A.

WriteReg(CFG1, 0x20)

0x8C

0xA1

0x20

0x82

WriteReg(CFG5, 0x06)

0x99

0xCA

0x06

0x66

ResetStatus

0xD0

0x32

N.A.

ReadReg(CFG3)

0xBE

0x3F

N.A.

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