STGAP1AS datasheet(50/70 Pages) STMICROELECTRONICS

50 / 70 page

Programming manual

STGAP1AS

50/70

DocID029344 Rev 2

The device register can be written through the WriteReg command when the device is set in

the configuration mode only (refer to Section 9.1.1), otherwise the write command is ignored

and the SPI_ERR flag is forced low.

The WriteReg command is followed by the data to be written into the target register. The

CRC code following the data is based on both command and data bytes. In this way, in case

of communication error, a data byte cannot be decoded as a command and vice-versa (refer

to Section : CRC protection on page 46).

9.1.3

ReadReg command

All the registers of the device can be read anytime, and this requires two accesses (CS must

be asserted LOW and HIGH twice). In the first access the SPI host issues the ReadReg

command (including the register address) and the CRC of the first byte, which will be

ignored if SPI CRC is not enabled. After the command is received and decoded by the

device, the register value and the respective CRC code is prepared for the transmission.

The CRC polynomial used by the device during the transmission is different from the one

used by the host, but the CRC code is not inverted before transmission (refer to Section :

CRC protection

The time required to obtain the reading result changes according to the side where the

The reading of a remote register (isolated side), if no communication error occurs between

the two sides of the device, is available in 30 µs.

After the read result is ready it is stored in the SPI output buffer, and the host MCU will

receive it as soon as it will send a new SPI command. Any command can be used for this

purpose, including a NOP or the ReadReg command for the next register to be read.

Some status and configuration registers contain a reserved bit whose content is not

predictable. In order to clearly identify the content of relevant information, the value read

from each register should be masked with the appropriate masking code (see “Mask code”

in Table 23).

Table 17. ReadReg command synopsis

Byte

3(1)

1. Proper time have to be waited in order to allow the device to prepare the data.

To device

101A AAAA(2)

2. The command byte where AAAAA is the address of the target register.

CCCC CCCC(3)

3. The CRC byte of the command, if the CRC check is disabled this byte is ignored.

0000 0000

CCCC CCCC(4)

4. The CRC byte of the NOP command.

From device

0000 0000

DDDD DDDD(5)

5. Data read from the target register.

KKKK KKKK(6)

6. The CRC byte of the data.

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