VG6640 datasheet(30/126 Pages) STMICROELECTRONICS

30 / 126 page

Control interface

VG6640, VD6640

30/126

DocID023597 Rev 11

4.1

Any internal register that can be written to, can also be read from. There are also read only

registers that contain device status information, for example, design revision details.

A read instruction from an unused register location returns the value 0x00. A read

instruction from a reserved address may return any value.

A write instruction to a reserved or unused register location is illegal and the effect of such a

write is undefined.

A read or write to an unused register location must not cause the I2C read or write message

to be aborted. Therefore an I2C read or write message to an unused register location must

complete in the same way as if the read or write message had addressed a used register

location.

It is the responsibility of the companion device to only write to register locations which have

been defined.

Note that some registers marked as Read Only (RO) or Do Not Write (DNW) can be written

to and may read back with the data written but may also have unpredictable effects on the

operation of the sensor.

4.1.1

Data alignment within registers

Registers that are larger than 8 bits long are stored most significant byte first, i.e the MSB is

placed in the location with the lowest index.

If the width of an internal register is narrower than the 8-bit, 16-bit or 32-bit register which

reports it’s value, then the register value is right aligned within the register and the unused

MS bits are padded with zeroes.

Figure 15. Right alignment for packing 10-bit data into two 8-bit registers

6 5 4

9 8

6 5 4

Hi Byte (MSB)

Lo Byte (LSB)

Internal 10-bit Register Value

Top bits

padded with

zeroes

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