ISM303DAC datasheet(46/80 Pages) STMICROELECTRONICS

46 / 80 page

Digital interfaces

ISM303DAC

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DocID030988 Rev 2

6.2

SPI bus interface

The ISM303DAC SPI is a bus slave. The SPI allows writing to and reading from the

registers of the device.

The serial interface interacts with the application using 3 wires: CS, SPC, SDI and SDO

(refer to Table 20).

Figure 16. Read and write protocol

CS is the serial port enable and it is controlled by the SPI master. It goes low at the start of

the transmission and goes back high at the end. SPC is the serial port clock and it is

controlled by the SPI master. It is stopped high when CS is high (no transmission). SDI and

SDO are respectively the serial port data input and output. Those lines are driven at the

falling edge of SPC and should be captured at the rising edge of SPC.

Both the read register and write register commands are completed in 16 clock pulses or in

multiples of 8 in case of multiple read/write bytes. Bit duration is the time between two falling

edges of SPC. The first bit (bit 0) starts at the first falling edge of SPC after the falling edge

of CS while the last bit (bit 15, bit 23, ...) starts at the last falling edge of SPC just before the

rising edge of CS.

bit 0: RW bit. When 0, the data DI(7:0) is written into the device. When 1, the data DO(7:0)

from the device is read. In latter case, the chip will drive SDO at the start of bit 8.

bit 1-7: address AD(6:0). This is the address field of the indexed register.

bit 8-15: data DI(7:0) (write mode). This is the data that is written into the device (MSb first).

bit 8-15: data DO(7:0) (read mode). This is the data that is read from the device (MSb first).

In multiple read/write commands additional blocks of 8 clock periods will be added. For the

accelerometer when the CTRL2_A (21h) (IF_ADD_INC) bit is ‘0’, the address used to

read/write data remains the same for every block. When the CTRL2_A (21h) (IF_ADD_INC)

bit is ‘1’, the address used to read/write data is increased at every block.

The function and the behavior of SDI and SDO remain unchanged.

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