AN2960 datasheet(14/41 Pages) STMICROELECTRONICS

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Digital interfaces

AN2960

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Doc ID 15557 Rev 1

4.2

SPI bus interface

The LIS331DL SPI is a bus slave. The SPI allows writing and reading to the registers of the

device.

The serial interface interacts with the outside world through 4 wires: CS, SPC, SPDI and

SPDO.

4.2.1

Read & write protocol

Figure 4.

Read & write protocol

CS is the chip select and is controlled by the SPI master. It goes low at the start of the

transmission and returns high at the end. SPC is the serial port clock and 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. These 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 cases of multiple byte read/write. 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, etc.) 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 the latter case, the chip drives SDO at the start of bit 8.

bit 1: MS bit. When 0, the address remains unchanged in multiple read/write commands.

When 1, the address is auto-incremented in multiple read/write commands.

bit 2-7: address AD(5:0). 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 to 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, further blocks of 8 clock periods are added. When the MS

bit is 0, the address used to read/write data remains the same for every block. When the MS

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

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

SPC

SDI

SDO

AD5 AD4 AD3 AD2 AD1 AD0

DI7

DI6

DI5

DI4

DI3

DI2

DI1

DI0

DO7 DO6 DO5 DO4 DO3 DO2 DO1 DO0

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