L9950XP datasheet(24/39 Pages) STMICROELECTRONICS

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Functional description of the SPI

L9950 - L9950XP

24/39

Doc ID 10311 Rev 11

Functional description of the SPI

4.1

Serial Peripheral Interface (SPI)

This device uses a standard SPI to communicate with a microcontroller. The SPI can be

driven by a microcontroller with its SPI peripheral running in following mode: CPOL = 0 and

CPHA = 0.

For this mode, input data is sampled by the low to high transition of the clock CLK, and

output data is changed from the high to low transition of CLK.

This device is not limited to microcontroller with a build-in SPI. Only three CMOS-compatible

output pins and one input pin will be needed to communicate with the device. A fault

condition can be detected by setting CSN to low. If CSN = 0, the DO pin will reflect the

status bit 0 (fault condition) of the device which is a logical or of all bits in the status registers

0 and 1. The microcontroller can poll the status of the device without the need of a full SPI

communication cycle.

Note:

In contrast to the SPI standard the least significant bit (LSB) will be transferred first

(see Figure 3).

4.2

Chip Select Not (CSN)

The input pin is used to select the serial interface of this device. When CSN is high, the

output pin (DO) will be in high impedance state. A low signal will activate the output driver

and a serial communication can be started. The state when CSN is going low until the rising

edge of CSN will be called a communication frame. If the CSN input pin is driven above

7.5V, the L9950 will go into a test mode. In the test mode the DO will go from tri state to

active mode.

4.3

Serial Data In (DI)

The input pin is used to transfer data serial into the device. The data applied to the DI will be

sampled at the rising edge of the CLK signal and shifted into an internal 24 bit shift register.

At the rising edge of the CSN signal the contents of the shift register will be transferred to

Data Input Register. The writing to the selected Data Input Register is only enabled if exactly

24 bits are transmitted within one communication frame (i.e. CSN low). If more or less clock

pulses are counted within one frame the complete frame will be ignored. This safety function

is implemented to avoid an activation of the output stages by a wrong communication frame.

Note:

Due to this safety functionality a daisy chaining of SPI is not possible. Instead, a parallel

operation of the SPI bus by controlling the CSN signal of the connected ICs is

recommended.

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