ST7DALIF2 datasheet(102/171 Pages) STMICROELECTRONICS

102 / 171 page

Serial peripheral interface (SPI)

ST7DALIF2

102/171

Figure 46.

Single master/ single slave application

17.4.1

Slave select management

As an alternative to using the SS pin to control the Slave Select signal, the application can

choose to manage the Slave Select signal by software. This is configured by the SSM bit in

the SPICSR register (see Figure 48)

In software management, the external SS pin is free for other application uses and the

internal SS signal level is driven by writing to the SSI bit in the SPICSR register.

In Master mode:

SS internal must be held high continuously.

In Slave mode:

There are two cases depending on the data/clock timing relationship (see Figure 47):

If CPHA=1 (data latched on 2nd clock edge):

–SS internal must be held low during the entire transmission. This implies that in

single slave applications the SS pin either can be tied to VSS, or made free for

standard I/O by managing the SS function by software (SSM= 1 and SSI=0 in the

in the SPICSR register)

If CPHA=0 (data latched on 1st clock edge):

–SS internal must be held low during byte transmission and pulled high between

each byte to allow the slave to write to the shift register. If SS is not pulled high, a

Write Collision error will occur when the slave writes to the shift register (see Write

collision error (WCOL) on page 106).

Figure 47.

Generic SS timing diagram

8-BIT SHIFT REGISTER

SPI

CLOCK

GENERATOR

8-BIT SHIFT REGISTER

MISO

MOSI

MISO

SCK

SLAVE

MASTER

+5V

MSBit

LSBit

MSBit

LSBit

Not used if SS is managed

by software

MOSI/MISO

Master SS

Slave SS

(if CPHA=0)

Slave SS

(if CPHA=1)

Byte 1

Byte 2

Byte 3

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