ST72561J4-Auto datasheet(147/324 Pages) STMICROELECTRONICS

147 / 324 page

ST72561-Auto

Serial peripheral interface (SPI)

Doc ID 12370 Rev 8

147/324

Figure 71.

Single master/ single slave application

14.3.2

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 73).

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 72):

If CPHA = 1 (data latched on second 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

If CPHA = 0 (data latched on first 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)).

Figure 72.

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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