ST7MC2XX datasheet(98/309 Pages) STMICROELECTRONICS

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(cont’d)

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 regis-

ter (see Figure 58).

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.

–SS internal must be held high continuously

There are two cases depending on the data/clock

timing relationship (see Figure 57):

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 manag-

ing the SS function by software (SSM = 1 and

SSI = 0 in the in the SPICSR register)

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 reg-

ister. If SS is not pulled high, a Write Collision

error will occur when the slave writes to the

shift register (see Section 10.4.5.3).

MOSI/MISO

Master SS

Slave SS

(if CPHA = 0)

Slave SS

(if CPHA = 1)

Byte 1

Byte 2

Byte 3

SS internal

SSM bit

SSI bit

SS external pin

ST7MC2XX Datasheet(PDF) 98 Page - STMicroelectronics