STM8S datasheet(280/467 Pages) STMICROELECTRONICS

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Serial peripheral interface (SPI)

RM0016

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DocID14587 Rev 14

20.3.10

SPI low power modes

Using the SPI to wake up the device from Halt mode

When the microcontroller is in Halt mode, the SPI is still capable of responding as a slave

provided the NSS pin is tied low or the SSI bit is reset before entering Halt mode.

When the first sampling edge of data (as defined by the CPHA bit) is detected:

•

The WKUP bit is set in the SPI_SR register

•

An interrupt is generated if the WKIE bit in the SPI_ICR register is set.

•

This interrupt wakes up the device from Halt mode.

•

Due to the time needed to restore the system clock, the SPI slave sends or receives a

few data before being able to communicate correctly. It is then mandatory to use the

following protocol:

–

A specific value is written into the SPI_DR before entering Halt mode. This value

indicates to the external master that the SPI is in Halt mode

–

The external master sends the same byte continuously until it receives from the

SPI slave device a new value other than the unique value indicating the SPI is in

Halt mode. This new value indicates the SPI slave has woken-up and can

correctly communicate.

Restrictions in receive-only modes

The wakeup functionality is not guaranteed in receive-only modes (BDM = 0 and

RXONLY = 1 or BDM = 1 and BDOE = 0) since the time needed to restore the system clock

can be greater than the data reception time. A loss of data in reception would then be

induced and the slave device can not indicate to the master which data has been properly

received.

20.3.11

SPI interrupts

Table 45. SPI behavior in low power modes

Mode

Description

Wait

No effect on SPI.

SPI interrupt events cause the device to exit from Wait mode.

Halt

SPI registers are frozen.

In Halt mode, the SPI is inactive. If the SPI is in master mode, then

communication resumes when the device is woken up by an interrupt with

“wakeup from Halt mode” capability.

If the SPI is in slave mode, then it can wake up the MCU from Halt mode after

detecting the first sampling edge of data.

Table 46. SPI interrupt requests

Interrupt event

Event

flag

Enable

control

bit

Exit

from

Wait

Exit

from

Halt

Transmit buffer empty flag

TXE

TXIE

Yes

Receive buffer not empty flag

RXNE

RXIE

Yes

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