RM0312 datasheet(217/275 Pages) STMICROELECTRONICS

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RM0312

Serial peripheral interface (SPI)

223

Overrun condition

An overrun condition occurs, when the master device has sent data bytes and the slave

device has not cleared the RXNE bit resulting from the previous data byte transmitted.

When an overrun condition occurs:

•

OVR bit is set and an interrupt is generated if the ERRIE bit is set.

In this case, the receiver buffer contents will not be updated with the newly received data

from the master device. A read to the SPI_DR register returns this byte. All other

subsequently transmitted bytes are lost.

Clearing the OVR bit is done by a read access to the SPI_DR register followed by a read

access to the SPI_SR register.

20.3.9

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.

Table 42. 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.

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