STM8S datasheet(277/467 Pages) STMICROELECTRONICS

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RM0016

Serial peripheral interface (SPI)

287

20.3.7

Status flags

There are three status flags to allow the application to completely monitor the state of the

SPI bus.

Tx buffer empty flag (TXE)

When set, this flag indicates that the Tx buffer is empty and that the next data to be

transmitted can be loaded into the buffer. The TXE flag is reset when writing the SPI_DR

Rx buffer not empty (RXNE)

When set, this flag indicates that there is a valid received data in the Rx buffer. This flag is

reset when SPI_DR is read.

Busy flag (BSY)

This BSY flag is set and reset by hardware (writing to this flag has no effect). The BSY flag

indicates the state of the communication layer of the SPI.

When BSY is set, it indicates that the SPI is busy communicating. There is one exception in

master mode / bidirectional receive mode (MSTR=1 and BDM=1 and BDOE=0) where the

BSY flag is kept low during the reception.

The BSY flag is useful to detect the end of a transfer if the software wants to disable the SPI

and enters Halt mode (or disable the peripheral clock). This will avoid corrupting the last

transfer. For this, the procedure described below must be strictly respected.

The BSY flag is also useful to avoid write collisions in a multimaster system.

The BSY flag is set when a transfer starts with the exception of master mode / bidirectional

receive mode (MSTR = 1 and BDM = 1 and BDOE = 0).

It is reset:

•

when a transfer is finished (except in master mode if the communication is continuous)

•

when the SPI is disabled

•

when a master mode fault occurs (MODF = 1)

When communication is not continuous, the BSY flag is low between each communication.

When communication is continuous, in master mode, the BSY flag is kept high during the

whole transfers.

When communication is continuous, in slave mode, the BSY flag goes back to low state for

one SPI clock cycle between each transfer.

Note:

Do not use the BSY flag to handle each data transmission or reception. It is better to use

TXE and RXNE flags instead.

20.3.8

Disabling the SPI

When a transfer is terminated, the application can stop the communication by disabling the

SPI peripheral. This is done by resetting the SPE bit.

For some configurations, disabling the SPI and entering Halt mode while a transfer is on-

going, can cause the current transfer to be corrupted and/or it can happen that the BSY flag

becomes unreliable.

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