UM1865 datasheet(130/132 Pages) STMICROELECTRONICS

130 / 132 page

SPI interface

UM1865

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DocID027514 Rev 1

5.2.4

SPI operation with BlueNRG-MS sleep mode

The BlueNRG-MS device is designed with a deep sleep mode, in which it turns off most of

the internal functions to save power. The BlueNRG-MS goes into sleep mode automatically

when there is no Bluetooth activity ongoing. So it is very likely that the BlueNRG-MS is

sleeping when the master starts an SPI transaction.

The BlueNRG-MS wakes up as soon as the nCS line is put to low. But then it needs to

initialize and prepare the SPI buffers. This will take some time. So the following sequence is

likely to occur on the SPI bus:

The master puts the nCS line to low and the BlueNRG-MS wakes up. BlueNRG-MS is

expecting a command within 2 ms before going to sleep.

The master sends the header, but the BlueNRG-MS replies with the READY byte as

0x00 or 0xFF. This means the BlueNRG-MS SPI interface is still not initialized.

The master keeps polling the BlueNRG-MS, (releasing and asserting CS again). In

order to read again the header, the SPI master has to release the CS line and assert it

within 2ms from the previous CS assertion. After a while, the READY byte becomes

0x02 but both the buffer sizes are 0x00. This means the BlueNRG-MS SPI is initialized,

but the buffer is not yet prepared.

As soon as the buffer size becomes non-zero, the master can write data to the

BlueNRG-MS.

With the measurement, the master should allow about 0.5 ms for the BlueNRG-MS to be

completely ready for the SPI communication. Note that 0.5 ms is when non-retention RAM

of BlueNRG-MS must be initialized upon wakeup (i.e. device in mode1 - slave only mode).

The value can be 0.3 ms if non-retention RAM initialization is not required, which makes the

SPI ready faster.

This waiting operation is only needed when the SPI bus is idle for a while. If the master is

continuously writing, from the 2nd write transaction there is no need to wait, because the

BlueNRG-MS will not sleep if there is still SPI data to be processed. Also, there is no need

to wait when reading from the BlueNRG-MS, because it cannot sleep if it still holds data for

the master.

UM1865 Datasheet(PDF) 130 Page - STMicroelectronics