SPI slave mode configuration requires user definition of clk rate

[pxia.1]

 in stm32cubeMX configuration of SPI in slave mode, it asks for user parameter input for clk prescaler, which then derives the clk rate. My understanding for SPI operated in SLAVE mode is that the master end, instead of the slave end, generates the clock.

Is there a way to better understand the purpose of this setting, highlighted in the screenshot? Thanks.

[KnarfB]

Cannot confirm your observation for STMCubeMX 5.4.0 and the few chips I have tried.

[pxia.1]

Thanks for looking into this problem. Here is my environment:

STM32H742ZGTx Device from STM32H7 series

Linux version CubeMX

• Version: 1.1.0
• Build: 4551_20191014-1140 (UTC)

[S.Ma]

For a slave, its maximum functional baud rate depends on its clock (hence its prescaler).

Typically if the SPI is programmed as master 10 MHz SCK frequency, if you turn it as slave with same prescaler settings, the slave will work with max 5MHz.

[waclawek.jan · Accepted solution]

> For a slave, its maximum functional baud rate depends on its clock (hence its prescaler).

I don't think this is the case. In slave mode, maximum input frequency is fPCLK/2, regardless of baudrate setting.

However, specifically in TI mode (and that appears to be selected in the OP), the baudrate setting determines the timing for threestating MISO, see the SPI chapter, SPI TI protocol in slave mode subchapter, in RM

JW

[pxia.1]

Thanks to everyone here to help me understand this issue from different angles 🙂

In the RM, SPI specific modes and control -- TI mode, here is the rationale to set the "Baud rate", now I understand it is in fact a signal integrity timing choice.

"In slave mode, the clock generator is used to define time when the slave output at MISO pin becomes to HiZ when the current transaction finishes. The master baud setting is applied and any baud rate can be used to determine this moment with optimal flexibility."