SM32F411, SPI slave without NSS still receives data

[BSchm.6]

Hi,

I have configured an SPI as receive-only slave in CubeMX and have set NSS to "Hardware input signal":

From logical point of view I now would expect, that there are no SPI data received, as long as NSS stays at high.

However, the CubeMX-generated code still jumps into the SPI-ISR SPI1_IRQHandler() also when NSS is not at LOW.

So my question: what am I doing wrong? How do I have to configure the SPI to respect the NSS/SYNC signal on receiving?

Thanks!

[waclawek.jan]

Which pin is set as NSS, how exactly is it set in GPIO, and what's its is state?

Generally, read out and check SPI and related GPIO registers content.

JW

[BSchm.6]

In case of SPI1, the NSS-pin is PA4. But this is not a GPIO-pin, CubeMX deals with it as SPI-NSS-pin where no possibilities exist to have the generic GPIO settings. And within the SPI-settings I can not change any parameters for this pin except than turning NSS on or off ("NSS signal type" is "Input Hardware" as shown in image above).

[waclawek.jan]

How is it connected? Read out its level in GPIO_IDR. Measure voltage directly on the pin. Read our and check SPI registered for other SS-related bits.

JW

[BSchm.6]

Once again: the NSS-input is NOT an GPIO-pin but a SPI-signal which should be handled by the SPI-hardware! So reading the GPIO-registers for PA4 does not make any sense as the GPI is set to "alternate function" and therefore this register does not reflect the state of the NSS-pin!

The problem is the SPI-hardware not handling the NSS-signal according to what it should do, this SPI-related and has nothing to do with some GPIOs.

In SR-register the flags BSY, OVR (may be because the software is halted in debugger while data transmission is still running), TXE and RXE are set.