Stm32 Spi Slave - How to detect Chip Select Edges?

[st239955_stm1_stmicro_com]

Posted on October 09, 2014 at 00:36

Using:

• Stm32F10x, F2xx, F4xx
• gcc-arm-none-eabi-4_8-2014q2

On my Stm32, I've running a spi slave and serving events (RX/TX) within an ISR.

Slave select management is configured to hardware:

    ...

    spiInitStruct.SPI_Mode = SPI_Mode_Slave;

    spiInitStruct.SPI_NSS = SPI_NSS_Hard;

    ...

This works fine so far. My ISR is called in case of RXNE and TXE events.

Now, I wan't also to be notified each time the chip select goes active or inactive.

Would that also be handled over the spi module (same ISR) or do I have to setup the chip select pin (e.g. GPIOB, GPIO_Pin_12 for SPI2) for interrupt handling with an additional ISR (and if so, how to do this)?

#ssi #spi #stm32

[Tesla DeLorean]

Posted on October 09, 2014 at 00:54

Don't know, EXTI?

[st239955_stm1_stmicro_com]

Posted on October 09, 2014 at 01:00

Does this mean, that I definitively must configure the chip select input for interrupt and set up an additional ISR?

[Tesla DeLorean]

Posted on October 09, 2014 at 01:16

It means I don't know....

The Hard NSS suggests a gating to cause the RXNE N-cycles into the transaction, ie slave accepting input based on a low -CS

If you want to get close and personal with the edges of the -CS pin, then I think you're going to have to use an EXTI based method to achieve that. It's not something I have tried, or plan on trying.

[st239955_stm1_stmicro_com]

Posted on October 09, 2014 at 06:15

Ok, I got it working using EXTI for now.

Unfortulately, because of SPI2's select is on PB12, there is no EXTI12_IRQn but I have to use EXTI15_10_IRQn instead.

Btw: I'm mainly interested if the chipselect has been deactivated between calls of my SPI2_IRQHandler(). So I don't really need a ISR call for every chipselect edge.

For this I'd now tried to work without EXTI15_10_IRQHandler(). Using EXTI_Mode_Event instead of EXTI_Mode_Interrupt and hoping that I could check some kind of »event bit« inside of my SPI2_IRQHandler().

But in the case of using EXTI_Mode_Event, there seems no event bit which I could read out (I'd also checked out

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/reference_manual/DM00031020.pdf

, pages 381 ff.).

Well, I could use EXTI_Mode_Interrupt and simply not enable the related EXTI15_10_IRQn. Now I can check EXTI_GetITStatus(EXTI_Line12) (and clear pending bit afterwards) and this works as expected.

But there's also a disadvantage: If I later want to use e.g. EXTI_Line11 for calling an isr, this isr will also called every time a transition occours on my chipselect line (permanently if the pending bit was not cleared inside of the isr). So, it seems to be a bad idea to do such things.

http://lmgtfy.com/?q=EXTI_Mode_Event

brings quite less helpful results and there are also no code examples inside of the standard peripheral library. My impression is, that EXTI_Mode_Event was only for waking up the Stm32 from sleep mode - nothing more or less. No way to check if such an event has been occoured or not (isn't it?).

[waclawek.jan]

Posted on October 09, 2014 at 10:56

PB12 is TIM1_BKIN and I2C2_SMBA, both should be usable as interrupt sources if those peripherals are unused. I admit that it's an unorthodox solution and there may be bumps ahead.

JW

[st239955_stm1_stmicro_com]

Posted on October 09, 2014 at 14:52

> … it's an unorthodox solution and there may be bumps ahead.

Within my application, the edges of the chip select are indicators for start and end of a transmission of a bunch of data. I think it's very common to use a chip select activation as some kind of »transmission start« when communicate over SPI.

May you know a better solution to detect chip select transitions between SPI2_IRQHandler() calls?