periph/spi: Switching between CPOL=0,1 problems on Kinetis with software CS

[jnohlgard]

(I did not yet verify this with a scope)
The software CS is causing problems when moving between different clock polarity settings.
We discovered that the LIS3DH driver on mulle requires setting the SPI mode to 0, but it should be using mode 3 according to the data sheet, as well as when using the old SPI driver and when using a different driver in Contiki. Clearly there is something different with the new RIOT SPI driver.

The current driver also works in mode 3 if the LIS3DH configuration is changed to use hardware CS on Mulle (PR #6572).

I believe that the problem arises from the fact that the CS pin is asserted before the new clock polarity setting is applied to the SPI module, resulting in the device picking up the clock polarity change as a normal clock pulse, when in fact it should be ignored. However, this does not explain why it used to work in the old periph driver, which also was using software CS where the CS pin was managed by the LIS3DH driver itself instead of deferred to the SPI bus driver.

This needs investigation with a scope or logic analyzer.

[jnohlgard]

Also, the LIS3DH driver works if we retry the who am I query during init, but that's just hiding the problem because the first transfer will have placed the bus in the correct clock polarity for the second transfer.

[haukepetersen]

Will look into this as soon as I can.

[miri64]

@haukepetersen did you?

[PeterKietzmann]

@MrKevinWeiss I just wanted you to be aware of this issue.

[MrKevinWeiss]

Thanks, I have kinetis boards and a scope. I will check it out!

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[kaspar030]

ping @MrKevinWeiss

[MrKevinWeiss]

Darn, I will try to remember when I'm back from vacation.

[PeterKietzmann]

@MrKevinWeiss obviously you didn't

[MrKevinWeiss]

True... I'll check it out, thanks!

[MrKevinWeiss]

I tested on the frdm-kw41z using test/periph_spi at 100 kHz
The procedure is:

> reboot
> init <args>
> send 123

args = init 0 0 0 -1 0 -> Mode 0, hw cs

args = init 0 0 0 2 19 -> Mode 0, sw cs

args = init 0 1 0 -1 0 -> Mode 1, hw cs

args = init 0 1 0 2 19 -> Mode 1, sw cs

args = init 0 2 0 -1 0 -> Mode 2, hw cs

args = init 0 2 0 2 19 -> Mode 2, sw cs

args = init 0 3 0 -1 0 -> Mode 3, hw cs

args = init 0 3 0 2 19 -> Mode 3, sw cs

[PeterKietzmann]

@MrKevinWeiss many thanks! What is your interpretation of the results? On first sight CS behavior looks normal to me, although it returns to high very fast in some configurations. Furthermore, the logic analyzer didn't decode all cases correctly. Did you just miss to switch the decoder mode?

[MrKevinWeiss]

I think there could be the problem when using soft CS and low power mode that @MichelRottleuthner pointed out on another platform (clock ends in the wrong polarity).

My interpretation is:
Mode0: POL=0 PHA=0
Mode1: POL=0 PHA=1
Mode2: POL=1 PHA=0
Mode3: POL=1 PHA=1

Which matches, the decoding was probably just a setting on the scope. My intention wasn't to show the decoding but the signals. Mode 0 and mode 3 both trigger the first rising edge, the only difference is the initial clock level, mode 3 starts high and mode 0 starts low.

I may have missed the initial capture of some of these actually and remember some problems that resulted in sending the command twice.

This may be because in mode 3, the clock pin starts low and when the CS pin goes low the device sees a clock pulse as the clock tries to go to its desired starting position. We don't see this the second time around because the SW CS leaves the clock high.

I would suggest setting the SCK based on the polarity setting during init.

[miri64]

So... is this a bug? Can it be fixed? Did we fix it?