stm32g0b1: spi: MOSI signal goes high before and after transmitting data

[simonguinot]

I noticed this issue while using the ws2812_spi LED strip driver on a nucleo_g0b1re board.

The led_ws2812 sample can be used to reproduce the issue. This patch is needed to add support for the nucleo_g0b1re board to the led_ws2812 sample.

When running this sample, then the MOSI signal goes high for 80 microseconds before and after transmitting data:

The issue is that this high "preamble" state is interpreted as a high bit by the first WS2812 LED of the strip. And then the signal of all the LEDs is shifted by one bit, which results in visual glitches.

Note that from a SPI protocol standpoint, the MOSI signal is correct because the clock signal is off during this extra "high" states. Thus, this won't affect a real SPI device. But the WS2812 strip LED controller is not a SPI device. It doesn't use the clock signal. SPI is used here as a hack on a single wire (MOSI) to generate a control signal for the LEDs.

In addition, the issue is not systematic. It depends on the last data sent over MOSI. If the last 4 bytes are zero, then there are no high "preamble" state when sending the next buffer of data.

I also tried disabling DMA and interrupts but this doesn't affect the issue.

[simonguinot]

Hi @erwango. Maybe do you have some ideas about how to fix this behaviour ?

[erwango]

@simonguinot Do you have more info to characterize this bug (G0 only, Zephyr only, long preamble also present with other SPI devices .. ) ?

[FRASTM]

I can see the the application running on g071rb nucleo board.
west build -p auto -b nucleo_g071rb samples/drivers/led_ws2812/
setting default frame format will display R, G, B colors

 		spi-one-frame = <0x70>;
		spi-zero-frame = <0x40>;

With

#define B1414_ZERO_FRAME 0xC0
#define B1414_ONE_FRAME   0xF0

--> another pattern, MOSI high during 80µs is visible when CONFIG_SPI_STM32_DMA=y
Without the CONFIG_SPI_STM32_DMA=y, all leds are white and high level of MOSI is around 50µs

[simonguinot]

Hi @FRASTM,

Thanks for testing.

I can see the the application running on g071rb nucleo board.
west build -p auto -b nucleo_g071rb samples/drivers/led_ws2812/
setting default frame format will display R, G, B colors

 		spi-one-frame = <0x70>;
		spi-zero-frame = <0x40>;

OK. And do you have a high "preamble" or not on the MOSI signal ?

--> MOSI is at high level during 60µs

With this sample, you won't see many visual artifacts due to the extra bit. So it may seem OK visually while it is not.

Please have a look at the signal.

If you need, I can provide a sample with a visual feedback when the bug is here.

With

#define B1414_ZERO_FRAME 0xC0
#define B1414_ONE_FRAME   0xF0

--> white led

The ws2812_spi driver supports two different devices: WS2812 and B1414. They have different timings. The definition above is for B1414 devices and a 4MHz SPI frequency. So if you have a WS2812 device, or if you are using a different frequency, then this definition is not correct. If you need I can help to provide a correct definition for your configuration.

[simonguinot]

@simonguinot Do you have more info to characterize this bug (G0 only, Zephyr only, long preamble also present with other SPI devices .. ) ?

Hi @erwango.

Thanks for looking into this. Unfortunately I don't have much more information.

I can tell I never experimented this issue with F0 MCUs.

I don't have built a firmware using STM32Cube (because I don't have the setup), so I don't know if the issue is rather in Zephyr, or in the HAL/SDK, or in the SPI controller itself.

Also I tried to play with the pinmux and the SPI mode configuration, but it didn't work.

Because the last data transmitted affects the issue, I suspect it could be something related with the TXFIFO and the SPI initialization/transmission.

And that's pretty much all I have.

[FRASTM]

If I keep the definition for the ws2812, the MOSI high level is seen during
~60µs when CONFIG_SPI_STM32_DMA=y
~30µs when CONFIG_SPI_STM32_DMA=n

[FRASTM]

MOSI signal on nxp FRDM_k64f board when testing led samples/drivers/led_ws2812/