ws2812bdemo having issues with string lengths >5 and <10

[romain145]

First of all, thanks for the great lib and examples! I'll try to contribute starting with this issue, and maybe a fix if that's at my level!

So, I noticed this strange issue of the SPI having different behaviour when changing the LED count.
Here are the scope captures with NR_LEDS set from 1 to 11:

For NR_LEDs between 6 and 9 the SPI behaviour is different, which causes issues with the WS2812B I'm using.

I noticed this in ws2812b_dma_spi_led_driver.h:

// Note first n LEDs of DMA Buffer are 0's as a "break"
// Need one extra LED at end to leave line high. 
// This must be greater than WS2812B_RESET_PERIOD.
#define WS2812B_RESET_PERIOD 2

What is the reason for keeping the line high? The 2812B's datasheet specifies that the line should be low for at least 50us after the last bit is sent.

[Randomblock1]

I was also running into this issue with 8 LEDS in the demo. If NR_LEDS is 7-9, LEDs 7 and 8 did not light up. Interestingly, it started working when NR_LEDS was >9, so I could just ignore the nonexistent LEDs. However, the better solution was to change DMALEDS in the driver code to literally any other valid value, since there's a buffer of 2, I chose 12. Somehow that worked.

Following the last guy's comment about putting the line low, I can keep everything as it was before but set the WS2812B_RESET_PERIOD to 0 and have the LEDS work fine as long as I set the SPI line low in the beginning of DMA start. If I don't set it low then the brightness is incorrect.

This is my first experience with this chip and this kind of C code so IDK what exactly is wrong, but there's clearly a bug in the DMA driver somewhere. Setting the SPI line low seems to fix it, IDK if that's the real solution but it works for me.