Update WS2812X ESP32 RMT timing for WS2815 compat

[rmounce]

Context

I'm using WLED and have been trying to troubleshoot flickering issues for a while now with my WS2815 LED strips from BTF-Lighting. I have about 6 metres of cable between my controller and each strip, and have taken the following measures so far:

• Added a 74AHCT125 level shifter
• Connected 5V supply directly to 5V pin on my ESP32 dev board (avoids voltage drop across input protection diode on MicroUSB input?)
• Add 110Ω / 220Ω series resistors in series with each output from the level shifter. The resistors are individually matched to each output using an oscilloscope.

These efforts cleaned up the data signal and reduced flickering considerably, but it never went away until I upgraded to WLED 0.15 beta. This finally fixed the flickering for one of my two LED strips! A bit more investigation showed that WLED changed to I2S by default for the first output, which has different timing to RMT. And reading the WS2815 datasheet showed that the RMT timing for WS2812B is out of the specs for WS2812.

Commit Summary

The existing values are out of spec for WS2815 which results in flickering, especially on longer cable runs.

These new RMT values are still closer to ideal WS2812B specs than the current I2S WS2812X timing (about 310us / 940us).

Datasheet specs:		T0H	T1H	T0L	T1L
WS2812B	250-550	650-950	700-1000	300-600
WS2815	220-380	580-1600	580-1600	220-420

Note for ESP32 WLED users interested in this PR

To resolve flickering issues in the meantime, upgrade to WLED 0.15 beta. For more than 1 output, this also adds APA106 support which uses compatible timings with WS2815 on the second output.

WLED 0.15 with WS2815 LED	Mode WS281X	Mode APA106
Output 1 (I2S)	Good	Bad
Output 2+ (RMT)	Bad	Good

[Makuna]

Further checks, While the WS2805/WS2814 might function, the WS2815 might have the same issue as previous WS2812 chips, in that the overall kbps must be no faster than 800kbps. Looking at the very latest update on the spec sheet, they added an addition that was not present before in the spec sheets I have. TDATA Data cycling time ≥1.25µs.
Normally this is defined by a line stating min/max data rate; but here they instead defined it by stating the total bit time must equal or exceed 1.25us.
With the WS2812, being even slightly faster than 800kbps caused issues only after approximately 60 leds in the strip, so after that 60 flickering would happen. If this holds for the WS2815, then a new speed will be needed unique to current sets. Esp32 I2s is already timed to 25% of the pulse, so 312us, but it is not accurate enough to be consistently below 400us.

[Makuna]

The Tm1914 which is already present is close, also try it. Increasing its reset to 300us and aliasing it to WS2815 would make it fully compatible.

[rmounce]

Hmm you're right with the newer WS2815 datasheet, they seem to be very inconsistent. I'll do some more research to summarise these and do some more scientific testing to determine the limits of my WS2815.

I haven't tried Tm1914 yet but I did try NeoEsp32RmtSpeedWs2811 which appeared to be even closer to the specs than Tm1914, but there was still flickering.

So my testing so far:

Stable

• NeoEsp32I2sSpeedWs2812x 312/938us, 938/312us
• NeoEsp32RmtSpeedApa106 350/1350us, 1350/350us

Unstable, occasional flickering

• NeoEsp32RmtSpeedWs2811 300/950us, 900/350us
• NeoEsp32RmtSpeedWs2812 400/850us, 800/450us

[rmounce]

Datasheet summary

	T0H	T1H	T0L	T1L	Tdata
WS2812B	250-550	650-950	700-1000	300-600
WS2811/WS2815	220-380	580-1600	580-1600	220-420
WS2815B-V1 2.0	220-380	580-1000	580-1000	580-1000	<=1250us

I noticed that WS2811 has exactly the same timing spec as the earlier WS2815 variants. I'm ignoring the T1L value for WS2815B-V1 2.0 and assuming that this is actually 220-420 like WS2811.

Additional testing

Constant 1250us Tdata, T1L = T0H+50

T0H	T1H	T0L	T1L	Tdata	Result
190	1010	1060	240	1250	Occasional corruption (every ~30 seconds)
230	970	1020	280	1250	Occasional corruption (every ~60 seconds)
270	930	980	320	1250	Stable (short test)
410	900	800	460	1250	Regular corruption (every few seconds)
430	770	820	480	1250	Constant corruption

I only realised after this testing that there is a 25us quantum with ESP32 RMT, so I think all of these values were actually rounded down to the nearest 25us resulting in a Tdata of 1225us for each test. Regardless, I think this confirmed the WS2812 RMT timing is extremely marginal with WS2815 LEDs and going slightly further out of spec by increasing T0H & T1L resulted in much more corruption.

Constant 350us T0H, 400us T1L

T0H	T1H	T0L	T1L	Tdata	Result
350	750	800	400	1150	Occasional corruption (every ~30 seconds)
350	800	850	400	1200	Stable (short test)
350	850	900	400	1250	Stable
350	900	950	400	1300	Stable

Constant 300us T0H, 350us T1L

T0H	T1H	T0L	T1L	Tdata	Result
300	750	800	350	1100	Stable
300	800	850	350	1150	Stable
300	850	900	350	1200	Stable
300	900	950	350	1250	Stable (default WS2811 RMT timing)

I think my initial test with NeoEsp32RmtSpeedWs2811 was flawed and that the corruption I saw was probably due to WiFi interrupt shortly after WLED startup, or I just made a typo. The datasheets & further testing show that the existing WS2811 timing seems to be good for my WS2815, and also within the published specs for WS2815B-V1 2.0 (ignoring an obviously wrong value in the datasheet).

So maybe there is not much to do here except add an alias from WS2815 to WS2811?

Instead I will turn my attention to WLED and try authoring a PR to split their "WS281X" mode into:

• WS2812X
• WS2811 / WS2815

[Makuna]

Yeah, I will be adding a WS2815 aliases into NeoPixelBus, see newly created issue to track this.

But in general, WLED will be minimizing the number of specific methods they use to a set that are considered "compatible" to reduce the option matrix, so in the future they may expose WS2815 but may only map to the WS2811 anyhow.

[blazoncek]

Just to chime in regarding WLED. We are constantly battling image size vs. feature set availability. Recent additions to WLED have pushed image size to nearly 100% available application partitions on ESP32 while we are still with ancient PlatformIO platform Espressif32 3.5.0. Updating to a newer platform increases image size by nearly 100kB which no longer fits on existing partitions.

@rmounce I've talked to @Makuna about the reduction of methods used to save some additional RAM and flash. Introducing new methods - or better - adding variation of existing methods will be very low on TODO list for WLED. PR or not. Exception being benign code size increase.

@Makuna would there be a simpler way to call some base class virtual functions instead of what we do now? I.e.:

case I_32_RN_NEO_3: (static_cast<NeoPixelBusLg<NeoGrbFeature, NeoEsp32RmtNWs2812xMethod, NeoGammaNullMethod>*>(busPtr))->Show(consistent); break;

[Makuna]

@Makuna would there be a simpler way to call some base class virtual functions instead of what we do now? I.e.:

case I_32_RN_NEO_3: (static_cast<NeoPixelBusLg<NeoGrbFeature, NeoEsp32RmtNWs2812xMethod, NeoGammaNullMethod>*>(busPtr))->Show(consistent); break;

Virtual functions add code and heap usage. The model I went with reduces it, so the library works on small AVRs (ATTINY with between 64-512 bytes of total RAM for stack and heap) the original target I built the library for. WLED and a few others are unique in that they require dynamic reconfiguration across all features. Most users don't need this.

I am investigating into "virtual" Features and Methods for those platforms that can dismiss the loss of 30 to 300 bytes of RAM and code space (because they would already spend it working around the lack of dynamic configuration). But some "reconfiguration" will still require memory churn, going from RGB to RGBW or even WS2812x to SM168xx requires back buffers to be released and reallocated. Not to mention one wire (NeoPixel) vs two wires (Dotstars).

[troyhacks]

Just to toss in on the "overall kbps must be no faster than 800kbps" timing, my I don't know what I'm doing experiments with NeoPixelBus show I can push RMT at 1.333MHz and it's stable... enough? These are allegedly WS2812b pixels, according to the AliExpress listing.

I also shortened the break, which I found entirely too long and that helps loop around faster too. 208 FPS on 256 pixels, 52 here on 1024 pixels in a chain. Probing the end of the chain show the "clock" was propagated all along the panels.

https://github.com/Makuna/NeoPixelBus/assets/5659019/c7dfddb6-fc56-4e28-a83d-303d03195ffd

[Makuna]

Just to toss in on the "overall kbps must be no faster than 800kbps" timing, my I don't know what I'm doing experiments with NeoPixelBus show I can push RMT at 1.333MHz and it's stable... enough? These are allegedly WS2812b pixels, according to the AliExpress listing.

Individual experience on the reset timing will vary, as true/real WS2812x (b,c,etc) officially need at least 280us. Clones/fakes can be all over the place, from 50us to 300us. This is why it was set to 300us.

The issue with overall speed comes from longer strips. If you have 300 pixels, with official/real WS2812x you will see glitches after about 60 pixels. And this was running at 833Kbps.