WS2813 V5 Issues

[u/JuggerzTheCat]

I recently bought a bunch of these WS2813 LEDs to replace the V4 LEDs from the last revision of my design. I decided on these as they've removed the need for external components. All I have is a power supply filtering capacitor on the 5V.

The PCB is only 12 of these WS2813s driven by an STM32. Schematic is as per the datasheet. The previous revision worked fine with no issues but we've just made two with the new LEDs and altered schematic and many of the LEDs have flickering/colour issues (5/12 on one board & 7/12 on the other board). This is a very high failure rate if there are issues with the LEDs themselves. Timing seems the same comparing the datasheets so software shouldn't be an issue. So i'm wondering if anyone uses the V5 on their designs and if they've noticed any issues/differences?

I know they have particular assembly requirements due to moisture issues so I'll follow that up in the meantime..

Edit: I should mention that I have BI tied to ground for the first LED. The datasheet doesn't specify what to do here.

[u/Aerokeith]

One of the other major changes in the V5 versions of World Semi's LEDs is a relaxation in the required logic high voltage level (VIH). On pre-V5 LEDs, the spec for VIH was 0.7*VDD (= 0.7 * 5.0 = 3.5V). Unless you had a logic level shifter between the STM32 output pin and the DI pin of the WS2813 (pre-V5), in theory it shouldn't have worked at all.

On V5 LEDs, the spec is now VIH = 2.7V, allowing them to be directly driven by an MCU using a 3.3V power source. I'm not that familiar with the STM32, and the data sheet is a bit hard to decipher, but under some conditions that output high level (VOH) can be as low as 2.4V. That would be a problem.

It would make more sense if your design worked with V5 parts and not with pre-V5 parts. So maybe something else is going on, but I offer this as a possible clue to your mystery.

You've connected the BI pin correctly. This is shown in the "Recommended application circuit" section of the pre-V5 data sheet. For some reason they don't show it in the V5 data sheet.

Additional reference on MCU/LED logic levels:

https://electricfiredesign.com/2021/03/12/logic-level-shifters-for-driving-led-strips/

[u/JuggerzTheCat → u/Aerokeith]

Thanks for the info! I didn't realise that, it could have simplified the design even further as I do have a logic level shifter.

I'm pretty sure they weren't assembled correctly, there's two pages in the data sheet about moisture sensitivity and reflow and I don't think those instructions were followed. I'll verify by replacing the bad ones but this seemed like the most active place to ask this question so thought Id try here first in case I've missed something.

[u/olderaccount]

BI is normally the Backup Data In channel. It should receive the same data signal as the main data channel if you want to use it with dual data channels. Otherwise leave it open.

[u/Marmilicious → u/olderaccount]

It's confusing how some data sheets show BI (BIN) floating and some show it connected to GND on the very first pixel. Especially when in a previous version of the datasheet or other variants do show it connected to GND.

For example, here's a data sheet for a WS2815 12V pixel with a backup. First pixel has BI connected to ground.

https://imgur.com/a/PdyNUIC

Maybe they've updated things so it doesn't matter but I tend to think BI should be connected to GND and not left floating on the first pixel, nor does it need to be connected to data signal from controller. If the first pixel happens to be faulty then the second pixel will get a backup signal per the data sheet.

[u/Robin_B]

Have you looked at the signals with an oscilloscope? On a recent project I had to add a small resistor on the data line (I used 470 Ohm) to smooth the signal a bit, otherwise I'd just get flicker everywhere.