Reading WS2812b leftover data through I2S

[Winz]

Hello there,

I am currently trying to read the leftover WS2812b LED strip data which was sent using a single channel I2S. The data sent from the board (through I2S) looks like this:

with around 34 mV for high and around 2~6 mV on low. I decided not to send FFF, but 888 instead.

I am aiming to calculate the length of the LED strip. Each LED chip will read the first 24 bits of the data, cascade (discard) it, then pass it to the next one. Therefore, the strategy is to get the data after being cascaded by the last chip and calculate the length of it (which later will be divided by 24 to see how many are left).

The question is: How will I be able to read those data? I tried reading as usual (set a buffer for the i2s, start it, listen for it (delay), stop it, then check its buffer), but I got my buffer filled with all FFF instead. and a thing is that... when I tried to read anything from the pin, although I am not sending any data (the LED strip is not powered on, but connected to other's ground), there will be data, which should not happen. I assigned pin 22, 23, 24, 25 to be the i2s input pin (which will be assigned and unassigned when used or unused), and they are configured as follows:

nrf_gpio_cfg_sense_input(DIN1_PIN, NRF_GPIO_PIN_PULLDOWN, NRF_GPIO_PIN_SENSE_HIGH);

Thank you in advance.

[Winz]

Hi Einar,

Sorry, please disregard 34 mV and 2~6 mV, it's wrong, it should be 2.79 V for High and -15.9 mV (or 0 V) for Low. This is the data signal created on WS2812B's Data out.

I connected the strip directly to the board's VDD to power it on, P0.11 for Data, and ground. I did not use any level converter (I know that I'm suggested to supply 5v and use a level converter, but 3.3 V is able to turn the LED on so, I'm trying it).

When there is no data being transferred through the WS2812b, the voltage of the DOUT is around -0.8 mV ~ -78.92 mV. At this state, if I call the board to read any data, by using

i2s_config_init(DIN2_PIN, 17);

where DIN2_PIN is defined by

#define DIN2_PIN 23

, followed by

i2s_start();
nrf_delay_us(I2S_DETECTION);

for (i = 1; i < I2S_BUFFER_SIZE; i++) {
	if (I2SRxBuffer[i] != 0 || I2SRxBuffer[i] != -1) {
		printf("isi: %x\r\n", I2SRxBuffer[i]);
		printf("as: %d\r\n", I2SRxBuffer[i]);
		if (I2SRxBuffer[i] > 0x11111110) {
			lengthz++;
		}
	}
}

i2s_stop();

, I2SRxBuffer[i] will always print ffffffff.

[Einar Thorsrud]

Hi,

OK. The logical levels for the nRF are supposed to be above 0.7 * Vdd for high and below 0.3 * Vdd (but > 0 V) for low. However, I would not expect that the logical levels are OK for the nRF, and the low negative voltage for logical '0' should be ok. But I would not do it like this in an actual end product.

I do not have any idea if the voltages are acceptable to the WS2812b, though. It is out of spec after all (according to the datasheet it should be between +3.5 and +5.3). Can you show us logic analyzer plots of the data out of the WS2812b? Is it all FF's there as well? If so then I would try using a level shifter and supplying the WS2812b with a voltage within spec before digging any further.

By the way, are you basing your endeavors on this blog post?

[Winz]

Hi Einar,

I believe that the voltages are acceptable to the WS2812b. Below a screenshot of the latest datasheet they provide:

It is written that the voltage received is between +3.5 ~ +5.3 V. nRF52DK supplies 3.8 V from the VDD, when powered by PC through a (well) powered USB, right? Therefore, I believe it should be good.

The input voltage for signal is 0.7 * VDD for High and 0.3 * VDD for low (same as nRF's logical levels). Therefore, I think it should work.

Here are the output data from the last LED

the above screenshot is the logic if powered from *VDD*

the above screenshot is the logic if powered from *5V*

I did tried to decode it manually, and I received this, which is the correct data:

00010000 00000000 01010000

G = 0x10
R = 0x00
B = 0x50

color = #001050

Otherwise, yes, the I2S reading is all FF's, no matter if I supply it using VDD nor 5V (with level converter).

And yes, I am basing on that post

Thank you

[Einar Thorsrud]

Are you using the nRF52 DK (with the nRF52832 device, PCA10040) powered from USB? If so the Vdd will be well below 3 V, and there is certainly no way it can be 3.8 V unless you power it directly. And if you do, you are anyway supplying a higher voltage than the absolute maximum supply voltage of the nRF52832, which is 3.6 V.

[Winz]

Yes, that is right. Now that you mentioned about it, I checked and confirmed: you're right, it is not anywhere above 3 V. so... I suppose I am stuck with 5V and level converters.. okay..

That being said, do I need to regulate the signal down back to 3 V (step down) using level converter before connecting it to other board?

[Einar Thorsrud]

You need level shifters on signals going both ways in order to respect the limitations of both devices. It is described in the blog you are using as far as I can see.