NB: This was an experiment, and should not actually be used as a method to communicate with the WS2812b
Power WS2812b LEDs from your Raspberry Pi over SPI.
The LEDs run with a scan frequency of no less than 400Hz, with a data transfer rate of 800kbps.
The traditional method of communication with the WS2812b is usually PWM. The duty cycle to send a single bit is:
| Bit | Time High | Time Low | Duty % |
|---|---|---|---|
| 0 | 0.4us | 0.85us | 32% |
| 1 | 0.8us | 0.45us | 64% |
A visual of the protocol is below.
PWM LED (0): where
on time = 0.4us +/-150ns
off time = 0.85us +/-150ns
Volts
| START END
| v v
| ----------
| | |
| | |
| | |
| ---- -------------------
*-------------------------------- Time
PWM LED (1): where
on time = 0.8us +/-150ns
off time = 0.45us +/-150ns
Volts
| START END
| v v
| ------------------
| | |
| | |
| | |
| ---- -----------
*-------------------------------- Time
Looking at the above duty cycle lengths, we can approximate each time slice into another 3 periods, each 33% of the original wavelength in order to have constant high/low runs for the given period. This will allow us to continue using SPI, using a clock speed of approximately 3Mhz.
To interact over SPI, each traditional LED sequence (3 bytes, 8 bits per colour channel) needs to be converted to a 9 byte sequence (72 bits per colour channel) before being shifted over SPI. Each traditional PWM 'bit' gets turned into an equivalent 3 SPI bits, and the clock speed of the SPI interface set to achieve within the +/-150ns error margins.
With the traditional period broken into 3 time sections, each can be pulled high or low individually. We can therefore represent what would have been a PWM (1) with an SPI 110. And on the contrary, represent a PWM (0) with an SPI 100.
An example of a Raspberry Pi controlling a flexible panel of WS2812b's:
(Mario)
Licensed under the GNU GPLv3.0