View source for LEDCtl (WS2812b LED Controller)

[[Category:STM32]][[Category:STM32 Projects]][[Category:KiCAD]][[Category:STM32 Hardware Development]][[Category:Work in progress]]{{metadesc|WS2812b LED Controller}}
At the moment this document is a work in progress.  I'll add to it as I go along designing the device, jotting down design decisions as I go along.

[[LED]] strips and panels based on some form of the [[WS2812]] RGB LEDs have gained an enormous popularity the past couple of years with the price of the individual LEDs dropping well below 10 cent each.  These LEDS contain 3 individual LEDs: one red, one green and one blue and the intensity of each of these can be set at 256 different levels.

== Overall Design Parameters ==

The "device" should be able to drive 2-4 strings of up to 1000 leds on each string and the length and type of each string should be configurable.

The "device" will offer multiple ways of controlling LEDS:

* USB/CDC (emulated Serial over USB)
* Serial rx/tx lines
* I2C
* SPI

== Hardware Design and Implementation ==

=== Picking the CPU ===

It might be possible to find a better CPU in ST's enormous line of processors, but since we've used the [[STM32F103]] on our own [[Green Pill]] board, that processor will be the initial choice.

The "limiting" factor of this choice is the available memory.  The STM32F103 has got 20 kB of RAM.  Our "device" will need a "buffer" holding the RGB values of each LED.  So with 1 string of 1000 LEDS we will need around 3k of RAM for this purpose alone.  With 2 strings that value doubles to 6k (which might still be doable) but if we want to support 1000 LEDs on 4 strings each that will require more than half the RAM, most likely leaving too little for the software to run.

=== Schematics ===

The design of the schematics is largely based on the [[Green Pill]] with a few changes.

==== Rev. a ====

<div class="res-img">
[[File:ledctl rev a.svg|1800px]]
</div>

The primary changes are:

Using a 16 MHz crystal rather than the 8 MHz one used on the [[Green Pill]].  The 16 MHz crystal is in fact cheaper and more importantly takes less space (the 8 MHz crystal was used on the [[Green Pill]] to remain compatible with [[Blue Pill]] boards).

The two [[LED]] drive lines are open drain which is pulled up to the 5V power rail to be compatible with the [[WS2812]] LEDs.

Only relevant [[GPIO]] pins are pulled out to the header.

==== Rev. c ====

Two issues were identified with [[#Rev. a|rev. a]]:

# Floating [[Boot0]] pin
# Pull up resistor too weak

<div class="res-img">
[[File:ledctl rev c.svg|2100px]]
</div>

== Register Map ==

{|class=wikitable width=100%
|-
! Start
! End
! Width (B)
! Type
! Default
! Description
|-
| align=right | 0x0000
| align=right | 0x0000
| align=right | 1
| R
| align=right | 0x19
| Device ID.  Must be 0x19
|-
| align=right | 0x0001
| align=right | 0x000b
| align=right | 12
| R
| align=right | 
| Unique 96 bit device ID.
|}

== Software ==

=== Hayes Like Serial Interface ===

To control the attached LEDs a simple serial interface is used.

==== AT Commands ====

{|class=wikitable width=100%
|-
! Command
! Parameters
! Example
! Response
! Description
|-
| &F
| 
| AT&F
| OK
| Factory reset
|-
| ATLsnnn=rrrgggbbb
| s=string<br/>nnn=led<br/>rrr=3 digit value
| ATL0000=010010010
| OK
| Set a specific led on a specific string to a specific value
|-
| Sx=y
| y = value
| ATS0=64
| OK
| Set register to a certain value.  Example will set register 0 to a value of 64
|-
| Z
| 
| ATZ
| OK
| Reset device
|}

==== S Registers ====

{|class=wikitable width=100%
|-
! Register
! Valid values
! Default value
! Description
|-
| 0
| 0-999
| 1
| LEDs on string 0
|-
| 1
| 0-999
| 0
| LEDs on string 1
|-
| 2
| 0-99
| 31
| "Width" of a zero (percentage)
|- 
| 3
| 0-99
| 69
| "Width of a one (percentage)
|}

== Gallery ==

<gallery>
File:ledctl1.png
File:ledctl2.png
File:ledctl3.png
File:ledctl4.png
</gallery>