Add ifndef to WS2812 timing constraints (#14678)

* Add ifndef to WS2812 timing constraints

Due to the way that the PrimeKB Meridian PCB was designed, this change
is needed in order to properly adjust the LEDs.

Testing:
* Compiled primekb/meridian:default successfully
* Compiled random board (walletburner/neuron:default) successfully

* Fix linting errors

Missed some spacing

* More linting fixes

Spacing on the comments... really?

* Rename WS2812 timing parameters for clarity; add comments

* Add docs update for the WS2812 timing macros

* Fix typo on comment

* Add ifndef for WS2812_RES

* Update double backticks and table with parameters

* Move timing adjustments documentation to ws2812_drivers

* Move timings adjustment discussion to bitbang section

* Update T0H and T1H definitions in subtractions

* format

Co-authored-by: Gondolindrim <alvaro.volpato@usp.br>
Co-authored-by: zvecr <git@zvecr.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/docs/ws2812_driver.md b/docs/ws2812_driver.md
index 101798f211..03dc4f2fd1 100644
--- a/docs/ws2812_driver.md
+++ b/docs/ws2812_driver.md
@@ -62,6 +62,23 @@ Configure the hardware via your config.h:
 #define WS2812_TIMEOUT 100 // default: 100
 ```
 
+##### Adjusting bit timings (ChibiOS only)
+
+The WS2812 LED communication topology depends on a serialized timed window, lasting typically 1250ns in total, where a bit is interpreted as either 0 or 1 depending on for how much time the `RGB_DI` pin voltage is kept high and how much time it is kept low. The WS2812 datasheet specifies quantities defined as `T0H`, `T0L`, `T1H`, `T1L` (respectively, typically 350ns, 900ns, 900ns and 350ns); a bit is interpreted as zero if the voltage on the control pin is held high for `T0H` and then `T0L`, and the bit is interpreted as one if the voltage is held high for `T1H` and then low for `T1L`. Additionally, there is also a RESET time parameter whereby an LED color is reset if the voltage control pin is kept low for more than that parameter; in WS2812 that amount is 6000 nanoseconds.
+
+The WS2812 "bit-banged" ChibiOS driver does just that, in a simple way. It defines these values and governs the `RGB_DI` pin according to these times. There are, however, other LED parts that work in a communication topology similar to this but with different timing parameters; such is the case, for instance, of the SK6812. In order to better support such LEDs whilst keeping the WS2812 driver applicable, QMK allows you to tune these parameters through the definition macros:
+
+| Macro               |Default                                     |
+|---------------------|--------------------------------------------|
+|`WS2812_TIMING       |`1250`                                      |
+|`WS2812_T0H`         |`350`                                       |
+|`WS2812_T0L`         |`WS2812_TIMING - WS2812_T0H`                |
+|`WS2812_T1H`         |`900`                                       |
+|`WS2812_T1L`         |`WS2812_TIMING - WS2812_T1L`                |
+|`WS2812_RES`         |`6000`                                      |
+
+It must be noted, however, that this tuning is only available for the ARM family of microprocessors (since it's ChibiOS-dependent) and not available for PWM and SPI drivers -- so if you intend to use this be aware it will only work with the "bit-banged" driver which is knowingly slower and can possibly throttle the microcontroller if too many LEDs are used.
+
 ### SPI
 Targeting STM32 boards where WS2812 support is offloaded to an SPI hardware device. The advantage is that the use of DMA offloads processing of the WS2812 protocol from the MCU. `RGB_DI_PIN` for this driver is the configured SPI MOSI pin. Due to the nature of repurposing SPI to drive the LEDs, the other SPI pins, MISO and SCK, **must** remain unused. To configure it, add this to your rules.mk: