+++ /dev/null
-title: RGB Lamp Led Driver
-linktitle: rgb-leds
-parent: rgb-code
-ctime: 2012-03-24
-
-# PWM drive
-
-The LEDs in the module receive varying current to adjust their brightness via
-the hardware PWM features of the PIC16LF1933 in conjunction with Timer 2. The
-module has 4 capture/compare peripherals, which is perfect for the 4 LEDs in the
-module.
-
-Each PWM output controls a simple current limited 2-transister drive circuit.
-This circuit is superior to a simple single transistor switch, as the CE
-junction of one transistor is used as a fixed voltage drop controlling current
-through a resistor of known size. This is particularly nice given that
-different LED modules have differnt voltage drops, and voltage drop is also a
-function of current and temperature. A fixed current driver removes these
-variables.
-
-# Mixing color and brightness
-
-An interesting challenge is to manipulate the output of the discrete color LEDs
-both for the purpose of emitting a specific color and for controlling the
-brightness. The solution is to treat the pre-computed PWM output value as a
-fraction of the maximum value (255). In the same fashion, the color range and
-the brightness are also treated as a fraction of their respective maximums.
-Therefore, the rgb drive output is essentially the product of the color and
-brightness values. Simple, and it seems to work out pretty well.
-
- rgb = 255 * (color/color_max * bright/bright_max)
-
-Of course, for this processor the preference is to do this computation in
-integer math. The actual implementation looks more like this:
-
- color: 0..63
- bright: 0..BRIGHT_MAX
- rgb = (color << 4) * bright / 4 / BRIGHT_MAX
-
-# LEDs
-
-An interesting effect was uncovered during testing. A certain delta change in
-drive string of a LED at a relatively low average power level is readily
-perceptible vsually, while the same delta is hard to detect at higher power
-levels. Some research shows this to be a nature of the human eye -- it
-perceives changes at lower intensities more readily than at higher percentages
-[(1)](http://neuroelec.com/2011/04/led-brightness-to-your-eye-gamma-correction-no/).
-To get a perceived linear output, the PWM value must be adjusted according to
-CIE Lightness.
-
- L* = 116(Y/Yn)^1/3 - 16 , Y/Yn > 0.008856
- L* = 903.3(Y/Yn), Y/Yn <= 0.008856
-
-Using these formulas, a look-up table can be created to provide the
-compensation. It works pretty well. The rgb.c and rgb.h files in the
-[repository](http://oss/gitweb?p=rgblamp.git;a=summary) implement this feature.
-Note that there are two different look-up tables available, depending upon the
-resolution desired.
projects / oss-web.git / blobdiff