Almost 20 years I was designing product with 8 bit microcontrollers since they were the more cost effective option at the time.  They had integrated memories which was less common in 32 bit microcontrollers.  By today’s standards the 8 bits parts can be a respectable value but their compute and memory limit are too constraining for the broader functionality common in today’s developments.

In the realm of 32 bit microcontrollers the ARM Cortex M series has achieved dominance in the market. The number of proprietary core offerings continues to shrink as they are displaced by ARM. There are a number of advantages. Knowledge gained about an ARM Cortex M based part can be transferred to parts from other manufacturers. ARM is continually growing their development resources and libraries that are common to all Cortex M parts and development tools can be common.

The absolute lowest cost microcontrollers are still 8 bit core but it still may be wise to use an ARM Cortex core to reduce development time. The lowest cost 8 bit cores might be $0.25 while the lowest cost ARM Cortex is $0.35. For that extra $0.10 you will get extra compute and memory resources that may make you development faster and enable additional product features. For $1 your memory and compute resource will likely support the use of an RTOS which could further speed you development.

There are many options in Cortex M microcontrollers. There are STM32 from ST, ATSAM from Microchip, MSP432 from TI, and LPC from NXP and many others. I have the most experience with STM32 because they had the best library support for fast product development when the family was introduced in 2007. They maintain great library support though many manufactures have caught up in that respect. Nordic, for example, has more extensive libraries supporting Bluetooth connectivity.

If you pick a Cortex M based microcontroller for your product design you will have the broadest support in libraries and tools and most peripheral options from the greatest number of microcontroller manufacturers.

My experience in developing LED drives has given me some insights that could be of benefit in your lighting project.  Most of that experience is in full color light output with the rest in adjustable white lighting.  In addition to the drive hardware I have also developed fixture application software and communications stacks.

When designing electronics capable of accurate full color light output there are a number of things I consider.  First I want to know how colors are designated.  A common method is to pick red, green, and blue reference points that are convenient for the light sources in the fixture then linearly scale between them.  This results in perceptually non-linear color transitions and problems in color matching between different light models.  The method of designating color that appeals most to me is using the same color translation method as computers and portable devices.  The advantages are that the differences in color codes are perceptually linear and colors will match between fixtures using this method and the devices that control them.

Another method I like to apply to color translation is extending the color set point into the color space of the fixture.  Full color LED fixtures are always capable of generating more saturated color than computers or portable devices.  The color set point can be extended by determining the absolute hue and saturation value of the color code then applying that hue and saturation value to the full color space that the fixture is able to produce.  This way the fixture will make colors as fully saturated as it is capable when fully saturated color codes are entered and the hue will still match other devices using this method.

Selection of LEDs effects the range of color that can be generated and how objects illuminated by that light will look.  For a full-color light a minimum of three LEDs are required; red, green, and blue.  For a three LED system the main compromise is between the range of color that can be displayed and the brightness of the fixture.  If a deeper red and blue are used a wider range of color can be generated at the expense of lower perceived brightness since the human eye is not as responsive to light at the wavelength extremes.  When a three color system is set to white illuminated objects may appear color shifted since the light contains such a narrow spectrum of light.  A way to improve color range and light spectrum is to add LEDs to the system.  Adding a white LED will broaden the spectral content of the light and make object render more naturally.  Adding color LEDs like amber and cyan will increase the range of color that can be produced.

When the color target is known the ratio at which the LEDs must be driven can be calculated.  Most basically the intensity ratios can be calculated at the typical output colors and drive levels of the LED sources.  This will result in some color error since LED intensity does not vary linearly with drive current.  Applying an intensity correction for drive level reduces the amount of color error.  LED temperature will effect intensity as well so an additional intensity correction can be applied for temperature.  It is not only the intensity that can vary over temperature and drive level but color as well.  Green LEDs in particular will change color with temperature and drive level.  Another correction can be applied for color drift.

Individual LEDs of the same part number will vary as well.  LEDs within a lot will vary little but lot to lot variance can be significant.  Data from calibrating a fixture at a few points allows for gain and offset correction that will account for most of this variance.  If more accuracy is needed calibration data can be collected over a wider range of operating conditions but most applications do not need this level of accuracy.

For the driver to be able to attain all the color points and transition smoothly between colors and brightnesses it has to be able to source current over a wide range.  To have a light that will transition without perceptible steps the drive should cover a range from 100% current to 0.001%.  This is not possible with a constant current drive when driving at currents much less than 1% LEDs will stop making light.  Another problem with driving LEDs at low constant currents is that they can significantly shift in color.  In my designs, I will either use pulsed dimming exclusively or use a hybrid analog and pulsed drive.  When implementing an analog drive I will not run currents below 1% of the rated current to avoid the problems with color shift and light cut-off.

Adjustable white lights will have many of the same issues as color lighting but possibly not to the same degree.  Since the fixture will be operating over a much smaller color range the LEDs will also need to operate over a smaller intensity range.  The color source intensity will be small relative to the white source intensity so variations in color will be less noticeable.

I’ve been maintaining building the site www.embeddedee.com for the past few years. Since much of my work is related to LED design I decided to make a new website dedicated to that and keep general engineering topics at www.embeddedee.com. I plan to go through my posts there and copy the relevant ones here.

A review of LED function will help us understand when LED intensity will change.  An LED will not flow current or produce light when the voltage across it is below its forward voltage.  As an LED’s voltage approaches its rated forward voltage the LED current will start to flow current and the LED will produce light.  As the voltage is increased above the point where current starts to flow the current quickly increases.  As the LED’s voltage is forced beyond its rated forward voltage the current will continue to increase beyond its ratings to the point of LED failure.  This is why LED fixtures supplied by a voltage source have some additional device to limit LED current.

One common current limiting device is a resistor.  The voltage supplied to the fixture in excess of the LED forward voltage will be applied across the resistor.  The amount of current the resistor will allow to flow is proportional to the amount of voltage across it.  In this way, the brightness of the LED can be varied by varying the supply voltage.  This can be an asset in applications where you want to vary LED brightness by varying supply voltage.  It can be a detriment in applications where you want consistent light output among fixtures that might have slightly different supply voltages or the supply voltage varies over the length of the fixture.

Another way to limit current is with a current regulator.  Like a resistor limit, the voltage in excess of the LED forward voltage is applied across the regulator.  Unlike a resistor, a regulator flows a particular amount of current regardless of the voltage supplied.  This is good for applications where you want consistent brightness with some voltage variation.  Dimming by varying the supply voltage doesn’t work though.

Power dissipation is another issue to consider when comparing resistor limited fixtures to current regulated fixtures.  Since a current regulated fixture’s current is constant as the voltage varies the power dissipated in the LEDs is also constant.  Only the heat generated by the regulator increases with the supply voltage.  With a resistor limited fixture the current increases as the supply voltage increases above its intended operating point.  This increases the heat generated in the LEDs and the resistor.  To mitigate this effect a larger value resistor can be used but this increases the heat generated by the resistor under all operating conditions decreasing the efficacy of the fixture. Usually a resistor limited fixture will have lower efficacy than a current regulated fixture.

Current regulated fixtures can’t be dimmed by a DC voltage source but can still be dimmed by using an AC voltage source.  This is because the supply voltage is not always above the LED forward voltage.  With a 60Hz supply, the LEDs will flicker on and off at 120Hz as this is how often an AC supply crosses through zero volts.  When the voltage is supplied through a phase-cut dimmer the amount of time the supply is at zero volts increases and the time the LED is off increases and the brightness decreases.

Resistor limited fixtures can be dimmed by varying the supply voltage either DC or AC.  Current regulated fixtures can not be dimmed by varying a DC supply but can be by phase-cut dimming an AC supply.  The compromise for using a resistor limited fixture is that there may be less consistency between fixtures and lower efficacy compared to a current regulated supply.

Mixing LEDs to make full color lighting seems straight forward on the surface.  Turn on red for red, green for green, and both for yellow.  Many color fixtures work this way in fact.  There is a separate DMX channel for each color under control and the fixture turns each color on in proportion to its DMX value.  This method works if the application doesn’t require a particular repeatable color.

If you do need a particular color, adjusting color sliders and guessing at the output isn’t a great way to get there.  What makes for better control is a color picker as is used in photo editing where you can adjust easily understandable hue, saturation and brightness values.  It would also be a benefit if the color a light fixture produced with a particular value set was the same as is produced on your device or computer display with these values.

For a fixture to operate from hue, saturation and brightness it has to calculate the color target from these values then calculate the drive level for each of its sources to achieve the color target.  The fixture can use information about each of the emitters efficiency and color at different operating currents and temperatures to calculate the required drive level.  This method works with more than just RGB fixtures.  If the fixture also has white and amber emitters the color target defined by the hue, saturation and brightness values doesn’t change.  The fixture will combine its additional sources into the drive level calculations to utilize all the different color LEDs it might have.

You might want color translation in your light fixture if you want your fixture to emit a particular color, you want the color to be consistent between fixtures, and you want to set the color in a convenient way.