deploy.py auto-commit

author: jaseg <git@jaseg.de> 2023-12-28 14:56:21 +0100
committer: jaseg <git@jaseg.de> 2023-12-28 14:56:21 +0100
commit: 3ab2c03e01264a9c6e262feef6a7188cb8b30775 (patch)
tree: bb5961ba3403296ee8d2757d50d98f3241155c1e /blog/multichannel-led-driver/index.html
parent: c12db2ab4296bf6c6a4f4255caea0cd6790f49eb (diff)
parent: 8a673ca3322350bd7e1580a90f89d0bd0c5fa3bf (diff)
download: blog-3ab2c03e01264a9c6e262feef6a7188cb8b30775.tar.gz
blog-3ab2c03e01264a9c6e262feef6a7188cb8b30775.tar.bz2
blog-3ab2c03e01264a9c6e262feef6a7188cb8b30775.zip
1 files changed, 384 insertions, 0 deletions
diff --git a/blog/multichannel-led-driver/index.html b/blog/multichannel-led-driver/index.html
new file mode 100644
index 0000000..22fa408
--- /dev/null
+++ b/blog/multichannel-led-driver/index.html
@@ -0,0 +1,384 @@
+<!DOCTYPE html>
+<html><head>
+    <meta charset="utf-8">
+    <title>32-Channel LED tape driver | Home</title>
+    <meta name="description" content="">
+    <meta name="viewport" content="width=device-width, initial-scale=1">
+    <meta name="mobile-web-app-capable" content="yes">
+    <meta name="color-scheme" content="dark light">
+    <link rel="stylesheet" href="/style.css">
+</head>
+<body><nav>
+    <div class="internal">
+        
+        <a href="/" title="Home">Home</a>
+        <a href="/blog/" title="Blog">Blog</a>
+        <a href="/projects/" title="Projects">Projects</a>
+        <a href="/about/" title="About">About</a>
+    </div>
+    <div class="external">
+        <a href="https://git.jaseg.de/" title="cgit">cgit</a>
+        <a href="https://github.com/jaseg" title="Github">Github</a>
+        <a href="https://gitlab.com/neinseg" title="Gitlab">Gitlab</a>
+        <a href="https://chaos.social/@jaseg" title="Mastodon">Mastodon</a>
+    </span>
+</nav>
+
+    <header>
+        <h1>32-Channel LED tape driver</h1>
+<ul class="breadcrumbs">
+    <li><a href="/">jaseg.de</a></li>
+    <li><a href="/blog/">Blog</a></li><li><a href="/blog/multichannel-led-driver/">32-Channel LED tape driver</a></li>
+</ul>
+ <strong>2018-05-02</strong>
+    </header>
+    <main>
+        <div class="document">
+
+
+<div class="section" id="theoretical-basics">
+<h2>Theoretical basics</h2>
+<p>Together, a friend and I outfitted the small staircase at Berlin's Chaos Computer Club with nice, shiny RGB-WW LED tape
+for ambient lighting. This tape is like regular RGB tape but with an additional warm white channel, which makes for much
+more natural pastels and whites. There are several variants of RGBW tape. Cheap ones have separate RGB and white LEDs,
+which is fine for indirect lighting but does not work for direct lighting. Since we wanted to mount our tape in channels
+at the front of the steps, we had to use the slightly more expensive variant with integrated RGBW LEDs. These are LEDs
+in the 5050 (5.0mm by 5.0mm) form factor common with RGB LEDs that have a small section divided off for the white
+channel. The red, green and blue LED chips sit together in the larger section covered with clear epoxy and the white
+channel is made up from the usual blue LED inside a yellow phosphor in the smaller section.</p>
+<p>Since we wanted to light up all of 15 steps, and for greatest visual effect we would have liked to be able to control
+each step individually we had to find a way to control 60 channels of LED tape with a reasonable amount of hardware.</p>
+<p>LED tape has integrated series resistors and runs off a fixed 12V or 24V constant-voltage supply. This means you don't
+need a complex constant-current driver as you'd need with high-power LEDs. You can just hook up a section of LED tape
+to a beefy MOSFET to control it. Traditionally, you would do <em>Pulse Width Modulation</em> (PWM) on the MOSFET's input to
+control the LED tape's brightness.</p>
+<div class="section" id="pulse-width-modulation">
+<h3>Pulse Width Modulation</h3>
+<p><a class="reference external" href="https://en.wikipedia.org/wiki/Pulse-width_modulation">Pulse Width Modulation</a> is a technique of controlling the brightness of a load such as an LED with a digital signal.
+The basic idea is that if you turn the LED on and off much too fast for anyone to notice, you can control its power by
+changing how long you turn it on versus how long you leave it off.</p>
+<p>PWM divides each second into a large number of periods. At the beginning of each period, you turn the LED on. After
+that, you wait a certain time until you turn it off. Then, you wait for the next period to begin. The periods are always
+the same length but you can set when you turn off the LED. If you turn it off right away, it's off almost all the time
+and it looks like it's off to your eye. If you turn it off right at the end, it's on almost all the time and it looks
+super bright to your eye. Now, if you turn it off halfway into the cycle, it's on half the time and it will look to your
+eye as half as bright as before. This means that you can control the LED's brightness with only a digital signal and
+good timing.</p>
+<figure>
+    <img src="images/pwm_schema.jpg" alt="A visualization of PWM at different duty cycles.">
+    <figcaption>Waveforms of two PWM cycles at different duty cycles.</figcaption>
+</figure><p>PWM works great if you have a dedicated PWM output on your microcontroller. It's extremely simple in both hardware and
+software.  Unfortunately for us, controlling 32 channels with PWM is not that easy. Cheap microcontrollers only have <a class="reference external" href="https://www.nxp.com/parametricSearch#/&amp;c=c731_c380_c173_c161_c163&amp;page=1">a
+handful of hardware PWM outputs</a>, so we'd either have to do everything in software, bit-banging our LED modulation, or
+we'd have to use a dedicated chip.</p>
+<p>Doing PWM in software is both error-prone and slow.  Since the maximum dynamic range of a PWM signal is limited by the
+shortest duty cycle it can do, software PWM being slow means it has poor PWM resolution at maybe 8 bits at most.  Poor
+color resolution is not a problem if all you're doing is to fade around the <a class="reference external" href="https://en.wikipedia.org/wiki/HSL_and_HSV">HSV rainbow</a>, but for ambient lighting
+where you <em>really</em> want to control the brightness down to a faint shimmer you need all the color resolution you can get.</p>
+<p>If you rule out software PWM, what remains are dedicated <a class="reference external" href="http://www.ti.com/lit/ds/symlink/tlc5940.pdf">hardware PWM controllers</a>. Most of these have either of three
+issues:</p>
+<ul class="simple">
+<li>They're expensive</li>
+<li>They don't have generous PWM resolution either (12 bits if you're lucky)</li>
+<li>They're meant to drive small LEDs such as a 7-segment display directly and you can't just hook up a MOSFET to their
+output</li>
+</ul>
+<p>This means we're stuck in a dilemma between two poor solutions if we'd want to do PWM. Luckily for us, PWM is not the
+only modulation in town.</p>
+</div>
+<div class="section" id="binary-code-modulation">
+<h3>Binary Code Modulation</h3>
+<p>PWM is the bread-and-butter of the maker crowd. Everyone and their cat is doing it and it works really well most of the
+time. Unbeknownst to most of the maker crowd, there is however another popular modulation method that's mostly used in
+professional LED systems: Enter <a class="reference external" href="http://www.batsocks.co.uk/readme/art_bcm_1.htm">*Binary Code Modulation* (BCM)</a>.</p>
+<p>BCM is to PWM sort of what barcodes are to handwriting. While PWM is easy to understand and simple to implement if all
+you have is a counter and an IO pin, BCM is more complicated. On the other hand, computers can do complicated and BCM
+really shines in multi-channel applications.</p>
+<p>Similar to PWM, BCM works by turning on and off the LED in short periods fast enough to make your eye perceive it as
+partially on all the time. In PWM the channel's brightness is linearly dependent on its duty cycle, i.e. the percentage
+it is turned on. In PWM the duty cycle D is the total period T divided by the on period T_on. The issue with doing PWM
+on many channels at once is that you have to turn off each channel at the exact time to match its duty cycle.
+Controlling many IO pins at once with precise timing is really hard to do in software.</p>
+<p>BCM avoids this by further dividing each period into smaller periods which we'll call <em>bit periods</em> and splitting each
+channel's duty cycle into chunks the size of these bit periods. The amazingly elegant thing in BCM now is that as you
+can guess from the name these bit periods are weighted in powers of two. Say the shortest bit period lasts 1
+microsecond. Then the second-shortest bit period is 2 microseconds and the third is 4, the fifth 8, the sixth 16 and so
+on.</p>
+<figure>
+    <img src="images/bcm_schema.jpg" alt="A visualization of BCM at different duty cycles.">
+    <figcaption>Waveforms of a single 4-bit BCM cycle at different duty cycles. This BCM can produce 16 different
+    levels.</figcaption>
+</figure><p>Staggered like this, you turn on the LED for integer value of microseconds by turning it on in the bit periods
+corresponding to the binary bits of that value. If I want my LED to light for 19 microseconds every period, I turn it on
+in the 16 microsecond bit period, the 2 microsecond bit period and the 1 microsecond bit period and leave it off for the
+4 and 8 mircosecond bit periods.</p>
+<p>Now, how this is better instead of just more complicated than plain old PWM might not be clear yet. But consider this:
+Turning on and off a large number of channels, each at its own arbitrary time is hard because doing the timing in
+software is hard. We can't use hardware timers since we only have two or three of those, and we have 32 channels.
+However, we can use one hardware timer to trigger a really cheap external latch to turn on or off the 32 channels all at
+once. With this setup, we can only controll all channels at once, but we can do so with very precise timing.</p>
+<p>All we need to do is to set our timer to the durations of the BCM bit periods, and we can get the same result as we'd
+get with PWM with only one hardware timer and a bit of code that is not timing-critical anymore.</p>
+<div class="section" id="applications-of-binary-code-modulation">
+<h4>Applications of Binary Code Modulation</h4>
+<p>BCM is a truly wondrous technique, and outside of hobbyist circles it is in fact very widely known. Though we're using
+it to control just 32 channels here, you can do much more channels without any problems. The most common application
+where BCM is invariably used is <em>any</em> kind of LED screen. Controlling the thousands and thousands of LEDs in an LED
+screen with PWM with a dedicated timer for each LED would not be feasible. With BCM, all you need to dedicate to a
+single LED is a flipflop (or part of one if you're multiplexing). In fact, there is a whole range of <a class="reference external" href="http://www.vabolis.lt/stuff/MBI5026.pdf">ICs with no other
+purpose than to enable BCM on large LED matrices</a>. Basically, these are a
+high-speed shift register with latched outputs much like the venerable <a class="reference external" href="http://www.ti.com/lit/ds/symlink/sn74hc595.pdf">74HC595</a>, only their outputs are constant-current
+sinks made so that you can directly connect an LED to them.</p>
+</div>
+<div class="section" id="running-bcm-on-led-tape">
+<h4>Running BCM on LED tape</h4>
+<p>In our case, we don't need any special driver chips to control our LED tape. We just connect the outputs of a <a class="reference external" href="http://www.ti.com/lit/ds/symlink/sn74hc595.pdf">74HC595</a>
+shift register to one <a class="reference external" href="https://en.wikipedia.org/wiki/MOSFET">MOSFET</a> each, and then we directly connect the LED tape to these MOSFETs. The MOSFETs allow us to
+drive a couple of amps into the LED tape from the weak outputs of the shift register.</p>
+<p>The BCM timing is done by hooking up two timer channels of our microcontroller to the shift registers <em>strobe</em> and
+<em>reset</em> inputs. We set the timer to PWM mode so we can generate pulses with precise timing. At the beginning of each
+bit period, a pulse will strobe the data for this bit period that we shifted in previously. At the end of the bit
+period, one pulse will reset the shift register and one will strobe the freshly-reset zeros into the outputs.</p>
+<figure>
+    <img src="images/olsndot_output_schematic.jpg" alt="From left to right, we see the STM32, one of the shift
+    registers, and the LEDs and MOSFETs. The LED tape is driven to ground by the MOSFETs, which are in turn directly
+    driven from the shift register outputs. The shift register is wired up to the STM32 with its clock and data
+    inputs on SCK and MOSI and its RESET and STROBE inputs on channel 2 and 3 of timer 1.">
+    <figcaption>
+        The schematic of a single output of this LED driver. Multiple shift register stages can be cascaded.
+    </figcaption>
+</figure><p>Our implementation of this system runs on an <a class="reference external" href="http://www.st.com/resource/en/datasheet/stm32f030f4.pdf">STM32F030F4P6</a>, the smallest, cheapest ARM microcontroller you can get from
+ST. This microcontroller has only 16kB of flash and 1kB of RAM, but that's plenty for our use. We use its SPI controller
+to feed the modulation data to the shift registers really fast, and we use two timer channels to control the shift
+registers' reset and strobe.</p>
+<p>We can easily cascade shift registers without any ill side-effects, and even hundreds of channels should be no problem
+for this setup. The only reason we chose to stick to a 32-channel board is the mechanics of it. We thought it would be
+easier to have several small boards instead of having one huge board with loads of connectors and cables coming off it.</p>
+<p>The BOM cost per channel for our system is 3ct for a reasonable MOSFET, about 1ct for one eighth of a shift register
+plus less than a cent for one resistor between shift register and MOSFET. In the end, the connectors are more expensive
+than the driving circuitry.</p>
+</div>
+</div>
+</div>
+<div class="section" id="hardware-design">
+<h2>Hardware design</h2>
+<p>From this starting point, we made a very prototype-y hardware design for a 32-channel 12V LED tape driver. The design is
+based on the <a class="reference external" href="http://www.st.com/resource/en/datasheet/stm32f030f4.pdf">STM32F030F4P6</a> driving the shift registers as explained above. The system is controlled through an <a class="reference external" href="https://en.wikipedia.org/wiki/RS-485">RS485</a>
+bus that is connected up to the microcontroller's UART using an <a class="reference external" href="https://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf">MAX485</a>-compatible RS485 transceiver. The LED tape is
+connected using 9-pin <a class="reference external" href="https://en.wikipedia.org/wiki/D-subminiature">SUB-D</a> connectors since they are cheap and good enough for the small current of our short segments
+of LED tape. The MOSFETs we use are small <a class="reference external" href="http://www.nxp.com/documents/outline_drawing/SOT23.pdf">SOT-23</a> logic-level MOSFETs. In various prototypes we used both International
+Rectifier's <a class="reference external" href="https://www.infineon.com/dgdl/?fileId=5546d462533600a4015356686fed261f">IRLML6244</a> as well as Alpha &amp; Omega Semiconductor's <a class="reference external" href="http://aosmd.com/pdfs/datasheet/AO3400.pdf">AO3400</a>. Both are good up to about 30V/5A.  Since we're
+only driving about 2m of LED tape per channel we're not going above about 0.5A and the MOSFETs don't even get warm.</p>
+<div class="section" id="switching-nonlinearities">
+<h3>Switching nonlinearities</h3>
+<p>During testing of our initial prototype, we noticed that the brightness seemed to jump around when fading to very low
+values. It turned out that our extremely simple LED driving circuit consisting of only the shift register directly
+driving a MOSFET, which in turn directly drives the LED tape was maybe a little bit too simple. After some measurements
+it turned out that we were looking at about 6Vpp of ringing on the driver's output voltage. The picture below is the
+voltrage we saw on our oscilloscope on the LED tape.</p>
+<figure>
+    <img src="images/driver_ringing_strong.jpg" alt="Strong ringing on the LED voltage waveform edge at about
+        100% overshoot during about 70% of the cycle time.">
+    <figcaption>Bad ringing on the LED output voltage caused by wiring inductance. Note that the effect on the
+    actual LED current is less bad than this looks since the LED's V/I curve is nonlinear.</figcaption>
+</figure><div class="section" id="dynamic-switching-behavior-cause-and-effect">
+<h4>Dynamic switching behavior: Cause and Effect</h4>
+<p>A bit of <a class="reference external" href="http://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html">LTSpice</a> action later we found that the inductance of the few metres of cable leading to the LED tape is the
+likely culprit. The figure below is the schematic used for the simulations.</p>
+<figure>
+    <img src="images/driver_output_ltspice_schematic.jpg" alt="The LTSpice schematic of one output of the driver,
+    taking into account the shift register's output ESR and the wiring ESL.">
+    <figcaption>The schematic of the simulation in LTSpice</figcaption>
+</figure><p>As tested, the driver does not include any per-output smoothing so the ~.5A transient on each BCM cycle hits the cable
+in full.  Combined with the cable inductance, this works out to a considerable lag of the rising edge of the LED
+current, and bad ringing on its falling edge. Below is the voltage on the LED output from an LTSpice simulation of our
+driver.</p>
+<figure>
+    <img src="images/overshoot_sim_r0.svg" alt="The result of the LTSpice simulation of our driver output. The LED
+    current shows similar ringing to what we measured using the oscilloscope. Interestingly, the gate voltage shows
+    strong ringing, too.">
+    <figcaption>The result of our LTSpice simulation. This simulation assumes 1µH of wiring inductance and 50Ω of
+    output impedance on the part of the shift register. The ringing at the gate visible in the gate voltage graph is
+    due to feed-through of the ringing at the output through the MOSFET's parasitic Cgd.</figcaption>
+</figure><p>We were able to reduce the rining and limit the effect somewhat by putting a 220Ω series resistor in between the shift
+register output and the MOSFET gate. This resistor forms an RC circuit with the MOSFET's nanofarad or two of gate
+capacitance. The result of this is that the LED current passing the wire's ESL rises slightly more slowly and thus the
+series inductance gets excited slightly less, and the overshoot decreases. Below is a picture of the waveform with the
+damping resistor in place and a picture of our measurement for comparison. The resistor values don't agree perfectly
+since the estimated ESL and stray capacitance of the wiring is probably way off.</p>
+<figure>
+    <img src="images/driver_ringing_weak.jpg" alt="Weak ringing on the LED voltage waveform edge at about 30%
+    overshoot during about 20% of the cycle time.">
+    <figcaption>Adding a resistor in front of the MOSFET gate to slow the transition damped the ringing somewhat,
+    but ultimately it cannot be eliminated entirely. Note how you can actually see the miller plateau on the
+    trailing edge of this signal.
+    </figcaption>
+</figure><figure>
+    <img src="images/overshoot_sim_r100.svg" alt="The result of the LTSpice simulation of our driver output with an
+    extra 100 Ohms between shift register output and MOSFET gate. Similar to the oscilloscope measurement the
+    ringing is much reduced in its amplitude.">
+    <figcaption>The LTSpice simulation result with the same parameters as above but with an extra 100Ω between the
+    shfit register's output and the MOSFET's gate.</figcaption>
+</figure><p>A side effect of this fix is that now the effective on-time of the LED tape is much longer than the duty cycle at the
+shift register's output at very small duty cycles (1µs or less). This is caused by the MOSFET's <a class="reference external" href="https://www.vishay.com/docs/68214/turnonprocess.pdf">miller
+plateau</a>. For illustration, below is a graph of both the excitation waveform (the boxy line) and the resulting LED
+current (the other ones) both without damping (top) and with 220Ω damping (bottom). As you can see the effective duty
+cycle of the LED current is not at all equal to the 50% duty cycle of the excitation square wave.</p>
+<figure>
+    <img src="images/asymmetric_iled.svg" alt="The result of an LTSpice simulation of the LED duty cycle without and
+    with damping. Dampening widens the LED current waveform from 50% duty cycle with sharp edges to about 80% duty
+    cycle with soft edges.">
+    <figcaption>Simulated LED duty cycle with and without damping. The damping resistance used in this simulation
+    was 220Ω.</figcaption>
+</figure><figure>
+    <img src="images/asymmetric_vgate.svg" alt="The gate voltages in the spice simulation above. The undamped
+    response shows sharp edges with the miller plateau being a barely noticeable step, but with strong ringing on
+    the trailing edge. The damped response shows RC-like slow-edges, but has wide miller plateaus on both edges
+    adding up to about 50% of the pulse width.">
+    <figcaption>The MOSFET gate voltage from the simulation in the figure above. You can clearly see how the miller
+    plateau (the horizontal part of the trace at about 1V) is getting much wider with added damped, and how the
+    resulting gate charge/discharge curve is not at all that of a capacitor anymore.</figcaption>
+</figure><p>In conclusion, we have three major causes for our calculated LED brightness not matching reality:</p>
+<ul class="simple">
+<li>Ringing of the equivalent series inductance of the wiring leading up to the LED tape</li>
+<li>Miller plateau lag</li>
+<li>The damping resistor and the MOSFET gate forming an RC filter that helps with wire ESL ringing but worsens the miller
+plateau issue and deforms the LED current edges.</li>
+</ul>
+<p>Added up, these three effects yield a picture that agrees well with our simulations and measurements. The overall effect
+is neglegible at long period durations (&gt;10µs), but gets really bad at short period durations (&lt;1µs). The effect is
+non-linear, so correcting for it is not as simple as adding an offset.</p>
+</div>
+<div class="section" id="measuring-led-tape-brightness">
+<h4>Measuring LED tape brightness</h4>
+<p>In order to correct for the nonlinearities mentioned above, we decided to implement a lookup table mapping BCM period to
+actual timer setting. That is, each row of the table contains the actual period length we need to set the
+microcontroller's timer to in order to get our intended brightness steps.</p>
+<p>To calibrate our driver, we needed a setup for reproducible measurement of the relative brightness of our LED tape at
+different settings. Absolute brightness is not of interest to us as the eye can't perceive it. To perform the
+calibration, the LED driver is set to enable each single BCM period in turn, i.e. brightness values 1, 2, 4, 8, 16 etc.</p>
+<p>The setup we used to measure the LED tape's brightness consists of a bunch of LED tape stuck into a tin can for
+shielding against both stray light and electromagnetic interference and a photodiode looking at the LED tape. We used
+the venerable <a class="reference external" href="http://www.vishay.com/docs/81521/bpw34.pdf">BPW34</a> photodiode in our setup as I had a bunch leftover from another project and because they are quite
+sensitive owing to their physically large die area.</p>
+<figure>
+    <img src="images/linearization_setup.jpg" alt="The led measurement setup consists of several PCBs and a
+    breadboard linked with a bunch of wires and a big tin can to shield the LEDs and the photodiode. A large sub-D
+    connector is put into the top of the tin can as a feed-through for the LED tape's control signals and the
+    photodiode signal. In the background the control laptop is visible.">
+    <figcaption>The LED brighness measurement setup. The big tin can contains a bunch of LED tape and the
+    photodiode. The breadboard on the right is used for the photodiode preamplifier and for jumpering around the LED
+    tape's channels. The red board next to it is the buspirate used as ADC. The board on the bottom left is a
+    TTL-to-RS485 converter and the board in the middle is the unit under test.</figcaption>
+</figure><p>The photodiode's photocurrent is converted into a voltage using a very simple transimpedance amplifier based around a
+<a class="reference external" href="http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf">MCP6002</a> opamp that was damped into oblivion with a couple nanofarads of capacitance in its feedback loop. The <a class="reference external" href="http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf">MCP6002</a>
+is a fine choice here since I had a bunch and because it is a CMOS opamp, meaning it has low bias current that would
+mess up our measurements. For many applications, opamp bias current is not a big issue but when using the opamp to
+directly measure very small currents at its input it quickly swamps out the signal for most BJT-input types.</p>
+<p>The transimpedance amplifier's output is read from the computer using the ADC input of a buspirate USB thinggamajob. In
+general I would not recommend the buspirate as a tool for this job since it's ADC is not particularly good and it's
+programming interface is positively atrocious, but it was what I had and it beat first wiring up one of the dedicated
+ADC chips I had in my parts bin.</p>
+<p>The computer runs a small python script cycling the LED tape through all its BCM period settings and taking a brightness
+measurement at each step. Later on, these measurements can be plotted to visualize the resulting slope's linearity, and
+we can even do a simulation of the resulting brightness for all possible control values by just adding the measured
+photocurrents for a certain BCM setpoint just as our retinas would do.</p>
+<figure>
+    <img src="images/driver_linearity_raw.svg" alt="">
+    <figcaption>
+        A plot of the measured brightness of our LED tape for each BCM period. The brightness values are normalized
+        to the value measured at the LSB setpoint (brightness=1/65535). Ideally, this plot would show a straight
+        line with slope 1. Obviously, it doesn't. The bend in the curve is caused by the above-mentioned duty cycle
+        offset adding an offset to all brightness values. Shown is both the raw data (light), which has essentially zero
+        measurement error and a linear fit (dark).
+
+        The plot is in log-log to approximate how the human eye would perceive brightness, i.e. highly sensitive at
+        low values but not very sensitive at all at large values.
+    </figcaption>
+</figure><p>While it would be possible to fully automate the optimization of BCM driver lookup tables, we needed only one and in the
+end I just sat down and manually tweaked the ideal values we initially calculated until I liked the result. You can see
+the resulting brightness curve below.</p>
+<div class="subfigure">
+    <figure>
+        <img src="images/uncorrected_brightness_sim.svg" alt="">
+        <figcaption>
+            Calculated brightness curve for the uncorrected BCM setup. As you can see, at low setpoints the result
+            is about as smooth as sandpaper, which is well in line with our observations. At high setpoints the
+            offset gets swamped out and the nonlinearity in the low bits is not visible anymore.
+        </figcaption>
+    </figure>
+   <figure>
+        <img src="images/corrected_brightness_sim.svg" alt="">
+        <figcaption>
+            Brightness curve for the corrected BCM setup extrapolated using actual measurements. Looks as buttery
+            smooth in real life as it does in this plot.
+        </figcaption>
+        </figcaption>
+    </figure>
+</div></div>
+</div>
+<div class="section" id="controlling-the-driver">
+<h3>Controlling the driver</h3>
+<p>Now that our driver was behaving linear enough that you couldn't see it actually wasn't we needed a nice way to control
+it from a computer of our choice. In the ultimate application (our staircase) we'll use a raspberry pi for this. Since
+we already settled on an <a class="reference external" href="https://en.wikipedia.org/wiki/RS-485">RS485</a> bus for its robustness and simplicity, we had to device a protocol to control the driver
+over this bus. Here, we settled on a simple, <a class="reference external" href="https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing">COBS</a>-based protocol for the reasons I wrote about in <a class="reference external" href="serial-protocols">How to talk to your
+microcontroller over serial</a>.</p>
+<p>To address our driver nodes, we modified the Makefile to build a random 32-bit MAC into each firmware image. The
+protocol has only five message types:</p>
+<ol class="arabic simple">
+<li>A 0-byte <em>ping</em> packet, to which each node would reply with its own address in the
+first 100ms after boot. This can be used to initially discover the addresses of all nodes connected to the bus. You'd
+spam the bus with <em>ping</em> packets, and then hit reset on each node in turn. The control computer would then receive
+each device's MAC address as you hit reset.</li>
+<li>A 4-byte <em>address</em> packet that says which device that the following packet is for. This way of us using the packet
+length instead of a packet type field is not particularly elegant, but our system is simple enough and it was easy to
+implement.</li>
+<li>A 64-byte <em>frame buffer</em> packet that contains 16 bits of left-aligned brightness data for every channel</li>
+<li>A one-byte <em>get status</em> packet that tells the device to respond with...</li>
+<li>...a 27-byte status packet containing a brief description of the firmware (version number, channel count, bit depth
+etc.) as well as the device's current life stats (VCC, temperature, uptime, UART frame errors etc.).</li>
+</ol>
+<p>Wrapped up in a nice python interface we can now easily enumerate any drivers we connect to a bus, query their status
+and control their outputs.</p>
+</div>
+<div class="section" id="conclusion">
+<h3>Conclusion</h3>
+<div class="subfigure">
+    <figure>
+        <a href="images/olsndot_schematic.png">
+            <img src="images/olsndot_schematic.png" alt="A picture of the LED driver schematic">
+        </a>
+        <figcaption>The LED driver <a href="images/olsndot_schematic.png">schematic</a></figcaption>
+    </figure>
+    <figure>
+        <a href="images/olsndot_pcb.png">
+            <img src="images/olsndot_pcb.png" alt="A picture of the LED driver PCB layout">
+        </a>
+        <figcaption>The LED driver <a href="images/olsndot_pcb.png">PCB layout</a></figcaption>
+    </figure>
+</div><p>Putting some thought into the control circuitry and software, you can easily control large numbers of channels of LEDs
+using extremely inexpensive driving hardware without any compromises on dynamic range. The design we settled on can
+drive 32 channels of LED tape with a dynamic range of 14bit at a BOM cost of below 10€. All it really takes is a couple
+of shift registers and a mildly bored STM32 microcontroller.</p>
+<p>Get a PDF file of the schematic and PCB layout <a class="reference external" href="olsndot_v02_schematics_and_pcb.pdf">here</a> or download the CAD files
+and the firmware sources <a class="reference external" href="https://github.com/jaseg/led_drv">from github</a>. You can view the Jupyter notebook used to
+analyze the brightness measurement data <a class="reference external" href="http://nbviewer.jupyter.org/github/jaseg/led_drv/blob/master/doc/Run_analysis.ipynb">here</a>.</p>
+</div>
+</div>
+</div>
+    </main><footer>
+    Copyright © 2023 Jan Sebastian Götte
+    / <a href="/about/">About</a>
+    / <a href="/imprint/">Imprint</a>
+</footer>
+<script>
+            if(navigator.getEnvironmentIntegrity!==undefined)document.querySelector('body').innerHTML=`<h1>Your browser
+            contains Google DRM</h1>"Web Environment Integrity" is a Google euphemism for a DRM that is designed to
+            prevent ad-blocking, and which Google has forced into their browsers against widespread public opposition.
+                In support of an open web, this website does not function with this DRM. Please install a browser such
+            as <a href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> that respects your freedom and supports
+            ad blockers.`;
+        </script>
+    </body>
+</html>
author	jaseg <git@jaseg.de>	2023-12-28 14:56:21 +0100
committer	jaseg <git@jaseg.de>	2023-12-28 14:56:21 +0100
commit	3ab2c03e01264a9c6e262feef6a7188cb8b30775 (patch)
tree	bb5961ba3403296ee8d2757d50d98f3241155c1e /blog/multichannel-led-driver/index.html
parent	c12db2ab4296bf6c6a4f4255caea0cd6790f49eb (diff)
parent	8a673ca3322350bd7e1580a90f89d0bd0c5fa3bf (diff)
download	blog-3ab2c03e01264a9c6e262feef6a7188cb8b30775.tar.gz blog-3ab2c03e01264a9c6e262feef6a7188cb8b30775.tar.bz2 blog-3ab2c03e01264a9c6e262feef6a7188cb8b30775.zip