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| -rw-r--r-- | config.toml | 2 | ||||
| -rw-r--r-- | content/posts/led-characterization/index.rst | 4 | ||||
| -rw-r--r-- | content/posts/multichannel-led-driver/index.rst | 47 | ||||
| -rw-r--r-- | content/posts/serial-protocols/index.rst | 243 |
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diff --git a/config.toml b/config.toml index 10ba64a..aee1071 100644 --- a/config.toml +++ b/config.toml @@ -1,4 +1,4 @@ -baseURL = "https://jaseg.github.io/_testrepo/" +baseURL = "https://www.physik.tu-berlin.de/~jaseg/beta/" languageCode = "en-us" title = "jaseg.net" theme = "hugo-classic" diff --git a/content/posts/led-characterization/index.rst b/content/posts/led-characterization/index.rst index ebcc16c..7f9a754 100644 --- a/content/posts/led-characterization/index.rst +++ b/content/posts/led-characterization/index.rst @@ -1,7 +1,6 @@ --- title: "Led Characterization" date: 2018-05-02T11:18:38+02:00 -draft: true --- Preface @@ -475,7 +474,8 @@ Lab to XYZ is somewhat complex since it requires a floating-point power for gamm libc will have one of those so this is still no problem. Lch also requires floating-point sine and cosine functions, but these should still be no problem on most hardware. -My implementation of these conversions in the ESP8266 firmware of my `Wifi LED driver`_ can be found `on Github`_. +My implementation of these conversions in the ESP8266 firmware of my `Wifi LED driver`_ can be found `on Github`_. You +can view the Jupyter notebook most of the analysis above `here <http://nbviewer.jupyter.org/github/jaseg/led_drv/blob/master/doc/Spectrum%20Measurement.ipynb>`__. .. _`on Github`: https://github.com/jaseg/esp_led_drv/blob/master/user/led_controller.c .. _`project repo`: https://github.com/jaseg/led_drv diff --git a/content/posts/multichannel-led-driver/index.rst b/content/posts/multichannel-led-driver/index.rst index 8d5ad42..7de03d5 100644 --- a/content/posts/multichannel-led-driver/index.rst +++ b/content/posts/multichannel-led-driver/index.rst @@ -1,7 +1,6 @@ --- title: "32-Channel LED tape driver" date: 2018-05-02T11:31:14+02:00 -draft: true --- Theoretical basics @@ -238,20 +237,19 @@ driver. due to feed-through of the ringing at the output through the MOSFET's parasitic Cgd.</figcaption> </figure> -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 dampening 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. +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. .. raw:: html <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 dampened the ringing somewhat, + <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> @@ -270,28 +268,28 @@ probably way off. 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 `miller plateau`_. For illustration, below is a graph of both the excitation waveform (the boxy line) and the resulting LED -current (the other ones) both without dampening (top) and with 220Ω dampening (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. +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. .. raw:: html <figure> <img src="images/asymmetric_iled.svg" alt="The result of an LTSpice simulation of the LED duty cycle without and - with dampening. Dampening widens the LED current waveform from 50% duty cycle with sharp edges to about 80% duty + 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 dampening. The dampening resistance used in this - simulation was 220Ω.</figcaption> + <figcaption>Simulated LED duty cycle with and without damping. The damping resistance used in this simulation + was 220Ω.</figcaption> </figure> .. raw:: html <figure> - <img src="images/asymmetric_vgate.svg" alt="The gate voltages in the spice simulation above. The undampened + <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 dampened response shows RC-like slow-edges, but has wide miller plateaus on both edges + 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 dampening, and how the + 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> @@ -301,8 +299,8 @@ In conclusion, we have three major causes for our calculated LED brightness not * Ringing of the equivalent series inductance of the wiring leading up to the LED tape * Miller plateau lag -* The dampening 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. +* 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. 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 (>10µs), but gets really bad at short period durations (<1µs). The effect is @@ -341,9 +339,9 @@ sensitive owing to their physically large die area. </figure> The photodiode's photocurrent is converted into a voltage using a very simple transimpedance amplifier based around a -MCP6002_ opamp that was dampened into oblivion with a couple nanofarads of capacitance in its feedback loop. The -MCP6002_ 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 +MCP6002_ opamp that was damped into oblivion with a couple nanofarads of capacitance in its feedback loop. The MCP6002_ +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. The transimpedance amplifier's output is read from the computer using the ADC input of a buspirate USB thinggamajob. In @@ -452,6 +450,7 @@ using extremely inexpensive driving hardware without any compromises on dynamic 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. -Get a PDF file of the schematic and PCB layout `here <olsndot_v02_schematics_and_pcb.pdf>`_ or download the CAD files -and the firmware sources `from github <https://github.com/jaseg/led_drv>`_. +Get a PDF file of the schematic and PCB layout `here <olsndot_v02_schematics_and_pcb.pdf>`__ or download the CAD files +and the firmware sources `from github <https://github.com/jaseg/led_drv>`_. You can view the Jupyter notebook used to +analyze the brightness measurement data `here <http://nbviewer.jupyter.org/github/jaseg/led_drv/blob/master/doc/Run_analysis.ipynb>`__. diff --git a/content/posts/serial-protocols/index.rst b/content/posts/serial-protocols/index.rst index a5db824..5797947 100644 --- a/content/posts/serial-protocols/index.rst +++ b/content/posts/serial-protocols/index.rst @@ -1,8 +1,249 @@ --- title: "How to talk to your microcontroller over serial" date: 2018-05-19T08:09:46+02:00 -draft: true --- +Scroll to the end for the `TL;DR <Conclusion_>`_. +In this article I will give an overview on the protocols spoken on serial ports, highlighting common pitfalls. I will +summarize some points on how to design a serial protocol that is simple to implement and works reliably even under error +conditions. + +If you have done low-level microcontroller firmware you will regularly have had to stuff some data up a serial port to +another microcontroller or to a computer. In the age of USB, a serial port is still the simplest and quickest way to get +communication to a control computer up and running. Integrating a ten thousand-line USB stack into your firmware and +writing the necessary low-level drivers on the host side might take days. Poking a few registers to set up your UART to +talk to an external hardware USB to serial converter is a matter of minutes. + +This simplicity is treacherous, though. Oftentimes, you start writing your serial protocol as needs arise. Things might +start harmless with something like ``SET_LED ON\n``, but unless you proceed it is easy to end up in a hot mess of command +modes, protocol states that breaks under stress. The ways in which serial protocols break are manifold. The simplest one +is that at some point a character is mangled, leading to both ends of the conversation ending up in misaligned protocol +states. With a fragile protocol, you might end up in a state that is hard to recover from. In extreme cases, this leads +to code such as `this gem`_ performing some sort of arcane ritual to get back to some known state, and all just because +someone did not do their homework. Below we'll embark on a journey through the lands of protocol design, exploring the +facets of this deceptively simple problem. + +.. _`this gem`: https://github.com/juhasch/pyBusPirateLite/blob/master/pyBusPirateLite/BBIO_base.py#L68 + +Text-based serial protocols +=========================== + +The first serial protocol you've likely written is a human-readable, text-based one. Text-based protocols have the big +advantage that you can just print them on a terminal and you can immediately see what's happening. In most cases you can +even type out the protocol with your bare hands, meaning that you don't really need a debugging tool beyond a serial +console. + +However, text-based protocols also have a number of disadvantages. Depending on your application, these might not matter +and in many cases a text-based protocol is the most sensible solution. But then, in some cases they might and it's good +to know when you hit one of them. + +Problems +-------- + +Low information density +~~~~~~~~~~~~~~~~~~~~~~~ + +Generally, you won't be able to stuff much more than four or five bit of information down a serial port using a +human-readable protocol. In many cases you will get much less. If you have 10 commands that are only issued a couple +times a second nobody cares that you spend maybe ten bytes per command on nice, verbose strings such as ``SET LED``. But +if you're trying to squeeze a half-kilobyte framebuffer down the line you might start to notice the difference between +hex and base-64 encoding, and a binary protocol might really be more up to the job. + +Complex parsing code +~~~~~~~~~~~~~~~~~~~~ + +On the computer side of thing, with the whole phalanx of an operating system, the standard library of your programming +language of choice and for all intents and purposes unlimted CPU and memory resources to spare you can easily parse +anything spoken on a serial port in real time, even at a blazing fast full Megabaud. The microcontroller side however is +an entirely different beast. On a small microcontroller, printf_ alone will eat about half your flash. On most small +microcontrollers, you just won't get a regex library even though it would make parsing textual commands *so much +simpler*. Lacking these resources, you might end up hand-knitting a lot of low-level C code to do something seemingly +simple such as parsing ``set_channel (13, 1.1333)\n``. These issues have to be taken into account in the protocol design +from the beginning. If you don't really need matching parentheses, don't use them. + +Fragile protocol state +~~~~~~~~~~~~~~~~~~~~~~ + +Say you have a ``SET_DISPLAY`` command. Now say your display can display four lines of text. The obvious approach to this +is probably the SMTP_ or HTTP_ way of sending ``SET_DISPLAY\nThis is line 1\nThis is line 2\n\n``. This would certainly +work, but it is very fragile. With this protocol, you're in trouble if at any point the terminating second newline +character gets mangled (say, someone unplugs the cable, or the control computer reboots, or a cosmic ray hits something +and ``0x10 '\n'`` turns into ``0x50 'P'``). + +.. _SMTP: https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol +.. _HTTP: https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol + +Timeouts don't work +~~~~~~~~~~~~~~~~~~~ + +You might try to solve the problem of your protocol state machine tangling up with a timeout. "If I don't get a valid +command for more than 200ms I go back to default state." But consider the above example. Say, your control computer +sends a ``SET_DISPLAY`` command every 100ms. If in one of them the state machine tangles up, the parser hangs since the +timeout is never hit, a new line of text arriving every 100ms. + +Framing is hard +~~~~~~~~~~~~~~~ + +You might also try to drop the second newline and using a convention such as ``SET_DISPLAY`` is followed by two lines of +text, then commands resume.". This works as long as your display contents never look like commands. If you are only ever +displaying the same three messages on a character LCD that might work, but if you're displaying binary framebuffer +data you've lost. + +Solutions +--------- + +Keep the state machine simple +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Always use a single line of text to represent a single command. Don't do protocol states or modes where you can toggle +between different interpretations for a line. If you have to send human-readable text as part of a command (such as +``SET_DISPLAY``) escape it so it doesn't contain any newlines. + +This way, you keep your protocol state machine simple. If at any time your serial trips and flips a bit or looses a byte +your protocol will recover on the next newline character, returning to its base state. + +Encode numbers in hex when possible +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Printing a number in hexadecimal is a very tidy operation, even on the smalest 8-bit microcontrollers. In contrast, +printing decimal requires both division and remainder in a loop which might get annoyingly code- and time-intensive on +large numbers (say a 32-bit int) and small microcontrollers. + +If you have to send fractional values, consider their precision. Instead of sending a 12 bit ADC result as a 32-bit +float formatted like ``0.176513671875`` sending ``0x2d3`` and dividing by 4096 on the host might be more sensible. If you +really have to communicate big floats and you can't take the overhead of including both printf_ and scanf_ you can +use hexadecimal floating point, which is basically ``hex((int)foo) + "." + hex((int)(65536*(foo - (int)foo)))`` for four +digits. You can also just hex-encode the binary IEEE-754_ representation of the float, sending ``hex(*(int *)&float)``. +Most programming languages will have a `simple, built-in means to parse this sort of thing +<https://docs.python.org/3.5/library/struct.html>`__. + +.. _printf: http://git.musl-libc.org/cgit/musl/tree/src/stdio/vfprintf.c +.. _scanf: http://git.musl-libc.org/cgit/musl/tree/src/stdio/vfscanf.c +.. _IEEE-754: https://en.wikipedia.org/wiki/IEEE_754 + +Escape multiline strings +~~~~~~~~~~~~~~~~~~~~~~~~ + +If you have to send arbitrary strings, escape special characters. This not only has the advantage of yielding a robust +protocol: It also ensures you can actually see everything that's going on when debugging. The string ``"\r\n"`` is easy to +distinguish from ``"\n"`` while your terminal emulator might not care. + +The simplest encoding to use is the C-style backslash encoding. Host-side, most languages will have a `built-in means of +escaping a string like that <https://docs.python.org/3.5/library/codecs.html#text-encodings>`__. + +Encoding binary data +-------------------- + +For binary data, hex and base-64 are the most common encodings. Since hex is simpler to implement I'd go with it unless +I really need the 30% bandwidth improvement base-64 brings. + +Binary serial protocols +======================= + +In contrast to anything human-readable, binary protocols are generally more bandwidth-efficient and are easier to format +and parse. However, binary protocols come with their own version of the caveats we discussed for text-based protocols. + +The framing problem in binary protocols +--------------------------------------- + +The most basic problems with binary protocols as with text-based ones is framing, i.e. splitting up the continuous +serial data stream into discrete packets. The issue is that it is that you have to somehow mark boundaries between +frames. The simplest way would be to use some special character to delimit frames, but then any 8-bit character you +could choose could also occur within a frame. + +SLIP/PPP-like special character framing +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Some protocols solve this problem much like we have solved it above for strings in line-based protocols, by escaping any +occurence of the special delimiter character within frames. That is, if you want to use ``0x00`` as a delimiter, you would +encode a packet containing ``0xde 0xad 0x00 0xbe 0xef`` as something like ``0xde 0xad 0x01 0x02 0xbe 0xef``, replacing the +null byte with a magic sequence. This framing works, but is has one critical disadvantage: The length of the resulting +escaped data is dependent on the raw data, and in the worst case twice as long. In a raw packet consisting entirely of +null bytes, every byte must be escaped with two escape bytes. This means that in this case the packet length doubles, +and in this particular case we're even less efficient than base-64. + +Highly variable packet length is also bad since it makes it very hard to make any timing guarantees for our protocol. + +9-bit framing +~~~~~~~~~~~~~ + +A framing mode sometimes used is to configure the UARTs to transmit 9-bit characters and to use the 9th bit to designate +control characters. This works really well, and gives plenty of control characters to work with. The main problem with +this is that a 9-bit serial interface is highly nonstandard and you need UARTs on both ends that actually support this +mode. Another issue is that though more efficient than both delmitier-based and purely text-based protocols, it still +incurs an extra about 10% of bandwidth overhead. This is not a lot if all you're sending is a little command every now +and then, but if you're trying to push large amounts of data through your serial it's still bad. + +COBS +~~~~ + +Given the limitations of the two above-mentioned framing formats, we really want something better. The `Serial Line +Internet Protocol (SLIP)`_ as well as the `Point to Point Protocol (PPP)`_, standardized in 1988 and 1994 respectively, +both use escape sequences. This might come as a surprise, but humanity has actually still made significant technological +progress on protocols for 8-bit serial interfaces until the turn of the millennium. In 1999, `Consistent Overhead Byte +Stuffing (COBS)`_ (`wiki <https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing>`__) was published by a few +researchers from Apple Computer and Stanford University. As a reaction on the bandwidth doubling problem present in +PPP_, COBS *always* has an overhead of a single byte, no matter what or how long a packet's content is. + +COBS uses the null byte as a delimiter interleaves all the raw packet data and a `run-length encoding`_ of the non-zero +portions of the raw packet. That is, it prepends the number of bytes until the first zero byte to the packet, plus one. +Then it takes all the leading non-zero bytes of the packet, unmodified. Then, it again encodes the distance from the +first zero to the second zero, plus one. And then it takes the second non-zero run of bytes unmodified. And so on. At +the end, the packet is terminated with a zero byte. + +The result of this scheme is that the encoded packet does not contain any zero bytes, as every zero byte has been +replaced with the number of bytes until the next zero byte, plus one, and that can't be zero. Both formatter and parser +each have to keep a counter running to keep track of the distances between zero bytes. The first byte of the packet +initializes that counter and is dropped by the parser. After that, every encoded byte received results in one raw byte +parsed. + +While this might sound more complicated than the escaping explained above, the gains in predictability and efficiency +are worth it. An implementation of encoder and decoder should each be about ten lines of C or two lines of Python. A +minor asymmetry of the protocol is that while decoding can be done in-place, encoding either needs two passes or you +need to scan forward for the next null byte. + +.. _`Point to Point Protocol (PPP)`: https://en.wikipedia.org/wiki/Point-to-Point_Protocol +.. _PPP: https://en.wikipedia.org/wiki/Point-to-Point_Protocol +.. _`Serial Line Internet Protocol (SLIP)`: https://en.wikipedia.org/wiki/Serial_Line_Internet_Protocol +.. _`Consistent Overhead Byte Stuffing (COBS)`: http://www.stuartcheshire.org/papers/COBSforToN.pdf +.. _`Point-to-Point Protocol (PPP)`: https://en.wikipedia.org/wiki/Point-to-Point_Protocol +.. _`run-length encoding`: https://en.wikipedia.org/wiki/Run-length_encoding + +State machines and error recovery +--------------------------------- + +In binary protocols even more than in textual ones it is tempting to build complex state machines triggering actions on +a sequence of protocol packets. Please resist that temptation. As with textual protocols keeping the protocol state to +the minimum possible allows for a self-synchronizing protocol. A serial protocol should be designed such that if due to +a dropped packet or two both ends will naturally re-synchronize within another packet or two. A simple way of doing that +is to always transmit one semantic command per packet and to design these commands in the most idempotent_ way possible. +For example, when filling a framebuffer piece by piece, include the offset in each piece instead of keeping track of it +on the receiving side. + +.. _idempotent: https://en.wikipedia.org/wiki/Idempotence#Computer_science_meaning + +Conclusion +========== + +Here's your five-step guide to serial bliss: + +1. Unless you have super-special requirements, always use the slowest you can get away with from 9600Bd, 115200Bd or + 1MBd. 8N1 framing if you're talking to anything but another microcontroller on the same board. These settings are + the most common and cover any use case. You'll inevitably have to guess these at some point in the future. +2. If you're doing something simple and speed is not a particular concern, use a human-readable text-based protocol. Use + one command/reply per line, begin each line with some sort of command word and format numbers in hexadecimal. You get + bonus points if the device replies to unknown commands with a human-readable status message and prints a brief + protocol overview on boot. +3. If you're doing something even slightly nontrivial or need moderate throughput (>1k commands per second or >20 byte of + data per command) use a COBS-based protocol. If you don't have a better idea, go for an ``[target MAC][command + ID][command arguments]`` packet format for multidrop busses. For single-drop you may decide to drop the MAC address. +4. Always include some sort of "status" command that prints life stats such as VCC, temperature, serial framing errors + and uptime. You'll need some sort of ping command anyway and that one might as well do something useful. +5. If at all possible, keep your protocol context-free across packets/lines. That is, a certain command should always be + self-contained, and no command should change the meaning of the next packet or line that is sent. This is really + important to allow for self-synchronization. If you really need to break up something into multiple commands, say you + want to set a large framebuffer in pieces, do it in a idempotent_ way: Instead of sending something like ``FRAMEBUFFER + INCOMING:\n[byte 0-16]\n[byte 17-32]\n[...]\nEND OF FRAME`` rather send ``FRAMEBUFFER DATA FOR OFFSET 0: [byte + 0-16]\nFRAMEBUFFER DATA FOR OFFSET 17: [byte 17-32]\n[...]\nSWAP BUFFERS\n``. |