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I will +summarize some points on how to design a serial protocol that is simple to implement and works reliably even under error +conditions.</p> +<p>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, an old-school 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.</p> +<p>This simplicity is treacherous, though. Oftentimes, you start writing your serial protocol as needs arise. Things might +start harmless with something like <tt class="docutils literal">SET_LED ON\n</tt>, but as the code grows 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 <a class="reference external" href="https://github.com/juhasch/pyBusPirateLite/blob/dece35f6e421d4f6a007d1db98d148e2f2126ebb/pyBusPirateLite/base.py#L113">this gem</a> 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.</p> +<div class="section" id="text-based-serial-protocols"> +<h2>Text-based serial protocols</h2> +<p>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.</p> +<p>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.</p> +<div class="section" id="problems"> +<h3>Problems</h3> +<div class="section" id="low-information-density"> +<h4>Low information density</h4> +<p>Generally, you won't be able to stuff much more than four or five bit of information down a serial port using a +single byte of 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 +<tt class="docutils literal">SET LED</tt>. 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.</p> +</div> +<div class="section" id="complex-parsing-code"> +<h4>Complex parsing code</h4> +<p>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, <a class="reference external" href="http://git.musl-libc.org/cgit/musl/tree/src/stdio/vfprintf.c">printf</a> 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 <em>so much +simpler</em>. Lacking these resources, you might end up hand-knitting a lot of low-level C code to do something seemingly +simple such as parsing <tt class="docutils literal">set_channel (13, <span class="pre">1.1333)\n</span></tt>. These issues have to be taken into account in the protocol design +from the beginning. For example, you don't really need matching parentheses, don't use them.</p> +</div> +<div class="section" id="fragile-protocol-state"> +<h4>Fragile protocol state</h4> +<p>Say you have a <tt class="docutils literal">SET_DISPLAY</tt> command. Now say your display can display four lines of text. The obvious approach to this +is probably the <a class="reference external" href="https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol">SMTP</a> or <a class="reference external" href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol">HTTP</a> way of sending <tt class="docutils literal">SET_DISPLAY\nThis is line 1\nThis is line 2\n\n</tt>. 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 <tt class="docutils literal">0x10 '\n'</tt> turns into <tt class="docutils literal">0x50 'P'</tt>).</p> +</div> +<div class="section" id="timeouts-don-t-work"> +<h4>Timeouts don't work</h4> +<p>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 <tt class="docutils literal">SET_DISPLAY</tt> command every 100ms. If in one of them the state machine tangles up, the parser hangs since the +timeout is never hit, because a new line of text is arriving every 100ms.</p> +</div> +<div class="section" id="framing-is-hard"> +<h4>Framing is hard</h4> +<p>You might also try to drop the second newline and using a convention such as <tt class="docutils literal">SET_DISPLAY</tt> 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.</p> +</div> +</div> +<div class="section" id="solutions"> +<h3>Solutions</h3> +<div class="section" id="keep-the-state-machine-simple"> +<h4>Keep the state machine simple</h4> +<p>In a text-based protocol, 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 <tt class="docutils literal">SET_DISPLAY</tt>), escape it so it doesn't contain any newlines.</p> +<p>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.</p> +</div> +<div class="section" id="encode-numbers-in-hex-when-possible"> +<h4>Encode numbers in hex when possible</h4> +<p>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.</p> +<p>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 <tt class="docutils literal">0.176513671875</tt> sending <tt class="docutils literal">0x2d3</tt> 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 <a class="reference external" href="http://git.musl-libc.org/cgit/musl/tree/src/stdio/vfprintf.c">printf</a> and <a class="reference external" href="http://git.musl-libc.org/cgit/musl/tree/src/stdio/vfscanf.c">scanf</a> you can +use hexadecimal floating point, which is basically <tt class="docutils literal"><span class="pre">hex((int)foo)</span> + "." + <span class="pre">hex((int)(65536*(foo</span> - <span class="pre">(int)foo)))</span></tt> for four +digits. You can also just hex-encode the binary <a class="reference external" href="https://en.wikipedia.org/wiki/IEEE_754">IEEE-754</a> representation of the float, sending <tt class="docutils literal"><span class="pre">hex(*(int</span> <span class="pre">*)&float)</span></tt>. +Most programming languages will have a <a class="reference external" href="https://docs.python.org/3.5/library/struct.html">simple, built-in means to parse this sort of thing</a>.</p> +</div> +<div class="section" id="escape-multiline-strings"> +<h4>Escape multiline strings</h4> +<p>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 <tt class="docutils literal">"\r\n"</tt> is easy to +distinguish from <tt class="docutils literal">"\n"</tt> while your terminal emulator might not care.</p> +<p>The simplest encoding to use is the C-style backslash encoding. Host-side, most languages will have a <a class="reference external" href="https://docs.python.org/3.5/library/codecs.html#text-encodings">built-in means of +escaping a string like that</a>.</p> +</div> +</div> +<div class="section" id="encoding-binary-data"> +<h3>Encoding binary data</h3> +<p>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.</p> +</div> +</div> +<div class="section" id="binary-serial-protocols"> +<h2>Binary serial protocols</h2> +<p>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.</p> +<div class="section" id="the-framing-problem-in-binary-protocols"> +<h3>The framing problem in binary protocols</h3> +<p>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.</p> +<div class="section" id="slip-ppp-like-special-character-framing"> +<h4>SLIP/PPP-like special character framing</h4> +<p>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 <tt class="docutils literal">0x00</tt> as a delimiter, you would +encode a packet containing <tt class="docutils literal">0xde 0xad 0x00 0xbe 0xef</tt> as something like <tt class="docutils literal">0xde 0xad 0x01 0x02 0xbe 0xef</tt>, 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.</p> +<p>Highly variable packet length is also bad since it makes it very hard to make any timing guarantees for our protocol.</p> +</div> +<div class="section" id="bit-framing"> +<h4>9-bit framing</h4> +<p>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.</p> +</div> +<div class="section" id="cobs"> +<h4>COBS</h4> +<p>Given the limitations of the two above-mentioned framing formats, we really want something better. The <a class="reference external" href="https://en.wikipedia.org/wiki/Serial_Line_Internet_Protocol">Serial Line +Internet Protocol (SLIP)</a> as well as the <a class="reference external" href="https://en.wikipedia.org/wiki/Point-to-Point_Protocol">Point to Point Protocol (PPP)</a>, 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, <a class="reference external" href="http://www.stuartcheshire.org/papers/COBSforToN.pdf">Consistent Overhead Byte +Stuffing (COBS)</a> (<a class="reference external" href="https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing">wiki</a>) was published by a few +researchers from Apple Computer and Stanford University. As a reaction on the bandwidth doubling problem present in +<a class="reference external" href="https://en.wikipedia.org/wiki/Point-to-Point_Protocol">PPP</a>, COBS <em>always</em> has an overhead of a single byte, no matter what or how long a packet's content is.</p> +<p>COBS uses the null byte as a delimiter interleaves all the raw packet data and a <a class="reference external" href="https://en.wikipedia.org/wiki/Run-length_encoding">run-length encoding</a> 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.</p> +<p>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.</p> +<p>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.</p> +</div> +</div> +<div class="section" id="state-machines-and-error-recovery"> +<h3>State machines and error recovery</h3> +<p>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 <a class="reference external" href="https://en.wikipedia.org/wiki/Idempotence#Computer_science_meaning">idempotent</a> 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.</p> +</div> +</div> +<div class="section" id="conclusion"> +<h2>Conclusion</h2> +<p>Here's your five-step guide to serial bliss:</p> +<ol class="arabic simple"> +<li>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. Using common values +like these makes it easier when you'll inevitably have to guess these at some point in the future ;)</li> +<li>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. Bonus +points for the device replying to unknown commands with a human-readable status message and printing a brief protocol +overview on boot.</li> +<li>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. A good starting point is a <tt class="docutils literal">[target <span class="pre">MAC][command</span> <span class="pre">ID][command</span> +arguments]</tt> packet format for multidrop busses. For single-drop you may decide to drop the MAC address.</li> +<li>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 command might as well do something useful.</li> +<li>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/line/command 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 <a class="reference external" href="https://en.wikipedia.org/wiki/Idempotence#Computer_science_meaning">idempotent</a> way: Instead of sending something like <tt class="docutils literal">FRAMEBUFFER +<span class="pre">INCOMING:\n[byte</span> <span class="pre">0-16]\n[byte</span> <span class="pre">17-32]\n[...]\nEND</span> OF FRAME</tt> rather send <tt class="docutils literal">FRAMEBUFFER DATA FOR OFFSET 0: [byte +<span class="pre">0-16]\nFRAMEBUFFER</span> DATA FOR OFFSET 17: [byte <span class="pre">17-32]\n[...]\nSWAP</span> BUFFERS\n</tt>.</li> +</ol> +</div> +</div> +</main> + + <footer> + +<script> +(function() { + function center_el(tagName) { + var tags = document.getElementsByTagName(tagName), i, tag; + for (i = 0; i < tags.length; i++) { + tag = tags[i]; + var parent = tag.parentElement; + + if (parent.childNodes.length === 1) { + + if (parent.nodeName === 'A') { + parent = parent.parentElement; + if (parent.childNodes.length != 1) continue; + } + if (parent.nodeName === 'P') parent.style.textAlign = 'center'; + } + } + } + var tagNames = ['img', 'embed', 'object']; + for (var i = 0; i < tagNames.length; i++) { + center_el(tagNames[i]); + } +})(); +</script> + + + <div id="license-info"> + ©2020 by Jan Götte. 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