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diff --git a/content/blog/serial-protocols/index.rst b/content/blog/serial-protocols/index.rst new file mode 100644 index 0000000..2f9bb2d --- /dev/null +++ b/content/blog/serial-protocols/index.rst @@ -0,0 +1,249 @@ +--- +title: "How to talk to your microcontroller over serial" +date: 2018-05-19T08:09:46+02:00 +--- + +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, 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. + +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 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 `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/dece35f6e421d4f6a007d1db98d148e2f2126ebb/pyBusPirateLite/base.py#L113 + +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 +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 +``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. For example, 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, because a new line of text is 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 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +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 ``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. Using common values + like these makes it easier when 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. Bonus + points for the device replying to unknown commands with a human-readable status message and printing 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. A good starting point is a ``[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 command 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/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 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``. + |