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diff --git a/content/posts/serial-protocols/index.rst b/content/posts/serial-protocols/index.rst deleted file mode 100644 index 5797947..0000000 --- a/content/posts/serial-protocols/index.rst +++ /dev/null @@ -1,249 +0,0 @@ ---- -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, 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``. - |