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