Marlin is targeting a plethora of different CPU architecture and platforms. Each of these platforms has its own serial interface. While many provide a Arduino-like Serial class, it’s not all of them, and the differences in the existing API create a very complex brain teaser for writing code that works more or less on each platform.
Moreover, many platform have intrinsic needs about serial port (like forwarding the output on multiple serial port, providing a serial-like telnet server, mixing USB-based serial port with SD card emulation) that are difficult to handle cleanly in the other platform serial logic.
Starting with version 2.0.9
, Marlin provides a common interface for its serial needs.
This interface is declared in Marlin/src/core/serial_base.h
Any implementation will need to follow this interface for being used transparently in Marlin’s codebase.
The implementation was written to prioritize performance over abstraction, so the base interface is not using virtual inheritance to avoid the cost of virtual dispatching while calling methods. Instead, the Curiously Recurring Template Pattern (CRTP) is used so that, upon compilation, the interface abstraction does not incur a performance cost.
Because some platform do not follow the same interface, the missing method in the actual low-level implementation are detected via SFINAE and a wrapper is generated when such method are missing. See the CALL_IF_EXISTS
macro in Marlin/src/core/macros.h
for documentation of this technique.
The different specificities for each architecture are provided by composing the serial type based on desired functionality.
In the Marlin/src/core/serial_hook.h
file, the different serial feature are declared and defined in each templated type:
BaseSerial
is a simple 1:1 wrapper to the underlying, Arduino compatible, Serial
’s class. It derives from it. You’ll use this if the platform does not do anything specific for the Serial
object (for example, if an interrupt callback calls directly the serial instance in the platform’s framework code, this is not the right class to use). This wrapper is completely inlined so that it does not generate any code upon compilation. BaseSerial
constructor forwards any parameter to the platform’s Serial
’s constructor.ForwardSerial
is a composing wrapper. It references an actual Arduino compatible Serial
instance. You’ll use this if the instance is declared in the platform’s framework and is being referred directly in the framework. This is not as efficient as the BaseSerial
implementation since static dereferencing is done for each method call (it’ll still be faster than virtual dispatching)ConditionalSerial
is working a bit like the ForwardSerial
interface, but it checks a boolean condition before calling the referenced instance. You’ll use it when the serial output can be switch off at runtime, for example in a telnet like serial output that should not emit any packet if no client is connected.RuntimeSerial
is providing a runtime-modifiable hooking method for its write
and msgDone
method. You’ll use it if you need to capture the serial output of Marlin, for example to display the G-Code parser’s output on a GUI interface. The hooking interface is setup via the setHook
method.MultiSerial
is a runtime modifiable serial output multiplexer. It can output (respectively input) to 2 different interface based on a port mask. You’ll use this if you need to output the same serial stream to multiple port. You can plug a MultiSerial
to itself to duplicate to more than 2 ports.Since all the types above are using CRTP, it’s possible to combine them to get the appropriate functionality. This is easily done via type definition of the feature.
For example, to create a single serial interface with 2 serial outputs (one enabled at runtime and the other switchable):
typedef MultiSerial< RuntimeSerial<Serial>, ConditionalSerial<TelnetClient> > Serial0Type;
To send the same output to 4 serial ports you could nest MultiSerial
like this:
typedef MultiSerial< MultiSerial< BaseSerial<Serial>, BaseSerial<Serial1> >, MultiSerial< BaseSerial<Serial2>, BaseSerial<Serial3>, 2, 1>, 0, 2> Serial0Type;
The magical numbers here are the step and offset for computing the serial port. Simplifying the above monster a bit:
MS< A = MS<a, b, offset=0, step=1>, B=MS<c, d, offset=2, step=1>, offset=0, step=2>
This means that the underlying multiserial A (with output to a,b
) is available from offset = 0 to offset + step = 1 (default value).
The multiserial B (with output to c,d
) is available from offset = 2 (the next step from the root multiserial) to offset + step = 3.
In practice, the root multiserial will redirect any index/mask offset
to offset + step - 1
to its first leaf, and any index/mask offset + step
to offset + 2*step - 1
to its second leaf.
By default, the serial base interface provide an emergency parser that’s only enable for serial classes that support it. Because of this condition, all underlying types take a first bool emergencyParserEnabled
argument to their constructor. You must take into account this parameter when defining the actual type used.
The following macros are defined (in serial.h
) to output data to the serial ports:
MACRO | Parameters | Usage | Example | Expected output |
---|---|---|---|---|
SERIAL_ECHO |
Any basic type is supported (char , uint8_t , int16_t , int32_t , float , long , const char* , …). |
For a numeric type it prints the number in decimal. A string is output as a string. | uint8_t a = 123; SERIAL_ECHO(a); SERIAL_CHAR(' '); SERIAL_ECHO(' '); |
123 32 |
SERIAL_ECHOLN |
Same as SERIAL_ECHO |
Do SERIAL_ECHO , adding a newline |
int a = 456; SERIAL_ECHOLN(a); |
456\n |
SERIAL_ECHO_F |
float or double |
Print a decimal value with a given precision (default 2) | float a = 3.1415; SERIAL_ECHO_F(a); SERIAL_CHAR(' '); SERIAL_ECHO_F(a, 4); |
3.14 3.1415 |
SERIAL_ECHOPAIR |
String / Value pairs | Print a series of string literals and values alternately | SERIAL_ECHOPAIR("Bob", 34); |
Bob34 |
SERIAL_ECHOLNPAIR |
Same as SERIAL_ECHOPAIR |
Do SERIAL_ECHOPAIR , adding a newline |
SERIAL_ECHOPAIR("Alice", 56); |
alice56 |
SERIAL_ECHOPAIR_P |
Like SERIAL_ECHOPAIR but takes PGM strings |
Print a series of PGM strings and values alternately | SERIAL_ECHOPAIR_P(GET_TEXT(MSG_HELLO), 123); |
Hello123 |
SERIAL_ECHOLNPAIR_P |
Same as SERIAL_ECHOPAIR_P |
Do SERIAL_ECHOPAIR_P , adding a newline |
SERIAL_ECHOLNPAIR_P(PSTR("Alice"), 78); |
alice78\n |
SERIAL_ECHOLIST |
String literal, values | Print a string literal and a list of values | SERIAL_ECHOLIST("Key ", 1, 2, 3); |
Key 1, 2, 3 |
SERIAL_ECHO_START |
None | Prefix an echo line | SERIAL_ECHO_START(); |
echo: |
SERIAL_ECHO_MSG |
Same as SERIAL_ECHOLN_PAIR |
Print a full echo line | SERIAL_ECHO_MSG("Count is ", count); |
echo:Count is 3 |
SERIAL_ERROR_START |
None | Prefix an error line | SERIAL_ERROR_START(); |
Error: |
SERIAL_ERROR_MSG |
Same as SERIAL_ECHOLN_PAIR |
Print a full error line | SERIAL_ERROR_MSG("Not found"); |
Error:Not found |
SERIAL_ECHO_SP |
Number of spaces | Print one or more spaces | SERIAL_ECHO_SP(3) |
|
SERIAL_EOL |
None | Print an end of line | SERIAL_EOL(); |
\n |
SERIAL_OUT |
SERIAL_OUT(myMethod) |
Call a custom serial method | SERIAL_OUT(msgDone); |
… |
This document was written by X-Ryl669 and is under CC-SA license