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.. highlight:: cpp
.. _lang-serialusb:
``SerialUSB``
=============
Used for communication between the Maple board and a computer.
.. contents:: Contents
:local:
Introduction
------------
In addition to three :ref:`serial ports <lang-serial>`, the Maple's
STM32 microprocessor includes a dedicated USB peripheral. This
peripheral is used to emulate a regular serial port for use as a
terminal (text read/write). The emulated terminal is relatively slow
and inefficient; it is best for transferring data at regular serial
speeds (kilobaud).
Library access to the emulated serial port is provided through the
``SerialUSB`` object. You can mostly use ``SerialUSB`` as a drop-in
replacement for ``Serial1``, ``Serial2``, and ``Serial3``.
.. warning:: The ``SerialUSB`` functionality includes a 50 millisecond
timeout for writes, and does not try to detect if the USB host is
"really" connected, or just enumerated and initialized.
This means that if you have a number of calls to one of the
``SerialUSB`` ``write()`` or ``print()`` functions in your code,
and you are not monitoring the emulated on a computer, your program
will run much, much slower than if it is being monitored or totally
disconnected (run off of a battery).
You can avoid this behavior by :ref:`deciphering the port status
using the DTR and RTS line status <lang-serialusb-safe-print>`; the
behavior of these control lines is platform dependent and we no
longer interpret them by default.
Library Documentation
---------------------
The ``SerialUSB`` object is an instance of the ``USBSerial`` class,
which is documented in this section. This means that you can use any
of these functions by writing
``SerialUSB.functionName(arguments...)``. For example, to print the
message "hello, world!", you can write ``USBSerial.println("hello,
world!")``.
.. cpp:class:: USBSerial
Emulated serial-over-USB class. ``SerialUSB`` is the predefined
(singleton) instance.
.. _lang-serialusb-begin:
.. cpp:function:: USBSerial::begin()
Set up the USB peripheral for emulated serial communication. The
peripheral is configured this way by default; calling this function
should only be necessary if you have disabled the peripheral using
``SerialUSB.end()``.
.. _lang-serialusb-end:
.. cpp:function:: USBSerial::end()
Disables the USB peripheral. Note that using this function will
terminate all USB communications between the Maple and the USB
host; in particular, it implies that you won't be able to upload
any new programs without resetting the board or using
:ref:`perpetual bootloader mode
<troubleshooting-perpetual-bootloader>`.
.. cpp:function:: unsigned int USBSerial::available()
Returns the number of bytes available for reading.
.. _lang-serialusb-read:
.. cpp:function:: unsigned char USBSerial::read()
Returns the next available, unread character. If there are no
available characters (you can check this with :cpp:func:`available
<USBSerial::available>`), the call will block until one
becomes available.
.. cpp:function:: USBSerial::print(unsigned char b)
Print the given byte over the USB connection.
.. cpp:function:: USBSerial::print(char c)
Print the given character over the USB connection. 7-bit clean characters
are typically interpreted as ASCII text.
.. cpp:function:: USBSerial::print(const char *str)
Print the given null-terminated string over the USB connection.
.. cpp:function:: USBSerial::print(int n)
Print the argument's digits over the USB connection, in decimal format.
Negative values will be prefixed with a ``'-'`` character.
.. cpp:function:: USBSerial::print(unsigned int n)
Print the argument's digits over the USB connection, in decimal format.
.. cpp:function:: USBSerial::print(long n)
Print the argument's digits over the USB connection, in decimal
format. Negative values will be prefixed with a ``'-'`` character.
.. cpp:function:: USBSerial::print(unsigned long n)
Print the argument's digits over the USB connection, in decimal
format.
.. cpp:function:: USBSerial::print(long n, int base)
Print the digits of ``n`` over the USB connection, in base ``base``
(which may be between 2 and 16). The ``base`` value 2 corresponds
to binary, 8 to octal, 10 to decimal, and 16 to hexadecimal.
Negative values will be prefixed with a ``'-'`` character.
.. cpp:function:: USBSerial::print(double n)
Print ``n``, accurate to 2 digits after the decimal point.
.. _lang-serialusb-println:
.. cpp:function:: USBSerial::println(char c)
Like ``print(c)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(const char *c)
Like ``print(c)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(unsigned char b)
Like ``print(b)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(int n)
Like ``print(n)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(unsigned int n)
Like ``print(n)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(long n)
Like ``print(n)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(unsigned long n)
Like ``print(n)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(long n, int base)
Like ``print(n, b)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println(double n)
Like ``print(n)``, followed by ``"\r\n"``.
.. cpp:function:: USBSerial::println()
Prints ``"\r\n"`` over the USB connection.
.. cpp:function:: USBSerial::write(unsigned char ch)
Sends one character over the USB connection. This function is
currently blocking, although nonblocking writes are a planned
future extension.
This is a low-level function. One of the ``print()`` or
``println()`` functions is likely to be more useful when printing
multiple characters, when formatting numbers for printing, etc.
.. cpp:function:: USBSerial::write(const char* str)
Send the given null-terminated character string over the USB
connection.
This is a low-level function. One of the ``print()`` or
``println()`` functions is likely to be more useful when printing
multiple characters, when formatting numbers for printing, etc.
.. cpp:function:: USBSerial::write(void *buf, unsigned int size)
Writes the first ``size`` bytes of ``buf`` over the USB connection.
Each byte is transmitted as an individual character.
This is a low-level function. One of the ``print()`` or
``println()`` functions is likely to be more useful when printing
multiple characters, when formatting numbers for printing, etc.
Examples
--------
.. _lang-serialusb-safe-print:
**Safe print**: This function should run smoothly and not block; the
LED should blink at roughly the same speed whether being monitored,
running from battery, or connected but not monitored. You may need to
experiment with the DTR/RTS logic for your platform and device
configuration. ::
#define LED_PIN BOARD_LED_PIN
void setup() {
/* Set up the LED to blink */
pinMode(LED_PIN, OUTPUT);
}
void loop() {
// LED will stay off if we are disconnected, and will blink
// quickly if USB is unplugged (battery power, etc.).
if(SerialUSB.isConnected()) {
digitalWrite(LED_PIN, 1);
}
delay(100);
// If this logic fails to detect if bytes are going to be read
// by the USB host, then the println() take a long time,
// causing a very slow LED blink. If the characters are
// printed and read, the blink will only slow a small amount
// when "really" connected, and will be fast fast when the
// virtual port is only configured.
if(SerialUSB.isConnected() && (SerialUSB.getDTR() || SerialUSB.getRTS())) {
for(int i = 0; i < 10; i++) {
SerialUSB.println(123456, BIN);
}
}
digitalWrite(LED_PIN, 0);
delay(100);
}
|
