esp8266/doc/reference.rst

Reference
=========

Interrupts
----------

Interrupts can be used on the ESP8266, but they must be used with care
and have several limitations:

* Interrupt callback functions must be in IRAM, because the flash may be
  in the middle of other operations when they occur.  Do this by adding
  the ``ICACHE_RAM_ATTR`` attribute on the function definition.  If this
  attribute is not present, the sketch will crash when it attempts to
  ``attachInterrupt`` with an error message.  

.. code:: cpp

    ICACHE_RAM_ATTR void gpio_change_handler(void *data) {...

* Interrupts must not call ``delay()`` or ``yield()``, or call any routines
  which internally use ``delay()`` or ``yield()`` either.
  
* Long-running (>1ms) tasks in interrupts will cause instabilty or crashes.
  WiFi and other portions of the core can become unstable if interrupts
  are blocked by a long-running interrupt.  If you have much to do, you can
  set a volatile global flag that your main ``loop()`` can check each pass
  or use a scheduled function (which will be called outside of the interrupt
  context when it is safe) to do long-running work.

* Memory operations can be dangerous and should be avoided in interrupts.
  Calls to ``new`` or ``malloc`` should be minimized because they may require
  a long running time if memory is fragmented.  Calls to ``realloc`` and
  ``free`` must NEVER be called.  Using any routines or objects which call
  ``free`` or ``realloc`` themselves is also forbidden for the same reason.
  This means that ``String``, ``std::string``, ``std::vector`` and other
  classes which use contiguous memory that may be resized must be used with
  extreme care (ensuring strings aren't changed, vector elements aren't
  added, etc.).

Digital IO
----------

Pin numbers in Arduino correspond directly to the ESP8266 GPIO pin
numbers. ``pinMode``, ``digitalRead``, and ``digitalWrite`` functions
work as usual, so to read GPIO2, call ``digitalRead(2)``.

Digital pins 0—15 can be ``INPUT``, ``OUTPUT``, or ``INPUT_PULLUP``. Pin
16 can be ``INPUT``, ``OUTPUT`` or ``INPUT_PULLDOWN_16``. At startup,
pins are configured as ``INPUT``.

Pins may also serve other functions, like Serial, I2C, SPI. These
functions are normally activated by the corresponding library. The
diagram below shows pin mapping for the popular ESP-12 module.

.. figure:: esp12.png
   :alt: Pin Functions

   Pin Functions

Digital pins 6—11 are not shown on this diagram because they are used to
connect flash memory chip on most modules. Trying to use these pins as
IOs will likely cause the program to crash.

Note that some boards and modules (ESP-12ED, NodeMCU 1.0) also break out
pins 9 and 11. These may be used as IO if flash chip works in DIO mode
(as opposed to QIO, which is the default one).

Pin interrupts are supported through ``attachInterrupt``,
``detachInterrupt`` functions. Interrupts may be attached to any GPIO
pin, except GPIO16. Standard Arduino interrupt types are supported:
``CHANGE``, ``RISING``, ``FALLING``. ISRs need to have
``ICACHE_RAM_ATTR`` before the function definition.

Analog input
------------

ESP8266 has a single ADC channel available to users. It may be used
either to read voltage at ADC pin, or to read module supply voltage
(VCC).

To read external voltage applied to ADC pin, use ``analogRead(A0)``.
Input voltage range of bare ESP8266 is 0 — 1.0V, however some many 
boards may implement voltage dividers. To be on the safe side, <1.0V 
can be tested. If e.g. 0.5V delivers values around ~512, then maximum 
voltage is very likely to be 1.0V and 3.3V may harm the ESP8266. 
However values around ~150 indicates that the maximum voltage is 
likely to be 3.3V.

To read VCC voltage, use ``ESP.getVcc()`` and ADC pin must be kept
unconnected. Additionally, the following line has to be added to the
sketch:

.. code:: cpp

    ADC_MODE(ADC_VCC);

This line has to appear outside of any functions, for instance right
after the ``#include`` lines of your sketch.

Analog output
-------------

``analogWrite(pin, value)`` enables software PWM on the given pin. PWM
may be used on pins 0 to 16. Call ``analogWrite(pin, 0)`` to disable PWM
on the pin.

``value`` may be in range from 0 to 255 (which is the Arduino default).
PWM range may be changed by calling ``analogWriteRange(new_range)`` or
``analogWriteResolution(bits)``.  ``new_range`` may be from 15...65535
or ``bits`` may be from 4...16.

The function ``analogWriteMode(pin, value, openDrain)`` allows to sets
the pin mode to ``OUTPUT_OPEN_DRAIN`` instead of ``OUTPUT``.

**NOTE:** The default ``analogWrite`` range was 1023 in releases before
3.0, but this lead to incompatibility with external libraries which
depended on the Arduino core default of 256.  Existing applications which
rely on the prior 1023 value may add a call to ``analogWriteRange(1023)``
to their ``setup()`` routine to return to their old behavior.  Applications
which already were calling ``analogWriteRange`` need no change.

PWM frequency is 1kHz by default. Call
``analogWriteFreq(new_frequency)`` to change the frequency. Valid values 
are from 100Hz up to 40000Hz.

The ESP doesn't have hardware PWM, so the implementation is by software. 
With one PWM output at 40KHz, the CPU is already rather loaded. The more 
PWM outputs used, and the higher their frequency, the closer you get to 
the CPU limits, and the fewer CPU cycles are available for sketch execution.

Timing and delays
-----------------

``millis()`` and ``micros()`` return the number of milliseconds and
microseconds elapsed after reset, respectively.

``delay(ms)`` pauses the sketch for a given number of milliseconds and
allows WiFi and TCP/IP tasks to run. ``delayMicroseconds(us)`` pauses
for a given number of microseconds.

Remember that there is a lot of code that needs to run on the chip
besides the sketch when WiFi is connected. WiFi and TCP/IP libraries get
a chance to handle any pending events each time the ``loop()`` function
completes, OR when ``delay`` is called. If you have a loop somewhere in
your sketch that takes a lot of time (>50ms) without calling ``delay``,
you might consider adding a call to ``delay`` function to keep the WiFi
stack running smoothly.

There is also a ``yield()`` function which is equivalent to
``delay(0)``. The ``delayMicroseconds`` function, on the other hand,
does not yield to other tasks, so using it for delays more than 20
milliseconds is not recommended.

Serial
------

The ``Serial`` object works much the same way as on a regular Arduino. Apart
from the hardware FIFO (128 bytes for TX and RX), ``Serial`` has an 
additional customizable 256-byte RX buffer. The size of this software buffer can
be changed by the user. It is suggested to use a bigger size at higher receive speeds.

The ``::setRxBufferSize(size_t size)`` method changes the RX buffer size as needed. This 
should be called before ``::begin()``. The size argument should be at least large enough
to hold all data received before reading.

For transmit-only operation, the 256-byte RX buffer can be switched off to save RAM by 
passing mode SERIAL_TX_ONLY to Serial.begin(). Other modes are SERIAL_RX_ONLY and 
SERIAL_FULL (the default).

Receive is interrupt-driven, but transmit polls and busy-waits. Blocking behavior is as follows:
The ``::write()`` call does not block if the number of bytes fits in the current space available
in the TX FIFO. The call blocks if the TX FIFO is full and waits until there is room before 
writing more bytes into it, until all bytes are written. In other words, when the call returns, 
all bytes have been written to the TX FIFO, but that doesn't mean that all bytes have been sent 
out through the serial line yet.
The ``::read()`` call doesn't block, not even if there are no bytes available for reading.
The ``::readBytes()`` call blocks until the number of bytes read complies with the number of 
bytes required by the argument passed in.
The ``::flush()`` call blocks waiting for the TX FIFO to be empty before returning. It is 
recommended to call this to make sure all bytes have been sent before doing configuration changes 
on the serial port (e.g. changing baudrate) or doing a board reset.

``Serial`` uses UART0, which is mapped to pins GPIO1 (TX) and GPIO3
(RX). Serial may be remapped to GPIO15 (TX) and GPIO13 (RX) by calling
``Serial.swap()`` after ``Serial.begin``. Calling ``swap`` again maps
UART0 back to GPIO1 and GPIO3.

``Serial1`` uses UART1, TX pin is GPIO2. UART1 can not be used to
receive data because normally it's RX pin is occupied for flash chip
connection. To use ``Serial1``, call ``Serial1.begin(baudrate)``.

If ``Serial1`` is not used and ``Serial`` is not swapped - TX for UART0
can be mapped to GPIO2 instead by calling ``Serial.set_tx(2)`` after
``Serial.begin`` or directly with
``Serial.begin(baud, config, mode, 2)``.

By default the diagnostic output from WiFi libraries is disabled when
you call ``Serial.begin``. To enable debug output again, call
``Serial.setDebugOutput(true)``. To redirect debug output to ``Serial1``
instead, call ``Serial1.setDebugOutput(true)``.

You also need to use ``Serial.setDebugOutput(true)`` to enable output
from ``printf()`` function.

Both ``Serial`` and ``Serial1`` objects support 5, 6, 7, 8 data bits,
odd (O), even (E), and no (N) parity, and 1 or 2 stop bits. To set the
desired mode, call ``Serial.begin(baudrate, SERIAL_8N1)``,
``Serial.begin(baudrate, SERIAL_6E2)``, etc.
Default configuration mode is SERIAL_8N1. Possibilities are SERIAL_[5678][NEO][12].
Example: ``SERIAL_8N1`` means 8bits No parity 1 stop bit.
 
A new method has been implemented on both ``Serial`` and ``Serial1`` to
get current baud rate setting. To get the current baud rate, call
``Serial.baudRate()``, ``Serial1.baudRate()``. Return a ``int`` of
current speed. For example

.. code:: cpp

    // Set Baud rate to 57600
    Serial.begin(57600);

    // Get current baud rate
    int br = Serial.baudRate();

    // Will print "Serial is 57600 bps"
    Serial.printf("Serial is %d bps", br);

| ``Serial`` and ``Serial1`` objects are both instances of the
  ``HardwareSerial`` class.
| This is also done for official ESP8266 `Software
  Serial <libraries.rst#softwareserial>`__
  library, see this `pull
  request <https://github.com/plerup/espsoftwareserial/pull/22>`__.
| Note that this implementation is **only for ESP8266 based boards**,
  and will not works with other Arduino boards.


To detect an unknown baudrate of data coming into Serial use ``Serial.detectBaudrate(time_t timeoutMillis)``. This method tries to detect the baudrate for a maximum of timeoutMillis ms. It returns zero if no baudrate was detected, or the detected baudrate otherwise. The ``detectBaudrate()`` function may be called before ``Serial.begin()`` is called, because it does not need the receive buffer nor the SerialConfig parameters.

The uart can not detect other parameters like number of start- or stopbits, number of data bits or parity.

The detection itself does not change the baudrate, after detection it should be set as usual using ``Serial.begin(detectedBaudrate)``.

Detection is very fast, it takes only a few incoming bytes.

SerialDetectBaudrate.ino is a full example of usage.

Progmem
-------

The Program memory features work much the same way as on a regular
Arduino; placing read only data and strings in read only memory and
freeing heap for your application.

In core versions prior to 2.7, the important difference is that on the
ESP8266 the literal strings are not pooled.  This means that the same
literal string defined inside a ``F("")`` and/or ``PSTR("")`` will take up
space for each instance in the code.  So you will need to manage the
duplicate strings yourself.

Starting from v2.7, this is no longer true: duplicate literal strings within
r/o memory are now handled.

There is one additional helper macro to make it easier to pass
``const PROGMEM`` strings to methods that take a ``__FlashStringHelper``
called ``FPSTR()``. The use of this will help make it easier to pool
strings. Not pooling strings...

.. code:: cpp

    String response1;
    response1 += F("http:");
    ...
    String response2;
    response2 += F("http:");

using FPSTR would become...

.. code:: cpp

    const char HTTP[] PROGMEM = "http:";
    ...
    {
        String response1;
        response1 += FPSTR(HTTP);
        ...
        String response2;
        response2 += FPSTR(HTTP);
    }

C++
----

- About C++ exceptions, ``operator new``, and Exceptions menu option
  
  The C++ standard says the following about the ``new`` operator behavior when encountering heap shortage (memory full):

  - has to throw a ``std::bad_alloc`` C++ exception when they are enabled

  - will ``abort()`` otherwise
  
  There are several reasons for the first point above, among which are:

  - guarantee that the return of new is never a ``nullptr``

  - guarantee full construction of the top level object plus all member subobjects

  - guarantee that any subobjects partially constructed get destroyed, and in the correct order, if oom is encountered midway through construction
  
  When C++ exceptions are disabled, or when using ``new(nothrow)``, the above guarantees can't be upheld, so the second point (``abort()``) above is the only ``std::c++`` viable solution.
  
  Historically in Arduino environments, ``new`` is overloaded to simply return the equivalent ``malloc()`` which in turn can return ``nullptr``.
  
  This behavior is not C++ standard, and there is good reason for that: there are hidden and very bad side effects. The *class and member constructors are always called, even when memory is full* (``this == nullptr``).
  In addition, the memory allocation for the top object could succeed, but allocation required for some member object could fail, leaving construction in an undefined state.
  So the historical behavior of Ardudino's ``new``, when faced with insufficient memory, will lead to bad crashes sooner or later, sometimes unexplainable, generally due to memory corruption even when the returned value is checked and managed.
  Luckily on esp8266, trying to update RAM near address 0 will immediately raise an hardware exception, unlike on other uC like avr on which that memory can be accessible.
  
  As of core 2.6.0, there are 3 options: legacy (default) and two clear cases when ``new`` encounters oom:
  
  - ``new`` returns ``nullptr``, with possible bad effects or immediate crash when constructors (called anyway) initialize members (exceptions are disabled in this case)

  - C++ exceptions are disabled: ``new`` calls ``abort()`` and will "cleanly" crash, because there is no way to honor memory allocation or to recover gracefully.

  - C++ exceptions are enabled: ``new`` throws a ``std::bad_alloc`` C++ exception, which can be caught and handled gracefully.
    This assures correct behavior, including handling of all subobjects, which guarantees stability.

  History: `#6269 <https://github.com/esp8266/Arduino/issues/6269>`__ `#6309 <https://github.com/esp8266/Arduino/pull/6309>`__ `#6312 <https://github.com/esp8266/Arduino/pull/6312>`__