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.
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.
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:
ADC_MODE(ADC_VCC);
This line has to appear outside of any functions, for instance right
after the #include lines of your sketch.
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 PWMRANGE, which is
equal to 1023 by default. PWM range may be changed by calling
analogWriteRange(new_range).
PWM frequency is 1kHz by default. Call
analogWriteFreq(new_frequency) to change the frequency.
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 object works much the same way as on a regular Arduino. Apart
from hardware FIFO (128 bytes for TX and RX) Serial has
additional 256-byte TX and RX buffers. Both transmit and receive is
interrupt-driven. Write and read functions only block the sketch
execution when the respective FIFO/buffers are full/empty. Note that
the length of additional 256-bit buffer can be customized.
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.
The method Serial.setRxBufferSize(size_t size) allows to define the
receiving buffer depth. The default value is 256.
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.
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
// 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.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.
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. 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.
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...
String response1;
response1 += F("http:");
...
String response2;
response2 += F("http:");
using FPSTR would become...
const char HTTP[] PROGMEM = "http:";
...
{
String response1;
response1 += FPSTR(HTTP);
...
String response2;
response2 += FPSTR(HTTP);
}