GPIO

From the documentation:

"On every boot/reset/wakeup, GPIO15 [8] MUST keep LOW, GPIO2 [4] MUST keep HIGH. GPIO0 [3] HIGH ->RUN MODE, LOW -> FLASH MODE. When you need to use the sleep mode,GPIO16 and RST should be connected, and GPIO16 will output LOW to reset the system at the time of wakeup."

So:

Pin 0 - is used to return from for deep sleep, so if you are using this mode, don't use it for anything else.

Pin 3 - if pulled low during reset, sends ESP8266 into flash programming mode. Pull-up.

Pin 4 - it's connected to the on-board led, and it is pulled up. Also remember that the led blinks during boot, so this pin receives signals.

Pin 8 - has pull-down as it must be low on boot.

gpio.mode()

Initialize pin to GPIO mode, set the pin in/out direction, and optional internal weak pull-up.

Sets a given pin as OUTPUT or INPUT

gpio.mode(7,gpio.OUTPUT)
gpio.mode(7,gpio.INPUT)

gpio.write()

Set digital GPIO pin value.

gpio.mode(7,gpio.OUTPUT)
gpio.write(7,gpio.HIGH)

gpio.read()

Read digital GPIO pin value. Returns 0 for low, 1 for high.

Blink with timer interrupts

"Blink" is the "hello world" of the embedded world. A blink program was introduced at the "First steps chapter", however, that program used a delay function that stopped all processing while waiting.

The following code uses a timer interrupt (code from adafruit):

-- Pin definition 
pin = 2
status = gpio.LOW
duration = 500    -- 0.5 second duration for timer

-- Initialising pin
gpio.mode(pin, gpio.OUTPUT)
gpio.write(pin, status)

-- Create an interval
tmr.alarm(0, duration, 1, function ()
    if status == gpio.LOW then
        status = gpio.HIGH
    else
        status = gpio.LOW
    end

    gpio.write(pin, status)
end)

To do

gpio.serout() Serialize output based on a sequence of delay-times in µs.

gpio.trig() Establish or clear a callback function to run on interrupt for a pin.