**Note:** This function can only be used in the condition that esp8266 PIN32(RST) and PIN8(XPD_DCDC aka GPIO16) are connected together. Using sleep(0) will set no wake up timer, connect a GPIO to pin RST, the chip will wake up by a falling-edge on pin RST.<br/>
option=0, init data byte 108 is valuable;<br/>
option>0, init data byte 108 is valueless.<br/>
More details as follows:<br/>
0, RF_CAL or not after deep-sleep wake up, depends on init data byte 108.<br/>
1, RF_CAL after deep-sleep wake up, there will belarge current.<br/>
2, no RF_CAL after deep-sleep wake up, there will only be small current.<br/>
4, disable RF after deep-sleep wake up, just like modem sleep, there will be the smallest current.
####Parameters
-`us`: number(Integer) or nil, sleep time in micro second. If us = 0, it will sleep forever. If us = nil, will not set sleep time.
-`option`: number(Integer) or nil. If option = nil, it will use last alive setting as default option.
Returns the current available heap size in bytes. Note that due to fragmentation, actual allocations of this size may not be possible.
####Syntax
`node.heap()`
####Parameters
`nil`
####Returns
number: system heap size left in bytes
####Example
```lua
heap_size = node.heap();
```
___
## node.key() --deprecated
Define action to take on button press (on the old devkit 0.9), button connected to GPIO 16.
This function is only available if the firmware was compiled with DEVKIT_VERSION_0_9 defined.
####Syntax
`node.key(type, function)`
####Parameters
-`type`: type is either string "long" or "short". long: press the key for 3 seconds, short: press shortly(less than 3 seconds)
-`function`: user defined function which is called when key is pressed. If nil, remove the user defined function. Default function: long: change LED blinking rate, short: reset chip
Redirects the Lua interpreter output to a callback function. Optionally also prints it to the serial console.
!!! note "Note:"
Do **not** attempt to `print()` or otherwise induce the Lua interpreter to produce output from within the callback function. Doing so results in infinite recursion, and leads to a watchdog-triggered restart.
####Syntax
`node.output(output_fn, serial_debug)`
####Parameters
-`output_fn(str)`: a function accept every output as str, and can send the output to a socket (or maybe a file).
-`serial_debug`: 1 output also show in serial. 0: no serial output.
####Returns
`nil`
####Example
```lua
function tonet(str)
sk:send(str)
end
node.output(tonet, 1) -- serial also get the lua output.
```
```lua
-- a simple telnet server
s=net.createServer(net.TCP)
s:listen(2323,function(c)
con_std = c
function s_output(str)
if(con_std~=nil)
then con_std:send(str)
end
end
node.output(s_output, 0) -- re-direct output to function s_ouput.
c:on("receive",function(c,l)
node.input(l) -- works like pcall(loadstring(l)) but support multiple separate line
end)
c:on("disconnection",function(c)
con_std = nil
node.output(nil) -- un-regist the redirect output function, output goes to serial
end)
end)
```
####See also
-`node.input()`
___
## node.readvdd33() --deprecated, moved to adc.readvdd33()
####See also
-`adc.readvdd33()`
___
## node.compile()
Compile a Lua text file into Lua bytecode, and save it as .lc file.
####Syntax
`node.compile(filename)`
####Parameters
-`filename`: name of Lua text file
####Returns
`nil`
####Example
```lua
file.open("hello.lua","w+")
file.writeline([[print("hello nodemcu")]])
file.writeline([[print(node.heap())]])
file.close()
node.compile("hello.lua")
dofile("hello.lua")
dofile("hello.lc")
```
___
## node.setcpufreq()
Change the working CPU Frequency.
####Syntax
`node.setcpufreq(speed)`
####Parameters
-`speed`: `node.CPU80MHZ` or `node.CPU160MHZ`
####Returns
number:target CPU Frequency
####Example
```lua
node.setcpufreq(node.CPU80MHZ)
```
___
## node.restore()
Restore system configuration to defaults. Erases all stored WiFi settings, and resets the "esp init data" to the defaults. This function is intended as a last-resort without having to reflash the ESP altogether.
This also uses the SDK function `system_restore()`, which doesn't document precisely what it erases/restores.
####Syntax
`node.restore()`
####Parameters
`nil`
####Returns
`nil`
####Example
```lua
node.restore()
node.restart() -- ensure the restored settings take effect
