When the EN (enable) pin is pulled low the DC/DC converter is turned off and the module does a hard reset. EN can be left unconnected for normal operation.
This is all that is told about NE pin in Hardware Overview section.
Does this pin related to usage of external power unit? Please give more documentation on the purpose of the EN pin.
How to ‘pull it low’? How to pull it high? For what operation it is used?
The 5V supplied to the M6ENano board (either from USB or separate power pins) is regulated down to 3V3 by the M6Nano chip. When Enable is pulled LOW the regulator is switched off. From the M6E hardware design document :
TTL input that turns the module off and reduces its power consumption to nearly zero.
Hi=Enable, Low=Shutdown module
If left unconnected, module will stay in ENABLE state.
Pulling low can be down by connecting to GND or connect to a GPIO pin off your processor board and set the digialpin low.
When Enable is pulled LOW the regulator is switched off
what happens with the RFID reader then? does it turns OFF? or pulling LOW the EN pin just removes regulation voltage protection?
Can EN pin be used as just the reader Reset function?
The 3v3 generated by the M6E chip is used by all the components that expect 3V3: The M6E nano chip reader will shut down, the level shifter will not work anymore etc.
EN is NOT regulation voltage protection it is ENABLE 3V3 voltage regulator. Indeed the EN can be used to hard- reset the M6E nano.
Please see the [data sheet, that has the answers you’re looking for.](https://cdn.sparkfun.com/assets/4/e/5/5/0/SEN-14066_datasheet.pdf)