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The ESP8266 Datasheet has a Pin 5 labelled as VDD_RTC and it describes its function as "NC (1.1V)".

I usually understand NC to mean no connection on most ICs, or on relays as normally closed. Neither of those seem to apply here. What does it mean in this case?

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    \$\begingroup\$ I share your confusion. It may be that it is a 1.1V output, but they don't want you to connect anything, or it may mean that if you don't use the 1.1V output, you shouldn't connect anything, or it may mean that there are two variants, one of which has a 1.1V output and the other of which the pin is not connected. \$\endgroup\$ – mkeith Dec 22 '15 at 19:46
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    \$\begingroup\$ The chip design may have necessitated a voltage be present on that pin--but the manufacture recommends it not be used due to some affect it may cause elsewhere in the hardware or the pin could be used in some special configurations (just not documented in the doc that you are viewing). \$\endgroup\$ – zeffur Dec 22 '15 at 20:23
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Since it's labelled as a type P, it should be a power input pin for RTC.

Extract from datasheet:

esp power management

It appears that if you keep RTC powered on, it will keep basic settings intact while in sleep.

Even though complete info is not present, I think pin 5 can be fed 1.1V externally to keep the RTC running even while your 3.3V supply is off.

So, usually, you aren't supposed to connect anything to it but you can connect a 1.1V (or maybe 1.1V is the minimum voltage required) power source to keep RTC running so that wifi configuration remains intact.

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If you view the Hardware user guide on page 12:

It can be seen that it is left Not Connected, the standard interpretation for NC.

There is a chance it could be used to connect a supercap/battery (at 1.1V) to supply the RealTimeClock but there is insufficient information to even consider trying this.

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  • \$\begingroup\$ This is a circuit diagram for a development board that the chip is mounted on. It may very well be that that's what the NC is referring to in the PDF I am referencing, but presumably it is connected to circuitry inside the ESP8266. \$\endgroup\$ – Octopus Dec 23 '15 at 2:34
  • \$\begingroup\$ While that is true, it shows precedence in the fact it is NC. Coupled with the lack of information as to how to utilise this pin as an external RTC powersource, its reasonable to deduce it is a pin that could have a function (esp with API to put the chip into low power mode) , but the presently provided infomation is insufficient to utilise any capability \$\endgroup\$ – JonRB Dec 23 '15 at 8:45
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Nobody has a definitive answer. I suggest you email feedback@espressif.com and ask them. It is a very simple question to answer for them.

It looks like it would be safe to leave it unconnected (based on @JonRB's answer). After you hear back from them you can answer your own question here (for the benefit of people searching for this answer).

--McKenzie

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Usually this means 'not connected' or 'no connection'

This may mean that the pin will read 1.1v in normal operation, but it's very confusing to the engineer or technician.

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    \$\begingroup\$ It's very possible it's a documentation error. \$\endgroup\$ – philbrooksjazz Dec 22 '15 at 20:08

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