NodeMCU Custom Builds for ESP8266

OK, just a quick blurb in case anyone else runs into this incredibly frustrating experience:

I’m playing with some sensors where I’m interested in really low power consumption. Basically, I want to go into deep sleep, which only draws <100uA, and wake up every minute or so, check an input, and report it to an MQTT broker if the state has changed. I also want to read from a DH11 temp/humidity sensor every 30 minutes or so, and report the values, regardless of whether the GPIO changed. I’m mainly working with NodeMCU right now, rather than “native” SDK code.

SO…the ESP8266 has a “deep sleep” mode, called using node.dsleep(us), which really just shuts off everything but the Real Time Clock (RTC), which drops XPD_DCDC (usually GPIO15) to gnd briefly when the timer expires. Tie that signal to RST, and the chip CPU will initialize when the timer expires and run your code from boot-up. NodeMCU has nifty rtctime and rtcmem modules now that let you read the Real Time Clock value, like after you wake up, and use 128 words of space in the RTC storage to persist info between wakeups.

OK, cool, but after flashing various verions of NodeMCU to my test chips, I get nothing but the following error, presented here for search engine to hit, whenever I try to access the rtctime or rtcmem functions:

“attempt to index global ‘rtctime’ (a nil value)” or “attempt to index global ‘rtcmem’ (a nil value)”

So, my suspicion is, the base NodeMCU firmwares are not being built with the relatively new support for the RTC modules built in. BINGO! After building a custom firmware that includes the module, these functions are available.  I will say, however, that the “rtctime” function desn’t appear to work as advertised. The docs seem to imply you will get the time, including the time that has elapsed while in deep sleep. Currently, I’m not seeing this behavior…it seems like it’s initialized at every boot. Persisting values via “rtcmem” is working well, though, so I can just do simple modulus on an incrementing number I store on every 1 minute “wake” to send temp data every 30 minutes or so.

This being said, a quick shout out to the excellent site, http://nodemcu-build.com/ 

This site lets you build a custom NodeMCU firmware image off the current master (or dev) branch, and include modules optionally, as you see fit. Check it out and donate (as I did) if you find it useful. It’s SO much easier that screwing around with build chains – jsut select the modules you want an click a button – you get an email when your build is ready (almost immediately for me)… and give this guy a few bucks, he deserves it for saving you the time!

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