Further improving powersaving with ESP8266 and LUA / NodeMCU

I already optimized my NodeMCU firmware - LUA code to send data and then go to sleep after less than 10 seconds. That now takes a bit more than 1.2 seconds. That was a great leap forward to run a mbile sensor on a single 14500 LiIon cell with 900mAh for over a month.

The first improvement was done by moving the DeepSleep call from the Close Connection handler within the "http-code" to the On Receive handler. This led to deep sleep at less than 50µA after about 1.2-1.6 seconds.

Going to sleep after reiception of the data has been
acknowledged by the server.

Today I tried to directly go to sleep after calling the send-command with the buffer of my data. This led to no data being sent at all - the sending is done asynchronously and the DeepSleep command just interrupts it before even being started.

The solution is a timer call. Maybe it's possible to shave off a few more ms, but I am sataisfied with the result so far.

Directly after the send-command add:
tmr.alarm(0,150,1,function() node.dsleep(deepsleep_time*1000) end)

So the ESP is going to sleep after 150ms after the send command. It works and successfully delivers the data to the server, so now the wakeup time is down to roundabout 0.6s. This means nearly doubling the runtime on a battery cell! (Another ~100ms are saved by using a ESP-01 with black PCB instead of ESP-12E, by the way.)

When calling deepsleep with a short timer after the send
command, the wakeup time becomes significantly shorter.

Update 24.09.2016: With trial-and-error I could go down to 120ms instead 150ms for the DeepSleep-timer-call. While this doesn't seem much, it is saving 5% of power. instead of 600ms, we now have 570ms of awake-time, summing up to 45,6mA used instead of 48mA. During the real 290s DeepSleep, the power used sums up to 7.25mA. Average power consumption is down to 181,88µA instead of 190,12µA.
Also, the LDO is now a MCP1700 with 3.0V output, which should give another reduction. It helps using the full voltage range from 4.2V down to 3.0V of the LiIon battery.

Different response times with different ESP modules

While optimizing the ESP-based temperature and humidity sensors for my home I stumbled upon a weird issue: The different modules show different times to connect to Wifi and send the data. This was a real issue with tha battery driven sensor as there the data was always sent after roundabout 4 seconds. Looking with a 1R and the scope at it, it became clear that the connection/wake time is much longer, even more than 10 seconds.

ESP-12F on an adapter PCB.

A bit could be solved in software. Going to deepsleep after reiception has been confirmed (on:receive-handler) brought the wake-time down close to the self-timed values of the board. Using a static IP borught the self-measured time down to 0.3-0.4s, being really awake for about 1.2s. The timing code used on my other modules revealed something:

ESP-01 old version (512kByte flash, blue PCB): 9-61s
ESP-01 new version (1 MByte flash, black PCB): 3,1s
ESP-12E: 2.95-3.05s
ESP-12F: 4.1s

It doesn't matter at which distance the sensors are to the base station. The connection time is quite stable around those values, only the old ESP-01 boards really vary between those values, most of the times at 15s though. The modules use the same firmware and the same code (one variable differs between the sensors which is their name which gets sent).

It is clear that the first versions may have a different µC stepping and/or worse layout than the newer modules. But I would have expected the ESP-12F to outperform all other boards, which clearly isn't the case. Maybe someone knows the reason for this behaviour. Leave a comment if you do!