Deep sleep wake ESP8266 with DS3231 RTC alarm

Question

I'm trying to use a DS3231 RTC to wake my ESP8266 at a specific hour, minute and second. My circuit is a very low power design.

I believe I need to reset my microcontroller (by momentarily pulling the RST pin low) when the RTC alarm signal goes from high to low (the SQW pin on the DS3231). However, when the DS3231 alarm is triggered, the signal stays low, so connecting the RTC SQW pin directly to the MCU RST pin won't work (because holding the RST pin low prevents the MCU from booting).

Desmos graph

Unfortunately, deep sleep doesn't support values greater than about 3.5 hours. I tried sleeping for an hour at a time until the desired time is reached, but found that it's not very accurate (often dozens seconds of drift on each wake, adding up to many minutes of drift after several hours).

Answer 1

You can probably get-away with a cobbled-together monostable like this: -

^{simulate this circuit – Schematic created using CircuitLab}

When the input drops low, the buffer output goes low but, then C1 is recharged via R1 (this sets the length of the pulse) and, after a short period of time, the buffer output goes high again. If the input returns high some time later, the capacitor is discharged fairly quickly by diode D1. All ready for the next event on the input.

R2 prevents excessive current flow into the buffer input when the input signal goes high.

Simulation: -

The top red waveform is the digital representation of the buffer output hence it is small in amplitude compared to the blue input that is regarded by micro-cap as an analogue input (because it connects to the capacitor).

Answer 2

There's two approaches; deep sleep wake and light sleep wake.

Deep sleep wake ESP8266 with DS3231 (monostable required)

Huge thanks Andy aka's monostable answer. This led to the eventual solution.

My solution is almost the same as Andy aka's monostable, except that I:

• Used a PNP transistor for the output buffer (the ESP8266 module has an internal pull-up on RST).
• Replaced the 10nF cap with a polarized 10uF cap, as the smaller cap discharged too quickly and so didn't pull the MCU RST pin low enough for long enough.

I also posted the MCU C++ code on Arduino SE.

^{simulate this circuit – Schematic created using CircuitLab}

Simulation:

Reality:

• CH1: RTC_SQW
• CH2: MCU_RST

Edit: 1uF cap also seems to do the job.

• CH1: RTC_SQW
• CH2: MCU_RST

Light sleep wake ESP8266 with DS3231 (lower part count)

As an alternative, for a lower part-count design: if very low power is not a requirement and light sleep is acceptable, you can wake the ESP8266 from light sleep using a GPIO pin where the input signal (unlike the RST pin), doesn't need to pulse.

esp_sleep_enable_gpio_wakeup()
esp_sleep_start()

Note: These functions don't appear to be available in the ESP class for Arduino.

There is a SO answer that describes using gpio_pin_wakeup_enable as an alternative (but that didn't actually seem to work for me).

This is similar to the esp_sleep_enable_ext0_wakeup function for ESP32, which unfortunately is not available for ESP8266. There doesn't seem to be a way to wake the ESP8266 from deep sleep with GPIO, and according to the docs: only a pulse on the RST pin can wake the ESP8266 from deep sleep.