I think that the RPi is not a particularly good ePaper platform. Mainly because ePaper are slow (refresh latency) and low power, and as such are perfect for controller devices that sleep much and update the screen with low frequency. Like something that fetches weather/stock/whatever data once an hour and displays that. Since many microcontrollers can consume in microamps when sleeping and ePaper can retain an image without power, you can (in the best case) make a device that consumes almost zero power.
The ESP8266 is better than the Pi, but has three downsides: 1) if you deep sleep it, you need to use a pin interrupt to reset it. This means that it "reboots", that is starts from the beginning. For some purposes this is not a problem but for others this can be a problem. The watchdog timer cannot be used from the lowest power state. 2) It does not come with an OS (single core cpu). Therefore the Wifi stack needs to be given time in the main program loop to use the cpu, otherwise it will crash. 3) while it uses less power (around 70 mA average with WIFI), the model that I have uses around 250 mA when transmitting. So for example, if you cannot use a static ip and you need dhcp, after it wakes from reboot it will consume 250 mA for like 4 seconds straight. This is not bad if you use a PSU, but if you want to make a battery enabled device you are going to need to sleep as much as possible and even then it is not going to last long if you need to send a lot of stuff when you wake.
The ESP32 has a OS, usually FreeRTOS, which is a real time operating system. It schedules tasks on the two cores in the background for you. This makes it easier to use, and it also has way more analog inputs (esp8266's have either none or one) and a RTC. It has Bluetooth low energy, but I have not been able to find any data on how much power that consumes compared to the WIFI (which I assume is identical to ESP8266). Also needs to be full reset when recovering from deep sleep.
Then there are ARM coretex-m0 microcontrollers with wifi. These usually can resume from where they left off. Consme very little power. For example Adafruit displays measurements on their page of their coretex m0 devices with WIFI which uses 12 mA with WFI on and practically zero when sleeping. These give you way much more computational power, are more power efficient and since the new Arduino's use the same architecture/chip, they are much better documented and have better Arduino IDE support than ESP32 last I checked. But this changes every week. However, these can be somewhat expensive. Adafruit feather m0 models are like 40 USD or more. You can get a coretex-m0+ dev board for less, but without WIFI/bluetooth/lora/sigfox etc.
Btw. I believe that this
CarlRJ wrote:The Arduino is not RTOS based. The Arduino doesn't really have an OS, it's basically just your-code-plus-libraries on top of bare metal.
is probably not true any more. Arduinos used to use 8-bit AVR microprocessors with bare metal code, but the new models seem to mostly use 32-bit microprosessors, either ARM (SAMD21) or x86 (Intel Curie) and can be used with OS's, though I have no idea if they come with an OS preinstalled:
https://create.arduino.cc/projecthub/fe ... ino-ebc3cc