After spending about a month tinkering with the M5Paper, a handful of shortcomings in the design have become apparent. This wishlist is what I would like to see in a future iteration of this otherwise great little device.
The first point I'd like to address is the core being used. While the ESP32 module used is fine for most purposes, Espressif has announced the ESP32-S3, which, among the upgraded cores, brings a few new features, most prominent being the built in USB stack. This would allow dropping the serial adapter chip, and using the MCU directly for USB functionality, including flashing, and even allowing the device to act as a USB peripheral (or host, for e.g. a keyboard, though power supply could be an issue).
The second point is power management. I don't want to 💩 on your work, but the M5Paper clearly received a less favourable treatment than the other M5 cores. Power supply is unnecessarily overcomplicated, and the design results in very little control over the power systems, and down the line, massive power drains unless the developer implements drastic limitations. Even the deep sleep mode of the ESP32 is drawing too much power, due to the choice of eink and touch driver (well, with the touch driver, the main issue is not having a separate power switch). Due to this, I'd like to see the return of the AXP192, which was proven quite capable in the M5Stack Core and M5Stick series. It's managed over I2C, provides better power management of components, better battery support (not to mention proper reporting of charging/discharging state and battery voltage/life reporting), and overall a better option for the overall power management. I believe it also provides drip charging, which would be crucial to extend the battery life when it's mainly mounted to a continuous power supply.
Third point is the eink driver. While the IT8951 is very capable, it was not designed with battery operated systems in mind. I've read multiple reports, not just on the M5Paper, but on various Waveshare units that also use the IT8951, that its low power/hibernate modes are unusable - the chip actually draws more power hibernated than it does in standby, and it's not a negligible amount (around 120-130mA). While this is fine for a mains powered project like a Raspberry Pi, it won't work well in the M5Paper. If I understand it right, the IT8951 basically obfuscates the waveshape messaging part of the eink protocol, providing a more manageable solution, at the price of considerably higher power usage.
The touch driver, that I'm actually happy with. The only negative I can say is that it has no separate power management, and such it cannot be disabled when not needed. With the AXP192, that should be doable, though.
Fourth point is the temperature/humidity sensor. Its unfortunate placement makes it practically useless - the proximity to the battery and the rest of the board makes temperature readings unreliable when the ambient temperature is lower than the internal temperature, unless the device was freshly powered on. The humidity sensor works, but due to the restricted airflow, which opens from the back, makes its use quite limited. Instead, I'd recommend replacing it with a combined 9DoF sensor (accelerometer, gyroscope, and magnetometer), which could provide better everyday usage, for example, for screen rotation. Though the built-in magnet might interfere.
And if we're talking about power, I think a similar POGO pin connector as the one on the M5Stack FIRE's charger base would be beneficial. A four pin connector, with I2C exposed, and the power lines connected to the PMIC, would allow for custom docking bases (e.g. a wall mount with an I2C temperature/humidity sensor, for a tear-away smart home control panel and thermostat, or a desk mount for a ticker style display for e.g. temperature). The M5Paper is clearly meant to be a semi-permanent mounted solution - most of the time it would sit in a dock, but the battery would allow for temporary mobility, such as the previously mentioned tear-away console, which mostly sits in a wall mount, but allows the user to pull it off and use it e.g. as a smart home remote.
Also another power point - a separate coin battery for the RTC. I understand that the current solution uses barely any power, however I'd feel safer if the RTC had a separate power supply in the form of a replaceable coin battery. If designed well, this would provide years of supply for the RTC (a standard CR2032 easily runs a ZigBee motion sensor for 6 to 12 months), and separate the power concerns.
Last, but not the least, the side buttons. The current solution is, honestly, quite bad. I'd rather have either three separate buttons, or a button and a full rotary encoder, or practically any other solution that provides 3-4 inputs (one of them being an interrupt/wake signal for the PMIC and the ESP32).
Otherwise, the form factor and size are perfect. LilyGO came out with a similar board around the time the M5Paper was released, same eink display but different driver, similar touchscreen, and support for both 2-pin Li-Ion, and 18650 batteries (as two "separate" model - the only difference is the connector soldered onto the board). I'll be grabbing one soon, at least to compare it to the M5Paper. The fact that the touchscreen is only available separately, and it isn't packaged as nicely as the M5Paper makes it a harder sell, but I can see the advantage of the different design.
So, to list the overall changes recommended:
- ESP32-S3 instead of the current MCU
- Replace SLM6635 (ideally with AXP912)
- Replace IT8951 with a better option (maybe just a simple shift register, similar to how the LilyGO T5 4.7 solves it)
- Connect external components (eink, touch, each external port separately) to the PMIC for separate control of power
- Replace SHT30 with IMU
- Add a POGO pin connector for power/I2C, similar to M5Go
- Replace side button with a different implementation (either three separate buttons or a rotary dial and a button)
- Replaceable coin battery for BM8563