Smartwatch journey – Part 3

Comparing specs of the boards

I will list the main paramters which affects my decision.


  1. AtMega32u4 8-bit single core 16Mhz
  2. 32 KB of flash
  3. 2.5 KB of RAM
  4. BLE module on the board
  5. $22
  6. Low power consumption

ESP32 (WROOM-32)

  1. Xtensa 32-bit LX6 dual core 160/240 MHz
  2. 448 KB of flash + 4MB SPI flash
  3. 520 KB of RAM
  4. BLE, WiFi
  5. $5
  6. High power consumption

Red Bear Lab Nano v2 (kit with flasher)

  1. ARM Cortex-M4F 32-bit single core 64Mhz
  2. 512 KB of flash
  3. 64 KB of RAM
  4. BLE
  5. $32
  6. Low power consumption

The display I wanna use is 1.54″ 200×200 e-paper / e-ink tricolor display from Waveshare.  It requires 5+5 KB of RAM just for black and red buffer. Math behind: 200 width * 200 height = 40k pixels  / 8 bits= 5k * 2 colors = 10k

Yeah, the display works like so. You have to send whole buffer over SPI and then send command for refresh the display. I am unable to do this with BLEPad. I can partially render the text and send it over and over. Too restrictive. I have to save some RAM for notifications. One character = One byte. Not so much for storing the messages with a RAM of that size.

The winner is RedBear Lab Nano V2 which will be used in my smartwatch and I will call them just Red Bear to be short.

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