Tama board

Tamagotchi

Design of an E-Ink tamagotchi board thing that runs off a CR2032 coin cell battery. Uses a GDEW0154M09 E-Ink 1.5x1.5in display from GoodDisplay.

Layout is a bit messy about breaking ground plane and having shit VDD distribution, but it should be good enough :^).

Notes

• http://www.lowpowerlab.com/
• https://lowpowerlab.com/forum/low-power-techniques/

MCU Selection

Final choice

STM32G431

• 128Kb flash enough for bitmaps
• ultra low power consumption is STANDBY or SHUTDOWN
• Tiny package to fit on PCB (QFN32)

Requirements

• ultra low power
• 1x SPI
• bitmap ROM
  • display is 200x200 pixels, 5kB for full image, maybe 3kB conservative for run length encoded image?
  • Want at least 10 images, so 30kB ROM required

Going for only JLCPCB approved parts or minimal extended parts

For STM, this is:

• STM32 F103
  • RUN 8MHz 8.6mA all peripherals
  • RUN 8MHz 6.8mA no peripherals (closer to reality)
  • STANDBY 3.2uA
• STM32 L151 (no stock)
• STM32 F030
  • RUN 8MHz 4.4mA
  • STANDBY ~3uA
• STM32 G030
  • internal HSI oscillator
  • 64kB flash / 8k ram
  • RUN 1.3mA
  • STANDBY 1.4uA
• STM32 G431
  • 128Kb flash (no external memory needed)
  • RUN 8MHz 1.10mA
  • RUN 1MHz 0.2mA
  • STANDBY 175nA
  • SHUTDOWN mode (no RTC): 74nA
  • SHUTDOWN mdoe (RTC via external 32k xtal) : 0.5/0.7uA
    • Guess this included the power draw of the crystal
• STM8 S105K6T6
  • 32kB flash
  • 2kB ram
  • External EEPROM required for bitmaps?
• STM8 S003F3P6
  • 8kB flash
  • 1kB ram

G431

• STM32G431KBU3
• nice QFN32 tiny package
• Good SHUTDOWN current
• https://jlcpcb.com/partdetail/Stmicroelectronics-STM32G431KBU3/C1341901
• 29K in stock

F103

Heaps of stock and 10 year guarantee At minimum core speed 8MHz

• STOP mode retains ram and registers
  • wake up with EXTI pin
  • wakeup with WKUP pin
  • wakeup with RTC alarm
  • 3.2 uA (DECENT)

L151

** Not for new designs **

** JLC stock 1 (!) **

• 214 μA/MHz Run mode
• 9 μA Low-power run mode
• 1.2 μA Stop mode + RTC
• 0.9 μA Standby mode + RTC
• 0.3 μA Standby mode (3 wakeup pins)

Modes:

• Sleep
  • core stopped, peripheral can intterrupt/wakeup
• Low power run
  • core clocked at 65KHz
• Stop mode with RTC
  • RAM and registers preserved, wakeup via EXTI pin
  • wakeup with RTC timestamp (or without RTC
• Standby mode
  • Ram and regists lost
  • wakeup with external reset NST
  • wakeup rising edge of one of the three WKUP pins
  • wakeup on RTC alarm

Display

5x GDEW0154M09

• https://www.aliexpress.com/item/1005001670352746.html?spm=a2g0o.order_list.order_list_main.9.42ea18020hM7dh
• https://www.good-display.com/product/206.html

Battery / Power

MCU

• worst cast STM32G431\
• core
  • RUN 8MHz 1.10mA
  • RUN 26MHz 3.2mA (maximum of Range 2 PLL freqs)
• periph
  • FLASH 4.6uA/mHZ
  • GPIOA 0.73uA/mHZ
  • GPIOB 0.41uA/mHZ
  • SPI1 1.92uA/MHz
  • RTC 2.92uA/MHz
  • AHB bus matrix 4.07uA/MHz
  • AHB to APB1 bridge 0.32
  • AHB to APB2 bridge 0.30 TOTAL peripheral
  • 394uA at 26MHz
    TOTAL
  • 3.6mA RUNNING
  • 0.7uA SHUTDOWN

Screen

• Image update max 3mA RUNNING
• Standby panel 5uA SHUTDOWN

Battery

• 235 mAh to 2v (require boost converter)
• screen minimum supply voltage 2.3V
• MCU minimum supply voltage 1.7V
• At 2.3V, this gives about 200mAH of usable longevity from the coin cell

Totals

Assuming 100% boost convert efficiency (haha)

• RUNNING 6.6mA
  • 35 hours
• STANDBY 6uA
  • 4 years 3 weeks :^)

PCB assembly

Went relatively smoothly except

• U1 (STM32 ended up rotated 90 degrees counter clockwise from its desired position). Changed rotation from 180 to 90.
• Q1 (Sot23 mosfet) ended up 180 degrees out. Changed from 0 to 180
• D1 footprint was fucked because I got confused by the example footpint from the website.
  • Couldn't be bothered to regenerate and re-do order with new gerbers so just rotated the pick-and-place position of the diode without changing silkscreen etc.
  • JLC realized and tried to fix it by rotating it to the silkscreen-defined position, told them to put it back to the (seemingly incorrect) position.