Hardware modifications

• Remove R1 (to turn off the power LED)
• U1, Richtek RT9193, is not designed for low power. It has 130µA quiescent current. Replace it by a pin compatible Richtek RT9073N-33GB which only draws 3µA quiescent current and 80mV dropout voltage at 50mA output current. The NCP4684 (Imax=150mA) and NCP4681 are inferior alternatives.
• Connect VBAT to VDD

Low power modes

• Reference
• If application has a repeated loop and every loop takes > 200ms, then use "Standby mode" (start from reset every time), which is the lowest power mode.
  • Mode entry: WFI (Wait for Interrupt) or WFE (Wait for Event) while:
    • Set SLEEPDEEP in Cortex-M3 System Control register
    • Set PDDS bit in Power Control register (PWR_CR)
    • Clear WUF bit in Power Control/Status register (PWR_CSR)
  • all pins HiZ except WKUP-pin. WKUP pin is used for wakeup from Standby mode and forced in input pull down configuration (rising edge on WKUP pin wakes-up the system from Standby mode).
• Turn off peripherals (by clock gating) during run-mode.

Debugging

• In case you can't load your firmware into the Blue Pill (e.g. because it's in standby mode), then do the following:
  • Put jumper BOOT0 in the "1" position.
  • Push and release reset button.
  • Put jumper BOOT0 back to the "0" position.
  • Try to upload your firmware again.
• During debugging, the Blue Pill will consume 1.5mA. Stop debugging and disconnect SWD to measure sleep current of your application.

Hardware connections

• Be aware that all IO-pins, except the WKUP-pin and PA12 are Hi-Z.
  • Connect a pull-down resistor to the UART-TX pins to avoid sending spurious characters.
  • PA12 has a 4K7 pull-up resistor connected to it. Don't connect it to a peripheral that pulls that line low during Blue Pill sleep.
• The debug connection (SWD) on connector P2 is responsible for about 330µA current draw, even when not debugging.

Power supply

• The voltage on the 5V connection can be made as small as 2.5V.