MCBSTM32F400: Coin cell battery may discharge much too fast

Information in this knowledgebase article applies to:

• MCBSTM32F400

SYMPTOM

The CR1220 coin cell battery (BATT1) may discharge within a few hours or days after jumper J1 is set to position BAT.

CAUSE

When jumper J1 is set to position BAT the STM32F407 microcontroller power supply input VBAT is connected to the coin cell battery. In this case, the RTC, some port pins and a part of the on-chip RAM continues to work even if the main power supply is switched off.

Resistor R1 has not been populated on our MCBSTM32F400 boards by mistake until end of 2013. This resistor connects to the PRD_ON input of the microcontroller. When PDR_ON is floating, the on-chip power-on reset (POR) / power-down reset (PDR) circuitry is in an undefined state. It may happen that an internal switch which connects different power domains does not open when the main power supply is switched off. Then, the microcontroller draws about 2 mA from the coin cell battery which is then discharged quite fast.

RESOLUTION

Please try to populate a 0 Ohm resistor to position R1. It is also possible to connect both SMD pads with solder.

STATUS

R1 will be populated on boards that are produced/sold in 2014 and later.

MORE INFORMATION

• Refer to Schematics in the MCBSTM32F200/400 User's Guide.
• Refer to Power Supply in the MCBSTM32F200/400 User's Guide.
• Refer to STM32F407xx Datasheet

Last Reviewed: Friday, June 3, 2016