MCBSTM32F400: Coin cell battery may discharge much too fast
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The CR1220 coin cell battery (BATT1) may discharge within a few hours or days after jumper J1 is set to position BAT.
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.
Please try to populate a 0 Ohm resistor to position R1. It is also possible to connect both SMD pads with solder.
R1 will be populated on boards that are produced/sold in 2014 and later.
Last Reviewed: Friday, June 3, 2016
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