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I wrote an application for the STM32F4DISCOVERY development board (part number MB997C) which places the MCU into standby mode, and measured its power consumption through the JP1/Idd jumper using a Keysight U1252B DMM.

I was expecting a low µA figure (since the data sheet claims a typical figure of 1.7 to 4 µA) and was surprised to get a reading of 285 µA. ST points out that the BOOT0 pull-up resistor uses up 500 µA (although, from looking at the circuit, this should be closer to 300 µA), but I had already taken out that resistor, so this isn’t the issue.

While the board is powered up by the mini-USB connector that connects to the onboard ST-Link, these measurements were taken without the debugger running — the computer should be used merely as a power supply in this situation. When actually connecting the debugger, the figures are higher. I also tried connecting the mini-USB cable to a USB charger rather than a computer, with no change. Next I tried to take out the jumpers that connect the onboard ST-Link to the MCU, and again, saw no change.

Looks like I’m not alone in this finding: these measurements, performed with a Keysight 34461A DMM, show exactly the same current consumption for the same board (see the fourth figure from the top).

I reviewed the schematics and can’t find any other sources for this current leakage from the VDD rail (obviously there are other circuits on the board, but they’re connected to the 3V rail, and the JP1/Idd jumper only measures the VDD rail).

Can anyone help explain what could be contributing to a drain of ~100x the expected current?

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  • \$\begingroup\$ Didn't specs specify consumption "per MHz"? Try clocking it down before going into standby \$\endgroup\$ – stiebrs Nov 11 '19 at 12:36
  • \$\begingroup\$ @stiebrs not for Standby mode, as most of the chip is powered down in this mode, including the core clock, the HSE/HSI oscillators and even the core's voltage regulator. Regardless, the MCU is configured during Run mode to use the HSI oscillator straight to HCLK (no PLL) at 16 MHz, which is less than 1/10th of the nominal frequency. \$\endgroup\$ – swineone Nov 11 '19 at 13:39
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Turns out the extra consumption was due to resistor R34 (pull-up for BOOT1). The user manual incorrectly claims SB19 shouldn't be connected at manufacturing, which wouldn't incur any extra power consumption. However, my board had it, as well as another, newer, board (part number MB997D, which is now called STM32F407G-DISC1).

Removing this resistor brought power consumption down to 3 uA.

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