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LoRaSensorTile power usage

A project log for Hackable CMWX1ZZABZ (LoRa) Devices

Useful STM32L082-based devices with embedded LoRa radio modem programmable with the Arduino IDE via USB connector, i.e, hackable.

Kris WinerKris Winer 01/20/2018 at 05:080 Comments

The LoRaSensorTile has a lot of sensors so the current is no longer dominated by the CPU current alone. I used an ST power monitor to measure the current usage as a function of time for different conditions. The ST power monitor supplies 3.30 V which I applied to the 3V3 and GND inputs of the LoRaSensorTile. The currents reported below might be a bit higher when using a 4 V LiPo battery.

A portion of the current history as a function of time for 32 MHz clock speed is shown here:

You can see that every second the when the RTC alarm goes off data is read and the led blinks, etc before the STM32L082 returns to the STOP mode, with stop current of ~12 uA; this includes the ~2 uA for the STM32L082 as well as the current drawn by the I2C sensors, battery charger, and SPI flash, which are absent in the Grasshopper.

The average current as a function of clock speed is plotted below:

You can see the advantage of dropping the CPU speed as low as possible when deploying devices in the field. The current used by everything except the CPU is about 70 uA (0 CPU intercept). This means the CPU uses ~92 uA at 32 MHz, ~49 uA at 16 Mhz, and ~12 uA at 4.2 MHz; CPU current usage is proportional to clock speed, as expected. At just over 80 uA average current usage at 4.2 MHz the LoRaSensorTile would last for more than a year on a couple of AAA batteries.

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