MSP-Dropstick testing

Since having built a simple testing rig on breadboard for interfacing TMP117 sensors with the MSP430 MCU, we've produced a PCB prototype to better evaluate possible LoRa radio solutions. After all, the purpose of the device is to autonomously report temperature readings wirelessly, while still having a fairly low power-consumption.

Device diagram

Above is the device diagram for the Dropstick device. A short version of the Dropstick's theory of operation:

"A self-powered wireless device to monitor road surface, and sub-surface temperatures in arctic conditions"

Which is used to pre-emptively monitor and prevent road surface damage caused by arctic conditions like permafrost.

MSP430? TMP117

While we considered other microcontrollers for this use-case, we chose the MSP430 platform and more specifically, the MSP430G2231 for both it's extremely low power consumption, automotive-grade qualification and being produced by the same manufacturer as the temperature sensors we use.

Seen above is a simple diagram of the different peripheral buses handled by the MCU.

As for the temperature sensors, the TMP117 was primarily chosen for its digital precision and configurable device address. Its low-power features also appealed to us, even though the sensors are fully powered down during the device's sleep mode.

Back to the prototype

We call the board the Dropstick Evaluation board MK.1

The board pictured above is revision no. 1 of the board, though we might not see another revision, since this one worked on first try (Hooray!) without any bodge wiring or piggyback-resistors.

With practically identical hardware compared to the breadboard prototype showcased in previous posts, the Evaluation Board MK.1 easily caught up with the ultra-low power consumption of the prototype and then some!

With the addition of a couple gates to mitigate an annoying current-leak with the microcontroller's USCI bus in I²C operation mode, and omission of redundant LED-s on the temperature sensors, the device's power consumption is all the way down to 400nA at 3.3-volts!

Once the transmission is over, the device goes into deep-sleep mode, where the extremely low currents of about 370-400nA are achieved. A timer checks the device status around every 25-seconds and consumes around 10μA during this check for about 2ms.

In the next steps we're going to test a handful of different LoRa radio modules and run the device in low temperature conditions while also monitoring its current draw. Stay tuned for updates!