Swap MKR 1300 LoRaWAN for Adafruit Feather M0 LoRa Radio

A project log for Low Power High Accuracy Weather Station

'Professional' wind instruments are mounted high on a pole in an exposed location with LoRa data transmission

Tegwyn☠TwmffatTegwyn☠Twmffat 04/15/2019 at 14:160 Comments

I've been really struggling getting the Arduino MKR1300 to work properly - A simple delay after powering up moisture pin seemed to fix it:

Old code:

digitalWrite(moisturePowerPin, HIGH);              // Powers up the moisture sensor for 0.1 seconds to prevent corrosion.
moisture = (analogRead(sensorPin)*0.120)-9; 

New code:

digitalWrite(moisturePowerPin, HIGH);              // Powers up the moisture sensor for 0.1 seconds to prevent corrosion.
delay (50);
moisture = (analogRead(sensorPin)*0.120)-19;  

Also, I think there was some type casting of variables problem which I fixed at the same time (eg we cant just cast integers to floats without using memcpy function in 32 bit system or similar) but on top of that there's a well known hardware fault with the device that stops it going into sleep mode. I say well known, but when I selected the device, I did not know about it! Nevertheless, swapping out for the Adafruit version was only a few hours work on DesignSpark PCB software and here's the new version:

As the name of this project suggests, having a low power consumption is vitally important, and from the Adafruit documentation it looks like those guys actually went through the various power profiles and it certainly looks like it works properly from the various graphs at: . The MCU on the feather has 256k of flash and 32k RAM memory so it's easily big enough for all my code. It's also pretty fast although speed is not really an issue. There are also 3 spare input pins which can be hacked if necessary in the future. PCB and Gerber files are on the GitHub page and have just been sent to JLC PCB for a 10 day turnaround for $10 including postage - they now have a swanky airmail service which should speed things up quite a bit.

Looking at the image above, you might notice a crazy small chip inside the yellow feather module box. This is a programmable watchdog that will trigger reset if the MCU crashes or starts producing erratic data. It works really well with the current MKR1300 setup, but will it work with the various Adafruit sleep modes? I'm guessing I can put the device into deep sleep for 3 seconds at a time, which is within the current watchdog timeframe. Should be interesting to see how it performs.

The current draw for the device is documented as being 11mA for the MCU and an additional 2mA for the radio. To transmit a data payload of 20 bytes, about 130mA is required for 70ms. When the radio is actively listening for data, the whole device consumes 40mA and when in deep sleep, about 300uA. Will the rain gauge work in deep sleep mode?