Keeping all points mentioned in the previous article with my mind, I looked for parts to use.
It took a few weeks to examine many possible combinations of GPS receiver module and CPU module. I finally picked the combination of Adafruit Ultimate GPS FeatherWing and Adafruit Feather M0 Adalogger
The reasons behind this decision are:
- It is ready-to-use; it has embedded patch antenna and coin battery holder.
- It has microSD holder.
- It can run with LiPo battery.
- It supports QZSS.
- Both modules are designed to work together. (This was important because I was quite new to current microcontroller development, though I have some experience more than a decade ago.)
Also, don't forget to buy female stacking pins.
Hardware development was quite straight forward thanks to Adafruit Feather's stacking architecture. As you can see in the photo, I put the CPU board on breadboard. Later, I added a toggle switch between EN pin and GND to power on / off the logger without disturbing charging from USB.
Software development was also easy thanks to Arduino IDE. I was really impressed how easy compared to my decade old experience. A decade ago (well, a little more than a decade), it took about 5 business days to make simple LED blink sample work on newly arrived hardware; which serial cable (straight / cross) to use was often unclear, compiling process was often not fully integrated (I had to execute linker manually with quite complicated options), writing compiled program to on-board ROM often failed for unclear reasons, and so on.
Actual code is available at GitHub. https://github.com/takyanagida/raw_nmea_logger
Some notes on the code:
- SD card flush is called not for every line of log to save battery consumption, but for every second to minimize data loss against empty battery.
- Battery alert threshold is determined as described in my Adafruit forum post.
Also, see README.md in the repository.
After some field testing of my first logger, I noticed that the stacked structure hides SD card LED and power LED. Also, I couldn't find a good case to put the logger into with the battery. So, I made the second one in flat structure.
Since CPU and GPS are connected through UART, only 4 wires (3V, GND, TX, RX) are required. Also, EN and GND are connected to a toggle switch same as the first one.
The green case is a cheap pill case found at a dollar store.
In the next articles, I will compare these two loggers' signal reception in different configurations (case, bag, pocket, etc.)