My newest design is a PC downloader utility called blsync, created as part of the Backwoods Logger project. BLsync is a command line program with a corresponding hardware adapter, and it enables snapshot lists and temperature / altitude / pressure graphs to be transferred from the Logger to a PC. Above is a quick graph I put together in Excel from blsync data, showing temperature and air pressure in my home over a 40 hour period. If I’d been hiking recently, I’d show a nice altitude over time graph too.

The Backwoods Logger is a programmable graphing altimeter / thermometer, developed as an open hardware project.  It’s designed for backpackers, runners, environmental scientists, and other people interested in environmental data logging over timescales from an hour to a few weeks. The Logger design is based on an Atmel ATmega328 microcontroller and Bosch BMP085 pressure / temperature sensor. You can build your own Logger following the published instructions, or purchase a pre-assembled Logger prototype. For additional details visit the project home page, or join the public discussion mailing list.

Bit Bang Serial

The Backwoods Logger wasn’t designed with a PC download function in mind, so retrofitting one after the fact was a challenge. The only external connector on the Logger is the 6-pin ISP port, used for programming the microcontroller firmware. After programming is complete, the pins MOSI, MISO, and SCK can be used as general-purpose I/Os or as an SPI interface. (The other three pins are VCC, GND, and RESET.) So what’s the simplest way to build a PC communication interface using those three pins?

I’m not aware of any straightforward way to build a PC interface using SPI, so my solution was to design a serial interface. Unfortunately the built-in USART that’s normally used for ATmega serial communication is hard-wired to use specific pins, and those pins aren’t connected to the ISP port. Instead of using the built-in USART, then, I wrote a pure-software serial port driver that works over the MOSI and MISO pins. It manually performs all the start bit and bit-to-bit timing functions necessary for a bidirectional serial connection. In order to reliably get the right serial baud rate, I also added some code to calibrate the internal RC oscillator using the external 32768 Hz crystal as a reference. It was all a little painful, but it works.

The Backwoods Logger comes in two versions that run at different clock speeds, which complicated the software serial driver a bit. The Logger Mini runs at 8 MHz, which provides 208 clock cycles between each bit at 38400 bps. That’s a fairly healthy amount of time to react to each bit and decide what to do with it. However, the Logger Classic runs at 1 MHz to minimize its power consumption, which provides only 26 clock cycles between bits. That’s not very much time for processing, and it means I wasn’t able to implement simultaneous two-way serial communication. At any given moment the Logger can either be receiving or sending serial data, but not both. There also needs to be a delay of about 200 microseconds between received bytes, in order to provide the Logger with sufficient time to process a byte before the next one arrives. There’s no receive buffer, so if the PC sends bytes with too short a byte-to-byte delay, data will be lost. The blsync program inserts the necessary delays when sending command bytes to the Logger. Since most of the data flow is in the other direction, this doesn’t impose any noticeable speed penalty. The complete set of graphs can be transferred from the Logger in about 300 milliseconds.

Physically, the blsync adapter is just a passive converter from the 3 x 2 ISP header to a 1 x 6 serial header, with RX connected to MOSI and TX connected to MISO. A MOSFET is used to perform level conversion (thanks Erik!), so either a 5V or 3.3V USB-Serial device can be used. If you know you’ll always be using a 3.3V USB-Serial device, you can omit the MOSFET and connect TX to MISO directly. The 1 x 6 header is designed to work with common USB-Serial devices such as the Adafruit FTDI Friend or Sparkfun FTDI Cable 3.3V.

I also designed a simple blsync adapter board, for a more rugged and permanent solution. It adds a pair of LEDs to indicate Logger and PC activity, but is otherwise the same as the circuit shown above.

Using BLsync

The blsync utility program can retrieve the Logger’s firmware version number, temperature / altitude / pressure graphs, and snapshot list. Data can be saved in CSV format for importing into Excel, or as raw binary.

The blsync utility is configured using command line options:

Usage: blsync -p port [-b speed] [-v] [-c] [-r] [-g filename] [-s filename]
    -p port       Port to use for Logger communication, such as COM1.
    -b speed      Bit rate for communication. Default is 38400.
    -v            Display the Logger firmware version number.
    -c            Save files in CSV format. This is the default.
    -r            Save files in raw binary format instead of CSV.
    -g filename   Sync the graph data, and save it to the named file.
    -s filename   Sync the snapshot data, and save it to the named file.

For example, to sync the temperature, altitude, and pressure graphs from the Logger connected to COM3, and save them in CSV format to the file graphs.csv, the command line would be:

blsync.exe -p COM3 -c -g graphs.csv

By importing the CSV file into your favorite spreadsheet or other analysis program, you can perform whatever analysis you desire on the collected Logger data. A hiker might create a graph showing his friends how high he climbed or how cold it was. A conservationist might use several Loggers to collect data for a study of temperature and pressure at multiple locations within a habitat, and how they relate to observed species observations. Through hardware and software extensions, humidity or other custom sensor data could be collected as well. Because the Backwoods Logger is an open source, open hardware project, the sky’s the limit on what data might be collected and what interesting things might be done with it.