On the SparkFun
Flashing Firmware web page (
https://github.com/sparkfun/OpenLog/wiki/Flashing-Firmware), they suggest using their
FTDI Basic and
Crossover Breakout for FTDI boards to connect to the
OpenLog board. I just used a RS-232 serial-to-TTL adapter I had instead (in the schematic on my web page for the portable serial data logger, you can see I used a MAX233 device - note, it is wired as a DTE so a null-modem adapter is needed to connect the correct signals on the DB-9 to the COM port on the PC). The signals that need to be connected are: Rx, Tx, DTR, and GND.
Anyway, after connecting the OpenLog to the PC, the Arduino software environment (Arduino 1.0 or later) is used to reprogram the OpenLog device similar to what would be done if you were programming/uploading to a standard Arduino board. In fact, the OpenLog appears to operate as an Arduino Uno, so, in the Arduino software environment, set the board under Tools->Board to Arduino Uno.
All of this is spelled out reasonably well in the
Updating Firmware on OpenLog v3 section on their
Flashing Firmware web page. If you follow their instructions, including obtaining the needed
SerialPort and
SdFat libraries, you should be able to recompile the
OpenLog_v3.ino from their GitHub site and load that on your OpenLog device as a start. From there, its just a matter of modifying the original source for the feature changes you need. (If you look at the source files on my web site, you will notice I split the original OpenLog_v3.ino into multiple .ino files to make it easier to manage as separate functional modules that all get compiled into one executable by the Arduino software environment.)