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Timestamp support for OpenLog

13/09/2013 2 comments

OpenLog board

OpenLog board (Image) © Sparkfun


I needed a tool that can play back recorded serial stream “in real time”, so that the playback perfectly reflects the stream that was originally sent. Rather than building my own, OpenLog project seemed to be a perfect starting point. The board itself is very small, about the size of a micro-sd socket that is on the bottom of the board. By default the device records serial data received to standard ftdi-pinout serial port. The device is Arduino UNO compatible and code can be compiled and uploaded with the Arduino IDE. The device is configured by editing CONFIG.TXT file on the sd-card. If the file is not present on the card, the device creates one with default settings at the startup.

Example of config file:

115200,26,3,0,1,1,0,1
baud,escape,esc#,mode,verb,echo,ignoreRX,timestamp

As you can see, I added the timestamp option at the end. If this option is enabled, each received byte in the log file is prefixed with 4-byte unsigned long timestamp. The timestamp is in little endian order and in milliseconds. Unsigned long type provides about 55 days before counter rolls over. The value can be decoded for example in python with struct.unpack function:

buf = f.read(4)
timestamp = struct.unpack("<L", buf)[0]
char = f.read(1)

The logged files will be four times larger than without timestamping, but with large sd-cards this does not really make a difference. I quickly tested the performance and the device still seemed to be able to log the data without data drops at 115200 bps speed.

The modified codes are in my git repository: https://github.com/JanneMantyharju/OpenLog
I also made a pull request to the official repository, but let’s see will they accept it.

The python script that plays back the stream in real time can be found from the examples folder.

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Categories: Electronics Tags: , , ,