Update sensor code with speed and filespace improvements.

[erichiggins]

Taking the advice offered in #1 , this updated version runs around 6x faster than before and trims down the output file.

Changes:

• Changed accelerometer from 4G to 8G range.
• Changed acceleration output from m/s² to G since it's metric/imperial agnostic.
• Called lis.read() directly instead of lis.getEvent() for a very small speed boost (but mostly to avoid conversion from G to m/s².
• Set accelerometer to faster data rate (unsure this has an effect due to issues in the library).
• Use SDFat library to support long filenames.
• Uses a full timestamp for the output filename (e.g. 201902171830.csv).
• Set I2C speed to fast mode.
• Define the column units (e.g. µs, G) in the header row to reduce disk usage.
• Dropped millis() from output (same info provided by micros()).
• Dropped battery voltage from output.
• Dropped acceleration range from output (inconsequential).
• Use unsigned long variables to reduce overhead.
• Call the unixtime() just once at start up and use micros() to estimate time afterward (shaved off 30ms per loop).
• Dropped the motion-detection aspect of data writing and simply write the data to the SD card every 800 loops.

Note: The process of writing to disk takes around 10 ms, which is still fairly slow. Ideally a buffered approach which only recorded movement would keep the output file significantly smaller. I hope to add that in a future iteration.

Chart Preview

Output Data Format

Here's a quick comparison of what the output data looks like before and after this change.

Before

timestamp	accel x	accel y	accel z	accel unit	sensor range	millis	micros	voltage
1535376330	1.59	9.96	-1.53	m/s^2	4	128808	128808823	4.15
1535376330	2.26	11.90	-2.95	m/s^2	4	128817	128817310	4.14
1535376330	2.68	10.86	-2.09	m/s^2	4	128823	128823514	4.14
1535376330	2.39	9.46	-1.21	m/s^2	4	128829	128829672	4.15
1535376330	1.88	8.47	-1.23	m/s^2	4	128835	128835808	4.14
1535376330	1.28	7.68	-1.42	m/s^2	4	128841	128841962	4.14

After

timestamp (s)	start (µs)	delta (µs)	accel x (G)	accel y (G)	accel z (G)
1547749706	123593	223	0.75	1.19	-0.19
1547749706	125917	251	0.80	1.19	-0.22
1547749706	126979	252	0.85	1.11	-0.21
1547749706	128040	252	0.85	1.09	-0.26
1547749706	129097	251	0.94	1.05	-0.22
1547749706	130158	252	0.97	1.02	-0.20

[erichiggins]

@jveitchmichaelis Have any feedback on this before I merge it?

[jveitchmichaelis]

Looks good to me!

[erichiggins]

@jveitchmichaelis FYI, I posted an article to share the benefits of this update and gave you a shout out. Hope you don't mind.