For dive computers where the reference time (epoch) of the device is
unknown, libdivecomputer uses the current time of the device (devtime)
and the host system (systime) to synchronize both clocks.
Currently, both timestamps are passed directly to the constructor of the
parser. With the new public function, the application can adjust the
timestamps afterwards.
Some dive computers store the depth as an absolute pressure value (in
bar). To convert to a depth value (in meters), the atmospheric pressure
and water density are required. For dive computers that do not have
those values available, libdivecomputer uses a default value. With the
new public api functions, applications can adjust those default values.
Some dive computers already provided a backend specific calibration
function. Those functions are now deprecated. They are kept around to
maintain backwards compatibility for now, but they will be removed in
the next version.
The byte at offset 0x1A appears to be some settings byte, containing not
only the sample rate index, but also some other values in the higher
bits. Except for the 4th bit, which contains the salinity setting, the
meaning of those higher bits remains unknown. Since the sample rate is
limited to only 4 possible values, 2 bits are sufficient for encoding
the sample rate index. The 3th bit might contain some other setting, or
be reserved for future sample rates.
With just 12 bits, the depth values are limited to at most 40.95m.
However for some deeper dives, this appears to be wrong. The next two
higher bits, which were previously unknown, are also part of the depth.
This increases the maximum depth to 163.83m.
The Sporasub SP2 uses a very simple communication protocol and memory
layout, but with some unusual aspects:
Dives are artifically limited to a maximum of 6000 samples.
Unlike all other dive computers, the dives are not stored in some kind
of ringbuffer structure. Once the memory is full, no new dives can be
recorded. The existing dives need to be erased first, and the dive
computer will start recording again at te start of the memory area. The
Sporasub application has an "Auto-clear watch memory after data
transfer" feature for this purpose.
I didn't implement a more efficient download algorithm because
downloading a full memory dumps takes less than 10 seconds.
