[Tinyos Core WG] Issue with sampling multiple sensors

[Philip Levis]

On May 22, 2007, at 2:09 PM, David Gay wrote:

> The thread that starts with
> http://mail.millennium.berkeley.edu/pipermail/tinyos-help/2007-May/ 
> 024986.html
> ("I can't sample X&Y acceleration at more than 31Hz!") brings up a
> limitation of ReadStream:
> how can one sample a group of sensors at high rates?
>
> It's easy enough to sample one sensor at a high rate, or several
> sensors in succession. But if I want 200Hz sampling of X&Y
> acceleration simultaneously, there doesn't seem to be a very good
> answer?
>
> Possible current solutions:
> - go low level, using Resource to reserve both sensors and ReadNow to
> sample them (i.e., do ReadStream by hand) - this requires the
> sensorboard to make *Now versions of each sensor available (the mts300
> doesn't...)
>
> - have the sensor provide all reasonable sensor groups (e.g., X, Y and
> X&Y) - painful for more than two sensors
>
> Possible future solutions:
> - some interface to sampling groups of sensors? could be icky...

Your example raises two separate issues:

1) Can I sample multiple sensors at a high rate?
2) Can I sample multiple sensors with low jitter between the readings?

I think that 1) is reasonably solvable in this case. In theory, a  
ReadStream should know the warmup times of a sensor and  
correspondingly control whether it goes to sleep. Currently, Accel  
uses a SplitControlPowerManager, so it powers down the sensor  
immediately on idle.

2) is a very different problem. Unlike 1), there isn't enough  
information from the application layer to infer this relationship.  
The ADC very well may interleave other ADC readings between the two.  
I'd therefore agree with Jan and Vlado on this one.

Phil



Phil