StopWatch activity

[Jim Gettys]

Note that X button and keyboard events have timestamps, in milliseconds.
(This wraps in some hundreds of days, but I doubt anyone will use the
stopwatch that long; you do have to worry in principle about doing
modulus arithmetic, though IIRC, X servers generally have been using
time since the server reset last for this value.

Theory goes that since we are using the kernel's evdev driver, that
these timestamps may even get done in the kernel at interrupt level, if
the implementation is as has been intended (I haven't looked to make
sure), as we did in the original X drivers in the mid '80s....

This would make the timing pretty independent of user space
considerations.

                                 - Jim


On Thu, 2007-11-15 at 09:22 +1100, James Cameron wrote:
> On Wed, Nov 14, 2007 at 12:03:08PM -0500, Benjamin M. Schwartz wrote:
> > It takes some time to process your mouse click, and under heavier CPU
> > load, that time may be long enough that the time label continues to
> > redraw before it can be stopped.
> 
> Good.  I suspected as such, based on your original announcement, so when
> I saw it I didn't dimiss it but tried some more.  Hopefully the kids
> will notice as well, and the teacher can have your notes on how to
> explain the effect.  ;-)
> 
> Please put your post in the source.
> 
> > Every digital stopwatch I have ever seen has precision to hundredths
> > of a second, no more and no less.
> 
> Yes, I suspect they mostly use the same internal design.  You are lucky,
> you don't have to.
> 
> > I don't know what you mean by a bar graph.  Younger children can't
> > read analog clocks (I recall being taught how to read them in second
> > grade).  Also, drawing clock faces is computationally expensive.
> 
> A bar graph ... okay, perhaps "fuel gauge" might be a better term, ...
> creates a linear single axis representation of time, which the human eye
> is *very* good at estimating and predicting against.  It is how a kid
> catches a ball.
> 
> A stopwatch clockface, represents the time by rotation of a marker
> around the circumference of a circle.  It does not have the same
> "problems" associated with 12-hour analog clocks, since there is
> normally only one hand, and the full circle doesn't represent half a
> day.
> 
> Analog day clocks can be taught to children, but not as easily as
> numbers and digital clocks, I agree.  On the other hand, the lack of
> momentum perception on a digital clock tends to teach children the art
> of estimating time without requiring an external beat.
> 
> Analog clocks remain the cheapest clock per square metre of visual
> coverage, so our target market will be exposed to them.  You cannot
> dismiss them based on your first-world understanding of digital clocks.
> 
-- 
Jim Gettys
One Laptop Per Child