Re: [patch 22/22] input: ads7846: introduce platform specific way to synchronize sampling

[Eric Miao <eric.y.miao@gmail.com>]

On Sat, Mar 14, 2009 at 9:09 AM, David Brownell <david-b@pacbell.net> wrote:
> On Thursday 12 March 2009, Eric Miao wrote:
>> >
>> > This should be a workaround for broken hardware and I don't expect
>> > this sync time on every boards.
>> >
>>
>> The sync time depends on the moment of this sync between two HSYNC,
>> I don't expect that to be very long,
>
> But how long is that?  1/(50 KHz) or so?
>

For Sharp Zaurus, let's say a poodle, with PXA255 and QVGA LCD
portrait, a typical of 320 lines per frame (although counting in blank
VSYNC will make this a bit larger), and 60Hz frame refresh rate:

1 HSYNC = 1/(60Hz * 320) = 52.08us

An average sync time would be half of this, say, 26us.

Having another interrupt to trigger the sampling will be a bit
trickier and considering this average of 26us on a 400MHz
PXA255, and another interrupt latency, I don't see too much
benefit of doing that.

And my personal feeling of the final result with the GUI seems
quite responsive, though I didn't stress test that.

>
>> yet the hardware is broken enough to
>> endure this latency.
>
> Actually I heard from someone else this morning that it's not just
> "broken" hardware that sees touchscreen ADC sampling noise related
> to hsync.  So this patch makes sense ... although there's no way to
> guarantee the SPI requests won't have queueing delays before they
> arrive at the touchscreen controller, possibly hitting the next
> hsync interval.
>
> It seems that it's surprisingly tricky to get clean samples from
> touchscreens.  There's physical oscillation to compensate, which
> changes capacitance until it damps down.  There's also electrical
> oscillation from charging the various sense lines.  Add to that a
> bunch of honest-to-goodness noise associated with hsync and vsync...
> yeech!  It would be a Good Thing if someone were to split out the
> sample filtering logic from this driver so it could be used with
> other touchscreen drivers.
>

Yes, sample filtering can actually be made input device wise.

There are actually some user-land solution to this, e.g. tslib.
Yet this sync issue seems annoyingly unavoidable even with
a fine grained filter, or maybe I didn't try my best to tweak those
filters to a maximum potential yet, and I gave up on that finally.

> - Dave
>
>



-- 
Cheers
- eric
--
To unsubscribe from this list: send the line "unsubscribe linux-input" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html