Import google/walt

Cloned from https://github.com/google/walt.git without modification.

Bug: 36896528
Test: N/A

1 files changed, 33 insertions, 0 deletions

diff --git a/docs/TapLatency.md b/docs/TapLatency.md
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+## Tap latency
+
+ * [Video demonstration of tap latency measurement](https://www.youtube.com/watch?v=1xAReF75Cts&list=PLd6Fi7WgXfcCEJg1FDqNCoQfpWo7W3J5a&index=2)
+ * [Detailed instructions on how to perform the measurement in usage doc](usage/WALT_usage.md#tap-latency).
+
+WALT uses a “stylus” equipped with an accelerometer. The finger is imitated by a flat metal
+tip that is grounded, pretty much any rigid tip can be used as long as it triggers the touch sensor.
+When the stylus “collides” with touch screen the accelerometer senses a shock (above 3g) which is
+timestamped by the Teensy. In order to generate a similar shock when picking the stylus up from the
+screen, the conductive surface and the accelerometer are mounted on a button of a retractable pen.
+On the way up, the spring keeps the button in contact with the touch screen for the first few mm of
+motion. This allows the hand holding the main part of the pen to gain some speed to which the button is
+then abruptly accelerated generating an easily detectable shock.
+
+Linux [Multi Touch (MT)](https://www.kernel.org/doc/Documentation/input/multi-touch-protocol.txt)
+implementation timestamps touch events in the kernel as they arrive from the hardware. On Android
+the MT events are then exposed in Java as
+[MotionEvent](http://developer.android.com/reference/android/view/MotionEvent.html) 
+and include the kernel timestamp. For tap, the relevant MotionEvent types are
+ACTION_DOWN and ACTION_UP.
+
+Sample measurements
+
+| Device       | OS version     | ACTION_DOWN [ms]| ACTION_UP [ms]| Kernel to Java [ms] |
+| :---         | :---           |           ---: |           ---: |                 ---: |
+| Nexus 5      | M4B30Z (6.0.1) |           26.9 |           15.9 |                  3.3 |
+| Nexus 5X     | NRD91P (7.0)   |           25.0 |           22.5 |                  2.4 |
+| Nexus 7      | LMY47Q (5.1)   |           29.6 |           31.0 |                  1.4 |
+| Nexus 9      | MMB29K (6.0.1) |           18.7 |           19.9 |                  1.3 |
+
+
+![Tap measurement](usage/images/tap.png)
+![Tap screenshot](Tap_screen_N7.png)