Synaptics TouchPad Interfacing Guide Second Edition
Copyright 2001 Synaptics, Inc. 510-000080 - A §2.1 Page 4
2. TouchPad Features
The Synaptics TouchPad is a pointing device for computers and other electronic devices.
To the user, the TouchPad is a flat, usually rectangular area of the computer which is
sensitive to finger touch. By putting the finger on the TouchPad sensor and moving the
finger around, the user can maneuver a cursor around the computer screen. By clicking a
button or tapping directly on the pad, the user can select and drag objects on the screen.
The TouchPad serves the same role in a computer system as a mouse or trackball, but its
compact size, low cost, and lack of moving parts makes it ideal for portable computers.
The Synaptics TouchPad’s advanced features make it the solution of choice for a variety
of applications above and beyond simple mouse replacement.
Synaptics offers a family of TouchPad models of various shapes and sizes, which connect
to the rest of the computer system (the “host”) using several different protocols.
However, there are also many things that all Synaptics TouchPads have in common:
They support the same features and modes; they offer roughly the same set of commands
and queries to the host; they operate according to the same principles. This first part of
the Interfacing Guide describes the common aspects of the Synaptics TouchPad.
2.1. Mouse-compatible Relative mode
When power is applied, the Synaptics TouchPad identifies itself to the host computer as a
regular mouse. This allows the TouchPad to be used with standard mouse drivers. This
mouse-compatible mode is called Relative mode because finger actions are reported to
the host in terms of relative mouse-like motions across the pad. The TouchPad reports
this relative motion to the host in mouse-compatible packets. The TouchPad generates
roughly 40–80 packets per second. Each packet reports the amount of motion in the X
(horizontal) and Y (vertical) directions that has occurred since the previous packet.
These amounts of motion are called deltas, and are written “∆X” and “∆Y”. The packet
also reports information about the left and right “mouse” buttons.
Field Size (bits) Range Meaning
Amount of horizontal finger motion
Amount of vertical finger motion
Left 1 0 or 1 State of left physical button or tap/drag gesture
Right 1 0 or 1 State of right physical button
Figure 2-1. Contents of Relative packet
Because the Relative packet is designed to be compatible with the existing mouse
protocol, the exact contents of the Relative packet vary from one protocol to another. See
the later sections of this Guide for details. (For example, in the PS/2 protocol, the packet
actually reports 9-bit deltas, plus a third “middle” button which is not supported by
current Synaptics products. Also, positive ∆Y values correspond to upward motion in the
PS/2 protocol, but to downward motion in the Serial and ADB protocols.)