1.2 Enabling / Disabling Keys
1 Overview
The NDIL parameter is used to enable and disable keys in the
matrix. Setting NDIL = 0 for a key disables it (Section 5.4). At
no time can the number of enabled keys exceed the maximum
specified for the device in the case of the QT60168.
QMatrix devices are digital burst mode charge-transfer (QT)
sensors designed specifically for matrix geometry touch
controls; they include all signal processing functions necessary
to provide stable sensing under a wide variety of changing
conditions. Only a few external parts are required for operation.
The entire circuit can be built within a few square centimeters of
single-sided PCB area. CEM-1 and FR1 punched, single-sided
materials can be used for possible lowest cost. The PCB’s rear
can be mounted flush on the back of a glass or plastic panel
using a conventional adhesive, such as 3M VHB 2-sided
adhesive acrylic film.
On the QT60168, only the first 2 Y lines (Y0, Y1) are
operational by default. On the QT60168, to use keys located on
line Y2, one or more of the pre-enabled keys must be disabled
simultaneously while enabling the desired new keys. This can
be done in one Setups block load operation.
2 Hardware & Functional
Figure 1-1 Field flow between X and Y elements
2.1 Matrix Scan Sequence
overlying panel
The circuit operates by scanning each key sequentially, key by
key. Key scanning begins with location X=0 / Y=0 (key #0). X
axis keys are known as rows while Y axis keys are referred to
as columns. Keys are scanned sequentially by row, for example
the sequence X0Y0 X1Y0 .... X7Y0, X0Y1, X1Y1... etc. Keys are
also numbered from 0..24. Key 0 is located at X0Y0. A table of
key numbering is located on page 22.
X
Y
element
element
Each key is sampled in a burst of acquisition pulses whose
length is determined by the Setups parameter BL (page 20),
which can be set on a per-key basis. A burst is completed
entirely before the next key is sampled; at the end of each burst
the resulting signal is converted to digital form and processed.
The burst length directly impacts key gain; each key can have a
unique burst length in order to allow tailoring of key sensitivity
on a key by key basis.
QMatrix parts employ transverse charge-transfer ('QT') sensing,
a technology that senses changes in electrical charge forced
across an electrode by a pulse edge (Figure 1-1). QMatrix
devices allow for a wide range of key sizes and shapes to be
mixed together in a single touch panel.
The devices use an SPI interface to allow key data to be
extracted and to permit individual key parameter setup. The
interface protocol uses simple single byte commands and
responds with single byte responses in most cases. The
command structure is designed to minimize the amount of data
traffic while maximizing the amount of information conveyed.
2.2 Disabling Keys; Burst Paring
Keys that are disabled by setting NDIL =0 (Section 5.4, page
19) have their bursts pared from the scan sequence to save
time. This has the consequence of affecting the scan rate of the
entire matrix as well as the time required for initial matrix
calibration.
In addition to normal operating and setup functions the device
can also report back actual signal strengths and error codes.
Reducing the number of enabled keys also reduces the time
required to calibrate an individual key once the matrix is initially
calibrated after power-up or reset, since the total cycle time is
proportional to the number of enabled keys.
QmBtn software for the PC can be used to program the
operation of the IC as well as read back key status and signal
levels in real time.
Keys that are disabled report as follows:
The QT60168 and QT60248 are electrically identical with the
exception of the number of keys which may be sensed.
Signal = 0
Reference = 0
Low-signal error flag (provided LSL >0)
Calibrating flag for key set only just after device reset or
after a CAL command, for one scan cycle only
Failed calibration error for key always set
Detect flag for key never set
1.1 Part differences
Versions of the device are capable of a maximum of 16 or 24
keys (QT60168, QT60248 respectively).
These devices are identical in all respects, except that each is
capable of only the number of keys specified. These keys can
be located anywhere within the electrical grid of 8 X and 3 Y
scan lines.
See also Section 4.16 notes.
2.3 Response Time
The response time of the device depends on the scan rate of
the keys (Section 5.11), the number of keys enabled (Section
5.4), the detect integrator settings (Section 5.4), the serial
polling rate by the host microcontroller, and the time required to
do FMEA tests at the end of each scan (~5ms).
Unused keys are always pared from the burst sequence in
order to optimize speed. Similarly, in a given part a lesser
number of enabled keys will cause any unused acquisition burst
timeslots to be pared from the sampling sequence to optimize
acquire speed. Thus, if only 14 keys are actually enabled, only
14 timeslots are used for scanning.
lQ
3
QT60248-AS R4.02/0405
QUANTUM [ QUANTUM RESEARCH GROUP ]
