at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 9

• Manufacturer Part Number: at42qt1481
• Description: 48-key Qmatrix Ic
• Manufacturer: ATMEL Corporation

2.5
2.6
9621B–AT42–06/11
Oscillator
Sample Capacitor; Saturation Effects
The oscillator can use either a quartz crystal or a ceramic resonator. In either case, XT1 and
XT2 must both be loaded with 22 pF capacitors to ground. Three-terminal resonators having
onboard ceramic capacitors are commonly available and are recommended. An external
TTL-compatible frequency source can also be connected to XT1 in which case, XT2 should be
left unconnected.
The frequency of oscillation should be 16 MHz ±1 percent for accurate UART transmission
timing.
The charge sampler capacitors on the Y pins (Cs0 – Cs5) should be NPO (preferred), X7R
ceramics or PPS film; NPO offers the best stability. The value of these capacitors is not critical
but 4.7 nF is recommended for most cases.
Cs voltage saturation is shown in
accumulation on Cs inducing conduction in the pin protection diodes. This badly saturated signal
destroys key gain and introduces a strong thermal coefficient which can cause phantom
detection.
The cause of this is either from the burst length being too long, the Cs value being too small, or
the X-Y transfer coupling being too large. Solutions include loosening up the interdigitation of
key structures, greater separation of the X and Y lines on the PCB, increasing Cs, and
decreasing the burst length.
Increasing Cs makes the part slower; decreasing burst length makes it less sensitive. A better
PCB layout and a looser key structure (up to a point) have no negative effects.
Cs voltages should be observed on an oscilloscope with the matrix layer bonded to the panel
material; if the Rs side of any Cs ramps is more negative than -0.25 volts during any burst (not
counting overshoot spikes which are probe artifacts), there is a potential saturation problem.
Figure 2-3
distortion is caused by excessive stray capacitance coupling from the Y line to AC ground; for
example, from running too near and too far alongside a ground trace, ground plane, or other
traces. The excess coupling causes the charge-transfer effect to dissipate a significant portion of
the received charge from a key into the stray capacitance.
This phenomenon is more subtle; it can be best detected by increasing BL to a high count and
watching what the waveform does as it descends towards and below -0.25V. The waveform
appears deceptively straight, but it slowly starts to flatten even before the -0.25V level is
reached.
A correct waveform is shown in
substantially straight (ignoring the downward spikes).
Unlike other QT circuits, the Cs capacitor values on QT1481 have no effect on conversion gain.
However, they do affect conversion time.
Unused Y lines should be left open.
shows a defective waveform similar to that of
Figure
Figure
2-4. Note that the bottom edge of the bottom trace is
2-2. This nonlinearity is caused by excessive voltage
Figure
2-2, but in this case the
AT42QT1481
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