at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 18

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

The host can try to recover from any safety critical failure by resetting the QT1481 using its RST
pin. The host should allow a grace period in consideration of the start-up and initialisation time
the QT1481 requires after reset to ability to communicate (see
Section 2.13 on page
14).
2.19
Frequency Hopping
This QT1481 supports frequency hopping, which tries to select a sampling frequency that does
not clash with noise at specific frequencies elsewhere in products or product operating
environments. It tries to hop away from the noise.
During the acquisition bursts, a sequence of pulses are emitted with a particular spacing, which
equates to a particular sampling frequency. If the latter should coincide with significant noise
generated elsewhere, touch sensing may be seriously impaired or false detections may occur.
To help combat such noise, the burst frequency can either be preset to one specific frequency
(with frequency hopping disabled), away from the noisy frequency, or frequency hopping can be
enabled and set to switch dynamically between three specific configured frequencies.
If frequency hopping is disabled (FHM = 0), the function FREQ0, together with DWELL, defines
the sampling frequency.
If frequency hopping is enabled (FHM = 1 or 2), the QT1481 continually monitors the noise
across all keys and switches frequencies dynamically to try and find the frequency with the lower
noise amplitude. Up to three different frequencies can be selected using the functions FREQ0,
FREQ1 and FREQ2. If frequency hopping between only two frequencies is desired, the same
frequency should be selected for two of these functions.
The signal levels vary slightly with frequency, and so the reference for each key must be varied,
in one way or another, during frequency hopping to compensate and to prevent false detections
and unresponsive keys. The mechanism used to adapt the references is different depending on
the frequency hop mode set with the FHM function.
If frequency hopping is enabled with FHM = 1, the QT1481 compensates for the variations in
signal level by recalibrating each key immediately after each frequency hop. A negative aspect
to this mode is the danger that a key is being touched at the time of the frequency hop. If this
were to happen, the touched key would be recalibrated to the touching finger and the detect
would be cancelled. However, on subsequent removal of the touch (for a time greater than or
equal to the the PRD function) the key would be recalibrated once again and the detect flagged
when the touch is re-established.
If frequency hopping is enabled with FHM = 2, the QT1481 adjusts each key's reference at each
frequency hop. The amount of the adjustment must first be configured using the setups CFO_1
and CFO_2. These entries are the amounts by which reference levels need to be adjusted to
track the signal changes when changing from the frequency set by FREQ0 to the frequency set
by FREQ1 or FREQ2. Each key uses its individual settings of CFO_1 and CFO_2.
Assuming FREQ0 defines the highest of the three selected frequencies, CFO_1 indicates the
offset to be applied when the frequency is changed between those defined by FREQ0 and
FREQ1. CFO_2 indicates the offset to be applied when the frequency is changed between those
defined by FREQ0 and FREQ2. The QT1481 uses combinations of these offset values, as
necessary, when switching between the three different frequencies.
AT42QT1481
18
9621B–AT42–06/11