at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 52

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

5.26
5.27
5.28
5.29
52
Frequency2 – FREQ2
Noise Threshold – NSTHR
Noise Integrator Limit – NIL
Calibrated Frequency Offset – CFO_1 and CFO_2
AT42QT1481
Up to three different operating frequencies may be configured, using FREQ0, FREQ1 and
FREQ2. These functions define the idle time between pulses in the burst. Larger values of
FREQ2 yield longer time between pulses and thus a lower fundamental frequency.
Together with DWELL, FREQ2 allows the fundamental frequency to be set in the range
31 kHz – 943 kHz. The frequency for a specific DWELL and FREQ2 combination should be
measured using an oscilloscope (temporarily disable frequency hopping to make the
measurement easier).
This function is programmed on a global basis. See
FREQ2 Default value:
FREQ2 Possible range:
NSTHR is used, if frequency hopping is enabled, by the frequency hopping algorithms to
determine if a signal delta should be considered as noise. A delta, of either polarity, greater than
or equal to NSTHR is considered as possible noise and forces the Noise Integrator counter to be
incremented. Two Noise Integrator counters are maintained, one for positive deltas and one for
negative.
This function is programmed on a global basis. See
NSTHR Default value:
NSTHR Possible range:
NIL is used if frequency hopping is enabled. To distinguish between short noise transients and
consistent noise, the QT1481 incorporates a “Noise Integrator” or NI counter mechanism. A
global counter is incremented each time a signal delta exceeds NSTHR. Two Noise Integrator
counters are maintained, one for positive deltas and one for negative. When both counters reach
the preset limit configured by NIL, the signals are declared to be noisy and a frequency hop is
considered.
The counters are reset to zero at the end of each matrix scan. This mechanism provides a
robust way of detecting strong noise while suppressing unnecessary frequency hopping.
This function is programmed on a global basis. See
NIL Default value:
NIL Possible range:
The Calibrated Frequency Offset (CFO) values are used when the frequency hop mode is set
to 2. They are used to adjust each key's reference whenever a frequency hop occurs, taking into
account any differences in calibrated signal at the different frequencies. The device uses the
highest selected frequency as a point of reference and calculates offsets in calibrated signals at
the other two frequencies relative to the highest one. The following explanation assumes that
FREQ0 defines the highest frequency, for convenience of this discussion, but that does not need
to be the case.
36 (delay cycles)
0 – 63 (Highest frequency to Lowest frequency)
0 (5 counts)
0 – 15 (5 – 50 counts)
3
0 – 15 (0 – 2 = factory use only)
Table 5-7 on page
Table 5-7 on page
Table 5-7 on page
57.
57.
57.
9621B–AT42–06/11