AT42QT2640_14 Datasheet, PDF (42/73 Pages) ATMEL Corporation – 64-key 8-slider/wheel QMatrix FMEA

5.5.3
Burst Length – BL
The signal gain for each key is controlled by circuit parameters as well as the burst length.
The burst length is simply the number of times the charge-transfer (QT) process is performed on a given X line. Each
QT process is simply the pulsing of an X line once, with corresponding Y lines enabled to capture the resulting
charge passed through the keys capacitance Cx.
QT2640 uses a fixed number of QT cycles which are executed in burst mode. There can be up to 64 QT cycles in a
burst, in accordance with the list of permitted values shown in Table 5-24 on page 56.
Increasing burst length directly affects key sensitivity. This occurs because the accumulation of charge in the charge
integrator is directly linked to the burst length. The burst length can be set for each key individually; charge is
selectively captured on all Y lines simultaneously during the burst on each X line.
Apparent touch sensitivity is also controlled by the Negative Threshold level (NTHR). Burst length and NTHR
interact. Normally burst lengths should be kept as short as possible to limit RF emissions, but NTHR should be kept
above 6 to reduce false detections due to external noise. The detection integrator mechanism also helps to prevent
false detections.
BL Default value:
1 (32 pulses)
BL Range:
0, 1, 2, 3 (16, 32, 48, 64 pulses / burst)
5.6 Addresses 1089 to 1216 – Calibrated Frequency Offset – CFO_1 and CFO_2
Table 5-6. Calibrated Frequency Offset
Address
Bit 7
Bit 6
Bit 5
1089

1152
1153

1216
Bit 4
Bit 3
CFO_1 (Key 0)

CFO_1 (Key 63)
CFO_2 (Key 0)

CFO_2 (Key 63)
Bit 2
Bit 1
Bit 0
CFO_1 and CFO_2 are only used if FHM = 2.
If frequency hopping is enabled with FHM = 2, the QT2640 adjusts each key reference at each frequency hop. The
amount of the adjustment must first be configured using the setups CFO_1 and CFO_2. These values 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.
FREQ0 must define the highest of the three selected frequencies. ie be configured with the lowest value. 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 QT2640 automatically uses combinations of these offset values, as necessary, when switching
between the three different frequencies.
The Calibrated Frequency Offset (CFO) values are used to adjust each key 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.
AT42QT2640 [PRELIMINARY DATASHEET]
42
9684DX–AT42–12/13