at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 40

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

5.2
5.3
5.4
40
Threshold Multiplier – THRM
Positive Threshold – PTHR
Drift Compensation – NDRIFT, PDRIFT
AT42QT1481
It is sometimes useful to be able to operate the QT1481 with much higher detect thresholds than
can be set with NTHR alone. The Threshold Multiplier (THRM) is a multiplier which extends the
range of NTHR and PTHR considerably. The operating detect threshold for a key is arrived at by
multiplying NTHR or PTHR for that key by THRM. Note that the detect threshold range is
extended at the expense of the step size.
each value of THRM.
Table 5-1.
This function is programmed on a global basis. See
THRM Possible range:
THRM Default value:
The positive threshold is used to provide a mechanism for recalibration of the reference point
when a key's signal moves abruptly to the positive. This condition is not normal, and usually
occurs only after a recalibration when an object is touching the key and is subsequently
removed. The desire is normally to recover from these events quickly.
Positive hysteresis: PHYST is fixed at 12.5 percent of the positive threshold value and cannot be
altered.
Positive threshold levels are programmed using the Setup process on a per-key basis. See also
Section 5.2 on page 40
Typical values:
Default value:
Signals can drift because of changes in Cx and Cs over time and temperature. It is crucial that
such drift be compensated for, or false detections and sensitivity shifts can occur.
Drift compensation
raw signal at a slow rate, but only while there is no detection in effect. The rate of adjustment
must be performed slowly, otherwise legitimate detections could be ignored. The QT1481 drift
compensates using a slew-rate limited change to the reference level; the threshold and
hysteresis values are slaved to this reference.
THRM
0
1
2
3
Extended Detect Threshold
(Figure 5-1 on page
1 to 4
(5 to 8 counts of threshold; 4 is internally added to PTHR to generate
the threshold)
2
(6 counts of threshold)
(Threshold Multiplier – THRM)
Multiplier
0, 1, 2, 3 (x1, x2, x4, x8)
0 (x1)
x1
x2
x4
x8
41) is performed by making the reference level track the
Table 5-1
Table 5-7 on page
shows the extended threshold range for
Extended Detect Threshold
57.
32 – 144
16 – 72
4 – 18
8 – 36
9621B–AT42–06/11