QT1100A-ISG View Datasheet(PDF) - Quantum Research Group
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QT1100A-ISG Datasheet PDF : 42 Pages
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4 Setup Block Functions
The Setups block controls internal operation including critical
functions such as sensitivity, filtering, sample rate, and
communications parameters. These functions are
summarized in Table 4-1, page 31.
4.1 NTHR - Negative Threshold Bits
Bytes 0 - 9, Bits 7..4
Default value:
9
Typical values:
See text
Disabling key(s):
0
See also Table 4-2, page 32.
The internal signal levels decrease when a key is touched.
This phenomenon is related to the charge-transfer
acquisition conversion mechanism used by the device.
Internally the device employs a 16-bit digital reference value
for each channel. This reference is determined during the
calibration process. After calibration, the reference is either
locked or can only move very slowly in response to
slow-moving changes in background levels of signal.
Against this reference, the actual signal can move very fast
in response to touch, but when it does so the internal
numerical signal value drops below the reference value.
The negative threshold (NTHR parameter) sets the device
sensitivity by controlling the distance that the signal has to
travel before creating a detection.
Each channel has its own NTHR setting; these are set in the
upper nibbles of Setup bytes 0..9. NTHR can control the
threshold in one of two ways, via the NTM bit contained in
byte 32 in the Setups block:
NTM bit = 0: When NTM is clear, NTHR key settings create
thresholds based on user-defined offsets from the
reference levels. The offsets are based on the setting of
NTHR, plus 5 counts. Thus the available threshold range
is from 5 to 20. If NTHR is 9, this will create a threshold
14 counts below the key’s reference level. Higher
numbers mean less sensitivity.
This method allows the sensitivity to be altered by
changing the value of Cs, as well as by changing the
value of NTHR. This allows a user to conveniently alter
key sensitivity without resort to an external EEPROM or
serial communications; the default key settings of N THR
mean that the Cs value can be altered proportionately to
increase sensitivity. Bigger Cs = higher gain.
NTM bit = 1: When NTM is set, the NTHR settings are
based on a percentage of the signal reference level. This
means that if the reference level doubles, the threshold
value also doubles. The reference value is directly
related to Cs and Cx. If Cs doubles but N THR is
determined as a ratio, the effect is that the device
sensitivity does not change at all. Due to the physics of
the acquisition process, increasing Cx will reduce
sensitivity even in this mode, just not as much as in the
NTM = 0 mode.
The NTHR value in this mode is set using a percen tage
calculation as defined in Table 4-2, page 32. If the setting
is set very sensitive but the burst length is short (due to a
small value of Cs and/or large value of Cx) then the
computed threshold may be too small to process reliably.
When this happens the sensitivity is limited internally to a
minimum value of 3 counts of signal.
Disabling of keys: To disable a key, set NTHR for that key
to 0; this will turn the bursts off for that key but will preserve
its timeslot, thus preserving all system timings which depend
on the burst spacing. If the system uses the external
EEPROM or UART or SPI communication, the NTHR
parameter must be used to disable a key. Using the SNS
pins to disable a key will result in an error report.
In stand-alone scanport mode with no EEPROM present,
keys must be disabled using default settings of SNS pins as
shown on page 4.
A key that is legally disabled cannot report an error.
See Section 2.16, page 14 for more information on error
reporting.
Typical values: For most touch applications where either an
EEPROM or a serial link is used, use NTM =1 and set
NTHR = 10 (1.37%) to begin. Each key needs to be tailored
due to inequalities in stray loading capacitance.
For most touch applications where ne ither an EEPROM nor
a serial link are used, the default setting is 9 + 5 = 14 counts
of signal change. Key sensitivity can be tailored for each key
individually by altering each Cs capacitor value.
4.2 NHYS - Negative Hysteresis Bits
Bytes 0 - 9, Bits 3..0
Default value:
3
Typical values:
3, 2
Hysteresis controls the level at which the detection process
ceases with respect to the threshold level NTHR. The
hysteresis is controlled by the lower 2 bits of the first 10
bytes of the Setups block.
The value is expressed as a percentage of the distance
measured from the threshold value back up towards the
reference. Thus given a scenario:
Signal reference = 732
NTHR = 12 counts
NHYS = 25%,
then the signal has to fall to 73 2 - 12 = 720 to cause a
detection. The signal has to then rise again to
720 + (12 * 0.25) = 723 for the detection to cease.
Each key can have its own hysteresis value.
Generally a low value of hysteresis (12.5%) is enough to
solve chatter-type problems. Excess hysteresis can cause
the sensor to ‘stick on’ especially if there are underlying
problems in wiring or the power supply.
Typical value: For most touch applications, use 12.5%.
4.3 NDCR / PDCR - Drift Comp Bits
NDCR: Bytes 10 - 19, Bits 7..4
PDCR: Byte 31, Bits 7..4
Default NDCR value:
7
Default PDCR value:
5
Typical values:
Tables 4-3 and4-4,
pages 32 and 33
Signals can drift because of changes in Cx and Cs over time
and temperature. It is crucial that such drift be compensated,
or false detections and sensitivity shifts can occur.
Drift compensation (Figure 4-1) is performed by making the
reference level track the 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
LQ
25
Copyright © 2003-2005 QRG Ltd
QT1100A-ISG R3.02/1105
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