at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 39

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

5. Setups
5.1
9621B–AT42–06/11
Negative Threshold – NTHR
The QT1481 calibrates and processes all signals using a number of algorithms specifically
designed to provide for high survivability in the face of adverse environmental challenges. It
provides a large number of processing options which can be user-selected to implement very
flexible, robust keypanel solutions.
User-defined Setups are employed to alter the algorithm to suit each application. The setups are
loaded into the QT1481 in a block load over one of the serial interfaces and stored in an onboard
EEPROM array. After a setups block load, the QT1481 should be reset to allow the new Setups
block to be shadowed in internal Flash ROM and to allow all the new parameters to take effect.
This reset can be either a hardware or software reset.
Refer to
Block length issues: The setups block is 350 bytes long (including the two CRC bytes) to
accommodate 48 keys. This can be a burden on smaller host controllers with limited memory. In
larger quantities the QT1481 can be procured with the setups block preprogrammed from Atmel.
If the application only requires a small number of keys (such as 16) then the setups table can be
compressed in the host by filling large stretches of the Setups area with nulls.
Many setups employ lookup-table (LUT) value translation. The Setups Block Summary on
page 57
Default values shown are factory defaults.
The negative threshold value is established relative to a key’s signal reference value. The
threshold is used to determine key touch when crossed by a negative-going signal swing after
having been filtered by the detection integrator. Larger absolute values of threshold desensitize
keys since the signal must travel farther in order to cross the threshold level. Conversely, lower
thresholds make keys more sensitive.
As Cx and Cs drift, the reference point drift-compensates for these changes at a user-settable
rate. The threshold level is recomputed whenever the reference point moves, and thus it is also
drift compensated.
The amount of NTHR required depends on the amount of signal swing that occurs when a key is
touched. Thicker panels or smaller key geometries reduce ‘key gain’ (signal swing from touch),
thus requiring smaller NTHR values to detect touch.
The negative threshold is programmed on a per-key basis using the Setup process. See
Table 5-7 on page 57
Typical values:
Default value:
Table 5-4 on page 54
shows all translation values.
and also
3 to 8
(7 to 12 counts of threshold; 4 is internally added to NTHR to generate
the threshold).
6
(10 counts of threshold)
for a list of all Setups.
Section 5.2 on page 40
(Threshold Multiplier – THRM)
AT42QT1481
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