Theory Of Operation - Quantum E240 User Manual

THEORY OF OPERATION

3
The E240 scans the keys using bursts of charge-transfer pulses. Immediately after power up, the
signals from these bursts are used to calibrate the baseline reference values of the keys. This
makes the device highly adaptive to mechanical differences (i.e. varying panel thicknesses). The
QT240 chip also performs drift compensation on a continuous basis, so that the signals are
constantly being adjusted at a slow rate to accommodate drift.
The calibration process (done at power-up) should be performed in a 'not disturbed'
neutral condition, i.e. all fingers away from the keys and with the board situated in the
location where it will be used. Changes in location can alter the amount of signal coupling
from the board back to the local environment and thereby change the sensitivity of the
keys. If the calibrations are in error, just leaving the board alone for up to 30 seconds will
allow the drift calibration process to settle the keys into the correct calibration values.
The QT240 features spread-spectrum pulse modulation which heavily suppresses external noise
effects, while also suppressing radiated emissions. This powerful feature provides for highly
reliable, compliant operation under a wide variety of operating conditions.
The QT240 chip also uses a 'Detect Integrator' to protect against false detection reporting by
verifying that a key is reporting touch for 6 bursts in a row; if the device fails to detect a touch
during any of these 6 signal samples the chip will refuse to report a key touch on an Out pin.
Combined with spread-spectrum operation, the detect integration process provides a powerful
mechanism to suppress false detections from external noise sources.
If a key is touched for longer than 10 seconds, the key is flagged as a 'stuck key' and the QT240
recalibrates it so that the corresponding LED will go out, and the key will operate again normally
while suppressing the object (ie, a touch) that caused the prolonged detection. Only the 'stuck
key' is recalibrated, as all electrodes operate independently. The 10s timeout can be changed to
60s or infinite with jumpers; see page 10 for details.
Downloaded from Elcodis.com electronic components distributor