at42qt2100 ATMEL Corporation, at42qt2100 Datasheet - Page 16

• Manufacturer Part Number: at42qt2100
• Description: Qtouch Touch Sensor Ic
• Manufacturer: ATMEL Corporation

4.3.7
4.4
4.4.1
4.4.2
4.4.3
16
Operating Modes
AT42QT2100
Faulty and Unused Keys
Introduction
Free Run Mode
LP Mode
Any sense channel that does not have its sense capacitor (Cs) fitted is assumed to be either
faulty or unused. A sensor fault is detected by an out-of-range signal count during calibration,
where the minimum allowed signal is 32 counts and the maximum is 8192. This channel takes
no further part in operation unless a host-commanded recalibration operation shows it to have
an in-range burst count again.
This is important for sense channels that have an open or short circuit fault across Cs. Such
channels would otherwise cause very long acquire bursts, and in consequence would slow the
operation of the device. Note that acquisition pulses will still be generated on these channels,
but no measurements of their state will be carried out. The burst will finish when all the
enabled channels on the burst group have completed acquisition.
Four basic operating modes are possible: Free Run, Low Power (LP), Sleep and Sync. Sleep
is a special case of LP mode, where the sleep time is infinite. Sync is a special case of LP
mode where the timing of host communications is used as the timing basis for the QT2100
acquisition cycle. In the absence of host communications, the device operates in its most
power-efficient low power mode, LP Mode 4. See below for further details.
In this mode the device operates continuously with short intervals between burst groups; there
are three bursts, one burst for each electrode group. DRDY goes high for approximately
500 µs between bursts and stays high during signal processing.
In this mode, the acquisition bursts are unsynchronized, making this mode unsuitable if
synchronization to mains frequency is needed.
LP mode is designed to allow low power operation while still retaining full operation but at a
slower speed. This mode is useful for devices that must use the touch keys to wake up a
product, yet minimize power consumption.
Several LP timings allow the user to trade power versus response time: the slower the
response time, the lower the power consumed.
In LP mode, the device spends a portion of the time sleeping between bursts; it wakes
periodically to measure all channels with a set of three acquisition bursts, then goes back to
sleep.
If a touch is detected, the device operates as in Free Run mode and attempts to perform the
detect integrator noise filter (DI) function to completion; if the DI filter fails to confirm a
detection the device goes back to sleep and resumes LP mode. During the DI function the
LPS bit will be cleared.
If a key is found to be in detection the CHANGE pin will go high and the part will remain in Free
Run mode. To go back into LP mode the host has to request LP mode again with an SPI
communications exchange after the touch detection has been cleared by removal of touch or
recalibration.
9554CX–AT42–11/11