2 Device Operation
2.1 Start-up Time
After a reset or power-up event, the device requires 350ms
to initialize, calibrate, and start operating normally. Keys will
work properly once all keys have been calibrated after reset.
2.5 DETECT Pin
DETECT represents the functional logical-OR of all eight
keys. DETECT can be used to wake up a battery-operated
product upon human touch.
DETECT is also required to indicate to a host when the
binary coded output pins (in that mode) are showing an
active key. While DETECT is active, the binary coded
outputs should be read at least twice along with DETECT to
make sure that the code was not transition ing between
states, to prevent a false reading.
The output polarity and drive of DETECT are governed
according to Table 1.4.
2.2 Option Resistors
The option resistors are read on power-up only. There are
two primary option mode configurations: full, and simplified.
In full options mode, eight 1M✡ option resistors are required
as shown in Figure 1.1. All eight resistors are mandatory.
To obtain simplified mode, a 1M✡ resistor should be
connected from SNS6K to SNS7. In simplified mode, only
one additional 1M✡ option resistor is required for the AKS
feature (Figure 1.2).
Note that the presence and connection of option resistors
will affect the required values of Cs; this effect will be
especially noticeable if the Cs values are under 22nF. Cs
values should be adjusted for optimal sensitivity after the
option resistors are connected.
2.6 SYNC/LP Pin
The SYNC / LP pin function is configured according to the
SL_0 and SL_1 resistor connections to either Vdd or Vss,
according to Table 1.5.
Sync mode:
Sync allows the designer to synchronize
acquire bursts to an external signal source, such as mains
frequency (50/60 Hz) to suppress interference. It can also be
used to synchronize two QT parts which operate near each
other, so that they will not cross-interfere if two or more of
the keys (or associated wiring) of the two parts are near
each other.
The SYNC input of the QT1080 is positive pulse triggered. If
the SYNC input does not change, the device will free-run at
its own rate after ~150ms.
A trigger pulse on SYNC will cause the device to fire two
acquire bursts in A-B sequence:
Burst A: Keys 0, 1, 4, 5
Burst B: Keys 2, 3, 6, 7
Low Power LP Mode:
This allows the device to enter a Low
Power mode with very low power consumption, in one of
three response time settings : 110ms, 200ms, and 360ms
nominal.
LP mode is entered by a positive >150µs trigger pulse on the
SYNC/LP pin. Once the LP pulse is detected , the device will
enter and remain in this microamp mode until it senses and
confirms a touch. Then it will switch back to normal (full
speed) mode on its own, with a response time of 30ms
typical (burst length dependent). The device will go back to
LP mode again if SYNC/LP is held high, or after another LP
pulse is received.
The response time setting is determined by option resistors
SL_1 and SL_0; see Table 1.5. Slower response times result
in lower power drain.
The SYNC/LP pulse should be >150µs in duration.
If the SYNC/LP pin is held high permanently, the device will
go into normal mode during a key touch, and return to
low-current mode when the detection ceases.
If the SYNC/LP pin is held low constantly, the device will
remain in normal mode (25ms typical response time)
continuously.
2.3 OUT Pins - Direct Mode
Direct output mode is selected via option resistors, as shown
in Table 1.4.
In this mode, there is one output for each key; each is active
when a touch is confirmed on the corresponding electrode.
Unused OUT pins should be left open.
If AKS is off, it is possible for all OUT pins to be active at the
same time.
Circuit of Figure 1.1:
OUT polarity and drive are governed
by the resistor connections to Vdd or Vss according to
Table 1.4. The drive can be either push-pull or open-drain,
active low or high.
Circuit of Figure 1.2:
In this simplified circuit, the OUT pins
are active high, push-pull only.
2.4 OUT Pins - Binary Coded Mode
Binary code mode is selected via option resistors, as shown
in Table 1.4.
In this mode, a key detection is registered as a binary code
on pins OUT_2, OUT_1 and OUT_0, with possible values
from 000 to 111. In practice, four lines are required to read
the code, unless key 0 is not implemented ; the output code
000 can mean either ‘nothing detecting’ or ‘key 0 is
detecting’. The fourth required line (if all eight keys are
implemented) is the DETECT signal, which is active-high
when any key is active.
The first key touched always wins and shows its output.
Keys that come afterwards are hidden until the currently
reported key has stopped detecting, in which case the code
will change to the latent key.
This mode is useful to reduce the number of connections to
a host controller, at the expense of being able to only report
one active key at a time. Note that in global AKS mode
(Section 2.7), only one key can report active at a time
anyway.
Circuit of Figure 1.1:
OUT polarity and drive can only be
push-pull and active high.
Circuit of Figure 1.2:
Binary coded not available.
lQ
7
QT1080 R11.06/0806
QUANTUM [ QUANTUM RESEARCH GROUP ]
