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7
SCD Pin Touch Key Status Request
Note that the MCU must transmits these pulses within 4
tSCD
An external capacitor, Cs, connected to the Cref pin, is
used to set the overall sensitivity of all pins. Some of the
most important factors affecting sensitivity are the fol-
lowing:
.
Maximum Key On Duration Time
To minimise the possibility of unintentional switch detec-
tions, such as undesired objects covering the sense
electrodes, the devices includes a Maximum Key-On
duration time function. To implement this function the
devices include an internal timer, which starts running
after each switch detection. If the key on time of a touch
key exceeds a value of about 40 seconds, then the de-
vice will be reset to its power-on condition and initiate a
new auto calibration. The output will then remain inac-
tive until the next switch detection.
·
Cref capacitor value - Cs
The purpose of the Cs capacitor is to allow for adjust-
ment of touch key sensitivity and power noise rejec-
tion. The optimal choice of sensitivity and power noise
rejection will be obtained when the value of the Cs ca-
pacitor is equivalent to the touch pad capacitance,
however the value can still be changed to obtain the
required sensitivity value. Higher Cs values will result
in higher levels of sensitivity. Recommended values
for Cs are between 0pF and 10pF.
·
Touch key pad size
Larger touch key sizes will increase sensitivity and of
course vice-versa, small electrode sizes will decrease
sensitivity.
Auto-calibration Function
The devices include a full auto-calibration function
which will be initiated after the device is powered-on. In
addition to the power-on calibration, if no switch detec-
tion has been made for more than about 2.5 seconds
then a further calibration procedure will be carried out.
The calibration is applied independently to each chan-
nel on the devices. By implementing this feature,
changes in the touch key environmental conditions are
automatically catered for dynamically.
·
·
Touch key pad insulating panel thickness
A thinner panel will result in higher sensitivity and of
course thicker panels will result in a lower sensitivities.
Touch key pad insulating panel material
The choice of the dielectric material for the panel will in-
fluence the sensitivity. Materials with higher dielectric
constants will result in higher sensitivities and lower di-
electric constants will result in lower sensitivities.
Touch
Key
Sensitivity Adjustment
Key
The sensitivity of the switch is a very important consider-
ation in most applications whose requirements will vary
according to the user application. The user should
therefore be aware of the factors which will affect the
overall sensitivity of their touch key application. Factors
to take into consideration include the electrode size and
the capacitance of the connection lines from the elec-
trode to the BS80xB device. Therefore the sensitivity
will vary according to the actual PCB layout and design.
Cs
Cref
0pF~
10pF
Rev. 1.00
8
July 7, 2009
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
