BS2824 Holtek Semiconductor Inc., BS2824 Datasheet - Page 38

• Manufacturer Part Number: BS2824
• Description: Touch Key Mcu
• Manufacturer: Holtek Semiconductor Inc.

BS2824/BS2854/BS2868
Touch Key MCU
Rev. 1.00
Maximum Key On Duration Time
Auto-calibration Function
Sensitivity Adjustment
To minimise the possibility of unintentional switch detections, 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 device will
be reset to its power-on condition and initiate a new auto calibration. The output will then remain
inactive until the next switch detection.
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 detection 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 channel on the devices. By implementing this feature, changes in the
touch key environmental conditions are automatically catered for dynamically.
The sensitivity of the switch is a very important consideration 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
electrode to the device. Therefore the sensitivity will vary according to the actual PCB layout and
design. 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 following:
The purpose of the Cs capacitor is to allow for adjustment of touch key sensitivity and power noise
rejection. The optimal choice of sensitivity and power noise rejection will be obtained when the
Cref capacitor value - Cs
SCD Pin Touch Key Status Request
0~2
3~6
7~10
11~14
>15
Clock Pulses
38
SCD Commands
For test purposes
One-key wake-up
For test purposes
Any-Key wake-up
Always normal operation
Operating Mode
January 27, 2010