BS801C/02C/04C/06C/08C
·
MCU reads the touch key status
command from the MCU. Any command sent to the
device from the MCU will override the original power
on configuration setup.
The MCU can send a command to the BS804C/
BS808C device to request the touch key status. The
protocol for reading the touch key status is as follows:
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The external MCU takes control of the SCD line and
first sends three rising edge pulses or more to the
BS804C/BS808C within a time of about 152ms.
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The MCU then changes its I/O pin to an input.
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The BS804C/BS808C device will then pull the SCD
line low, then high, the timings are given by syn-
chronise period. This allows the MCU to synchro-
nise itself for the data transfer.
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SCD Device Setup
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The BS804C/BS808C device will then transmit its
key status, given by D0~Dn
The desired Operating Mode to be setup depends
upon the number of pulses received by BS804C/
BS808C within a time period, 4 tSCD, as shown in the
table. After the programming clock cycles have been
transmitted and the SCD line returns high, the device
will return to normal operation.
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For the BS804C, the data format is D0~D3.
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For the BS808C, the data format is D0~D7.
¨
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The MCU can then read the key status taking care
to read the data in the centre of the transmitted data
pulse.
Clock Pulses
0~2
Operating Mode
Set by external pin
After the last data bit is transferred the BS804C/
BS808C device will return its SCD pin to an input
state.
3~6
For test purposes
One-Key state
The timings associated with the above protocol can be
described in multiples of the SCD clock periods. The
SCD clock period is about about 76ms.
7~10
11~14
>15
For test purposes
Any-Key state
A certain time is required for the MCU to transmit three
or more pulses to inform the device that it desires to
read the key status. After this time has elapsed the de-
vice will then transmit the data bits, D0~Dn.
BS804C and BS808C SCD Commands
Note that the MCU must transmits these pulses within 4
tSCD
.
Device
BS804C
BS808C
Time
Maximum Key On Duration Time
22 tSCD
38 tSCD
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.
Key Status Read Total Time
Each bit of transmitted data corresponds directly to a
touch input key. Therefore the status of touch key Key0
is represented by D0, Key1 by D1 and so on. A ²low² bit
means that a touch has been detected, a ²high² bit
means no touch has been detected.
Pin
Data Bit
D0
Dn Value
Key0
Key1
Auto-calibration Function
D1
1: Key Off
0: Key On
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.
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Keyn
Dn
·
MCU sends commands to BS804C/BS808C device
The MCU can send commands along the SCD line to
setup the device in different Operating State. To do
this the externally connected MCU takes control of the
SCD line, first forcing it low and holding it low for a
time of greater than 8 tSCD cycles. This action will tem-
porarily disable the device and enable it to receive the
Rev. 1.00
7
December 2, 2009
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
