BS801B/02B/04B/06B/08B
lows three functions to be implemented, two are initiated
by the external MCU and one by the BS80xB.
SCD Command
Wake-up or Interrupt
MCU
Read Touch Key
Status
Send command to
BS804B/BS808B
Direction
BS804B/BS808B
®
MCU
Step1: MCU send command
MCU
®
BS804B/BS808B
Step2: MCU read key status
BS804B/BS808B
®
MCU
MCU
®
BS804B/BS808B
read the key status. After this time has elapsed the de-
vice will then transmit the data bits, D0~Dn.
Device
BS804B
BS808B
Time
22 t
SCD
38 t
SCD
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
Key0
Key1
:
:
Keyn
Data Bit
D0
D1
:
:
Dn
1: Key Off
0: Key On
Dn Value
SCD Function Summary
·
BS804B/BS808B Wakes-up or Interrupts MCU
When any of the touch keys change state, a pulse will
be generated by the BS804B/BS808B device which
can be used to wake up or to interrupt an external
MCU. The MCU must ensure that its pin connected to
the SCD pin is setup as an input to receive this pulse.
The pulse width generated by the device is equal to a
t
SCD
/2 period where t
SCD
is about 76ms.
t
S
O /P
C D
/2
IN T
o r
I/O
·
MCU sends commands to BS804B/BS808B device
B S 8 0 4 B
B S 8 0 8 B
S C D
I/P
M C U
Wake-up/Interrupt Pulse to MCU
·
MCU reads the touch key status
The MCU can send a command to the BS804B/
BS808B device to request the touch key status. The
protocol for reading the touch key status is as follows:
¨
The MCU can send commands along the SCD line to
setup the device in different Operating Modes. 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 t
SCD
cycles. This action will tem-
porarily disable the device and enable it to receive the
command from the MCU. Any command sent to the
device from the MCU will override the original power
on configuration setup.
N u m b e r o f p u ls e s
w ith in 4 t
S C D
s e ts o p e r a tin g m o d e
S C D L in e
fro m M C U
1
T A
N o te : T A > 8 t
S
C D
The external MCU takes control of the SCD line and
first sends three rising edge pulses or more to the
BS804B/BS808B within a time of about 152ms.
The MCU then changes its I/O pin to an input.
The BS804B/BS808B 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.
The BS804B/BS808B device will then transmit its
key status, given by D0~Dn
-
-
2
n
4 t
S
P e r io d s
C D
¨
¨
SCD Device Setup
The desired Operating Mode to be setup depends
upon the number of pulses received by BS804B/
BS808B within a time period, 4 t
SCD
, 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.
Clock Pulses
0~2
3~6
7~10
11~14
>15
Operating Mode
Set by external pin
One-key wake-up
For test purposes
Any-Key wake-up
Always normal operation
BS804B and BS808B SCD Commands
¨
For the BS804B, the data format is D0~D3.
For the BS808B, the data format is D0~D7.
¨
The MCU can then read the key status taking care
to read the data in the centre of the transmitted data
pulse.
Af t e r t he l as t dat a b i t i s tr a n s f e r r e d th e
BS804B/BS808B device will return its SCD pin to
an input 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.
A certain time is required for the MCU to transmit three
or more pulses to inform the device that it desires to
Rev. 1.00
7
July 7, 2009
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
