BS818A-2 Serial Interface Application Description

BS818A-2 Serial Interface Application Description
BS818A-2 Serial Interface Application Description
D/N: HA0348E
Introduction
This text shows how the BS818A-2 serial interface is used.
Description
Transfer Format (Data & Clock)
Data Transfer Timing
Data Format
Start bit：When a key changes state, the Data pin outputs a low level to wake up the
master, which can then read the key status.
Bit 0 : Key1 sate – "0" = touched, "1" = not touched
Bit 1 : Key2 sate – "0" = touched, "1" = not touched
Bit 2 : Key3 sate – "0" = touched, "1" = not touched
Bit 3 : Key4 sate – "0" = touched, "1" = not touched
Bit 4 : Key5 sate – "0" = touched, "1" = not touched
Bit 5 : Key6 sate – "0" = touched, "1" = not touched
Bit 6 : Key7 sate – "0" = touched, "1" = not touched
Bit 7 : Key8 sate – "0" = touched, "1" = not touched
Bit 11~8 : Checksum – the total number of "0" is used to indicate how many keys have
been touched.
Bit 15~12 : Stop bits, always "1010B".
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BS818A-2 Serial Interface Application Description
Bit
Function
15 ~ 12
11 ~ 8
7~0
Stop Bit Check Sum
1010
1000
1010
0111
Descriptions
Key4 ~ Key1 State
0: Touched / 1: Nnot Touched
00000000
Check Sum = 1000, 8 keys are touched.
00000001, 00000010, 00000100, 00001000
Check Sum = 0111, 7 keys are touched.
00010000, 00100000, 01000000 or 10000000
00000011, 00000110, 00011000, 00110000
1010
0110
Check Sum = 0110, 6 keys are touched.
11000000 or 10000001 …
00000111, 00001110, 11100000, 10000011,
1010
0101
Check Sum = 0101, 5 keys is touched.
10000110, 10001100 or 10011000…
00001111, 00011110, 00111100, 01111000,
Data Stream
1010
0100
Check Sum = 0100, 4 keys is touched.
11110000 or 10000111 …
00011111, 00111110, 01111100, 11111000
1010
0011
Check Sum = 0011, 3 keys are touched.
100011111 or 110001111…
00111111, 01111110, 11111100,
1010
0010
Check Sum = 0010, 2 keys are touched.
01111110, 01111101or 01111011…
11111110, 11111101, 11111011, 11110111
1010
0001
Check Sum = 0001, 1 key is touched.
11101111, 11011111, 10111111 or 01111111
1010
0000
11111111
Check Sum = 0000, No key is touched.
Serial Data Stream Format for 8-key
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BS818A-2 Serial Interface Application Description
Program Flowchart
START
Initial
Clock Pin & Data Pin as Input
Set Clock Pin Low
Delay 30us
Set Clock Pin as Input
Read Data Pin State
Delay 30us
N
Get 16bits ?
Y
Error Function
N
Stop bit & Check
Sum Correct ?
Y
END
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BS818A-2 Serial Interface Application Description
Program Example
SET Clock_C
SET Data_C
CLR Receive_BufferH
CLR Receive_BufferL
MOV A,16
MOV Receive_Count, A
Receive_Loop:
CLR Clock
CLR Clock_C
MOV A,20
SDZ Acc
JMP $-1
;----Read bit
SET Clock_C
SET C
SNZ Data
CLR C
RRC Receive_BufferH
RRC Receive_BufferL
MOV A,20
SDZ Acc
JMP $-1
SDZ Receive_Count
JMP Receive_loop
;set Clock pin as input
;set Data pin as input
;clear the Data Receive Buffer
; receive 16 bits of data
;Clock = low
;-----Delay
;delay 30us
;Clock = high
;read data through the data pin
;
;
;rotate data to the Receive Buffer
;rotate data to the Receive Buffer
;-----Delay
;delay 30us
;-----Count down Receive_Count
;Receive 16 bits of data?
;------Verify Data
MOV
XOR
AND
SNZ
JMP
A,Receive_BufferH
A,10100000B
A,11110000B
Z
Receive_error
MOV A,Receive_bufferL
MOV Data_Buffer,A
CLR Key_pressed_Amount
MOV
RRC
SNZ
INC
SDZ
JMP
A,8
Data_Buffer
C
Key_Pressed_Amount
ACC
$-4
MOV A Receive_buffer
XOR A,Key_Press_Amount
AND A,00001111B
SZ Z
JMP Receive_error
RET
Receive_error:
;check stop bit = 1010 ?
;--;
;
;
;----;
;total number of touched keys
;
;
;
;
;---- -Verify Check Sum
;
;data correct
;--do error function
RET
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BS818A-2 Serial Interface Application Description
Additional Information
 Both the BS818A-2 Data pin and Clock pin have an integrated pull high resistor.
 When the touch key device detects that a touch key has been pressed, it will output a
low level on the Data pin which can be used to wake up the master MCU.
 The touch key status data can be read by using a polling method or waiting for the Data
pin to change to a low level.
 When a low clock signal is received, the key status data is prepared by the touch key
device. When the clock signal changes to a high level, the master reads the touch key
status data from the Data pin. Every 8 clock periods form a data group.
 After reading a data group, it is suggested to use the checksum bits to determine the
data integrity.
If there are any errors in reading the data, it is necessary to wait for about 6ms and
then read the data again.
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