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DECODING IR REMOTE CONTROLS

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```                       DECODING IR REMOTE CONTROLS
by Juergen Putzger

The origin of this posting was the question what to do with an old TV.
I suggested to use the infrared remote control as an input keyboard for a
microcontroller board and mentioned a piece of code I had written for
the 8052 microcontroller.  I was asked by some people to share my
information about remote controls, so here it is:

There are at least two international standards which are used by remote
controls to encode the commands, the RC5 and RECS 80 code. The RECS 80
code uses pulse length modulation. Each bit to be transmitted is encoded
by a high level of the duration T followed by a low level of duration 2T
representing a logical '0' or 3T representing a logical '1'.

T 2T T 3T  T 2T
_    _     _
| |  | |   | |
_| |__| |___| |__
0    1     0

Notice that a '1' takes more time to be transmitted than a '0'. The RC 5 code
instead has a uniform duration of all bits. A transition in the middle of the
time interval assigned to each bit encodes the logical value. A '0' is encoded
by a high to low transition and a '1' by a low to high transition. Therefore
we need additional transitions at the beginning of each bit to set the proper
start level if a series of equal bits is sent. We don't need this additional
transition if the next bit has a different value. This is also called a
'biphase' code.

|1.Bit|2.Bit|3.Bit|4.Bit|
__    __       __    __
|  |  |     |  |  |
|__|  |_____|  |__|
0     0     1     1

Instead of being fed direct into the IR emitter, most remote controls modulate
a 20-30 kHz carrier with this signal. A logic one is represented by a burst of
oscillations.

______/\/\/\/\_______/\/\/\/\________
0      1       0       1      0

The reason is, that you can use a filter tuned to the carrier frequency
to distinguish the signal from noise in the ambient light. Fluorescent lamps
are the main source of such noise. Photodiodes behind  an optical filter
which transmits infrared light but blocks visible light are used as detectors.
The signal from the photodiode is fed through a filter tuned to the carrier
fequency and then amplified. The amplified signal is demodulated just like

+
|
_|_  photodiode
/_\                 demodulator
|      |\
_|_ ____| \_____| |__ __|\|___ ____            L and C form a
|   |    | /     | |  |  |/|   |    signal      circuit resonant
|   /    |/          _|_       |    out         to the carrier
===  \    amplifier   /_\      ===
|C  / L               |        |
|___|_________________|________|____

It can be a lot of pain to design a sensitive receiver that does'nt
start to oscillate. It is also necessary to have some automatic gain
control to avoid overload of the amplifier at close distance to the
emitter. It is easier to use some integrated circuit that does
all of the job. The best i have ever seen (and used) is the SFH505A
manufactured by SIEMENS (no, I don't work for this company). It looks
like one of this three legged voltage regulators and uses a single 5V
supply. It incorporates an optical filter, the photodiode, a filter
tuned to about 30 kHz , the amplifier with automatic gain control and
the demodulator.

If you don't know which code your remote control is transmitting you can
identify it by viewing the output of your receiver with an oscilloscope.
The RECS 80 code uses high pulses of uniform length while the low pulses
differ in length. If there are high and low pulses of two different lengths
it might be RC5 code. Note that your receiver may invert the levels.

How are commands like volume control or channel selction encoded? In the
case of the RC5 code there is an international standard. Every command is
encoded by 14 bits. The first two bits S are startbits to allow the receiver
to adjust the automatic gain control and to synchronize. Next a bit T
follows, that toggles with every new keystroke. Next is the address A of
the  device which shall respond to the command. At last the command itself
follows.

| S | S | T | A4 | A3 | A2 | A1 | A0 | C5 | C4 | C3 | C2 | C1 | C0 |

0               TV1              0...9    Numbers 0...9 (channel select)
1               TV2              12       Standby
5               VCR1             16       Master Volume +
6               VCR2             17       Master Volume -
17               Tuner            18       Brightness +
18               Audio Tape       19       Brightness -
20               CD Player        50       Fast rewind
52       Fast run forward
53       Play
54       Stop
55       Recording

There are integrated decoder circuits which have inputs to select the device
address and parallel outputs activated by the commands. Since this is comp.
robotics the devices you wish to control will have a microcontroller on board
which can do all the decoding. Here is an input routine I have written for
the 8052 microcontroller family to receive RC5 codes. My cousin has written
a similar routine for the RECS80 code which i will try to make available also.
Perhaps we can start a collection of such routines and archive them somewhere.

Juergen Putzger (still looking for that public domain 8052 C-compiler....)

------------------------ source text begins here -------------------------

;  ---------==========----------==========---------=========---------
;         Interrupt Driven Receiving Routine for RC5 code
; written by Juergen Putzger ([email protected])
;  ---------==========----------==========---------=========---------

\$MOD52

INPUT   EQU     P3.2    ; Port3,Bit2 is used as input. The demodulated signal
; with active low level is connected to this pin
LF      EQU     0AH     ; Linefeed
CR      EQU     0DH     ; Carriage return
SPC     EQU     20H     ; Space
RB0     EQU     000H    ; Select Register Bank 0
RB1     EQU     008H    ; Select Register Bank 1  ...poke to PSW to use

DSEG            ; This is internal data memory
ORG     20H     ; Bit adressable memory

FLAGS:  DS      1
CONTROL BIT     FLAGS.0  ; toggles with every new keystroke
NEW     BIT     FLAGS.1  ; Bit set when a new command has been received

COMMAND: DS     1       ; Received command byte
BUFFER: DS      30      ; Buffer to store length of transmitted pulses
STACK:  DS      1       ; Stack begins here

CSEG            ; Code begins here

;---------==========----------==========---------=========---------
;              PROCESSOR INTERRUPT AND RESET VECTORS
;---------==========----------==========---------=========---------

ORG     00H    ; Reset
JMP     MAIN

ORG     0003H  ; External Interrupt0

;  ---------==========----------==========---------=========---------
;                            Output routines
;        Don岐 forget to set up the serial port and Baud rate !
;  ---------==========----------==========---------=========---------

N_OUT:  ADD     A,#30H  ;Convert BCD number to ASCII
C_OUT:  JNB     TI,\$    ;Wait until transmission completed.
CLR     TI      ;Clear interrupt flag.
MOV     SBUF,A  ;Write out character to serial port.
RET

BIN2BCD:                ;Convert 8 bit value in Acc to 3 digit BCD
MOV     B,#100
DIV     AB
CALL    N_OUT
XCH     A,B
MOV     B,#10
DIV     AB
CALL    N_OUT
XCH     A,B
CALL    N_OUT
RET

;  ---------==========----------==========---------=========---------
;  Interrupt routine is entered by the first high to low transition
;  at Port3-Bit2. Stores the length of all pulses occuring at this
;  pin in buffer. Analyzes the timing of the startbits to calculate
;  a threshold between short and long pulses. This routine is
;  independent of CPU speed. The device address and command are
;  extracted from the bit stream. Two flags are set upon exit,
;  the control bit which toggles with every new keystroke and the
;  NEW bit indicating that a new command has been received.
;  ---------==========----------==========---------=========---------

PUSH   PSW           ; save current registerset
MOV    PSW,#RB1
PUSH   ACC
MOV    R0,#BUFFER
REC:     MOV    A,#0
REC0:    INC    A             ; Measure duration of low-level
NOP
NOP                  ; Delay
NOP
NOP
JZ     TIMEOUT       ; End of transmission if duration exeeds 256 counts
JNB    INPUT,REC0
MOV    @R0,A
INC    R0
MOV    A,#0
REC1:    INC    A             ; Measure duration of high-level
NOP
NOP                  ; Delay
NOP
NOP
JZ     TIMEOUT       ; End of transmission
JB     INPUT,REC1
MOV    @R0,A
INC    R0
JMP    REC
TIMEOUT:
MOV    A,BUFFER      ; calculate threshold between short and long pulses
INC    R0            ; length of first low-pulse
ADD    A,BUFFER+1    ; plus length of first high-pulse
CLR    C
RRC    A             ; divided by two
MOV    R1,A
CLR    C
RRC    A             ; plus half of the time
MOV    R5,A          ; yields threshold
MOV    R0,#BUFFER
MOV    R1,#1         ; initial value
MOV    R2,#13        ; Number of bits to decode
DECODE:  MOV    A,@R0
INC    R0
CLR    C
SUBB   A,R5          ; compare length with threshold
MOV    A,#0
CPL    C             ; short=1
RLC    A
JNZ    NOSKIP
INC    R0            ; if short skip over next pulse
NOSKIP:  XRL    A,R1          ; new bit is calculated by XOR with previous bit
MOV    R1,A          ; Store new bit
RRC    A
MOV    A,R3          ; Store new Bit in R3/R4 by rotating
RLC    A
MOV    R3,A
MOV    A,R4
RLC    A
MOV    R4,A
DJNZ   R2,DECODE
MOV    A,R3
ANL    A,#00111111B  ; extract command from R3
MOV    COMMAND,A
MOV    A,R3
RLC    A   	      ; do some rotating to extract
XCH    A,R4
XCH    A,R4
RLC    A
XCH    A,R4
RLC    A
CLR    CONTROL
JNB    ACC.5,TZ      ; Check control bit
SETB   CONTROL
POP    ACC           ; Restore old registerset
POP    PSW
SETB   NEW           ; Set flag to indicate the new command
RETI

;  ---------==========----------==========---------=========---------
;  Main routine. Program execution starts here. Don't forget to add
;  code to initialize the serial port and Baud rate if your monitor
;  program doesn't do that for you. The Main loop waits until a command
;  has been received. Then the control bit, subaddress and command byte
;  are printed separated by spaces. Leading zeroes are not suppressed.
;  When a standby command (12) has been received, the main loop is
;  terminated and the program returns to the monitor.
;  ---------==========----------==========---------=========---------

MAIN:    MOV    TCON,#00H     ; MAKE SURE TIMERS ARE SHUT DOWN.
MOV    PSW,#RB0      ; Select register bank 0
MOV    SP,STACK
SETB   EX0           ; Enable external Interrupt0
CLR    IT0           ; triggered by a high to low transition
SETB   EA
CLR    NEW
LOOP:    JNB    NEW,LOOP      ; Wait until a command has been received
MOV    A,#CR
CALL   C_OUT         ; Ouput carriage return and linefeed
MOV    A,#LF
CALL   C_OUT
MOV    A,FLAGS
ANL    A,#00000001B
CALL   BIN2BCD       ; Output control Bit
MOV    A,#SPC
CALL   C_OUT
MOV    A,#SPC
CALL   C_OUT
MOV    A,COMMAND
CALL   BIN2BCD       ; Output command
MOV    A,COMMAND
CLR    C
SUBB   A,#0CH        ; compare for standby command
CLR    NEW
JNZ    LOOP          ; go on receiving
CLR    EX0           ; stop receiving
CLR    EA            ; and
JMP    8000H         ; return to monitor which has its entry point at 8000H

END

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