Decoding IR Remote Controls

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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 the carrier is demodulated in any AM radio receiver.

     + 
     | 
    _|_  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 | 

Some important addresses and commands:

Address:          Device:          Command: 
  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.

 ;  -==========--==========-=========- 
 ;         Interrupt Driven Receiving Routine for RC5 code 
 ; written by Juergen Putzger (juergen.putzger@physik.uni-regensburg.de) 
 ;  -==========--==========-=========- 
  
 $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 
 SUBAD:  DS      1       ; Device subaddress 
 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 
          JMP     RECEIVE 
  
 ;  -==========--==========-=========- 
 ;                            Output routines 
 ;        DonB4t 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. 
 ;  -==========--==========-=========- 
  
 RECEIVE: 
          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 
          ADD    A,R1 
          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 
          RLC    A             ;device address 
          XCH    A,R4 
          RLC    A 
          XCH    A,R4 
          RLC    A 
          CLR    CONTROL 
          JNB    ACC.5,TZ      ; Check control bit 
          SETB   CONTROL 
 TZ:      ANL    A,#00011111B  ; mask device address 
          MOV    SUBAD,A 
          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,SUBAD 
          CALL   BIN2BCD       ; Output subaddress 
          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 

Sources

  • Juergen Putzger

Category:InfraRed

 

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