Programming IR Remote Controls
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.
__ __ __ __
| | | | | |
|__| |_____| |__|
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.
_|_ ____| \_____| |__ __|\|___ ____ 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
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
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 (firstname.lastname@example.org)
INPUT EQU P3.2 ; Port3,Bit2 is used as input. The demodulated
; 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
ORG 0003H ; External Interrupt0
; Output routines
; Don$B4t(B forget to set up the serial port and Baud rate !
N_OUT: ADD A,#30H ;Convert BCD number to ASCII
C_OUT: JNB TI,$ ;Wait until transmission completed.
CLR TI ;Clear interrupt flag.
MOV SBUF,A ;Write out character to serial port.
BIN2BCD: ;Convert 8 bit value in Acc to 3 digit BCD
; 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
REC: MOV A,#0
REC0: INC A ; Measure duration of low-level
NOP ; Delay
JZ TIMEOUT ; End of transmission if duration exeeds
REC1: INC A ; Measure duration of high-level
NOP ; Delay
JZ TIMEOUT ; End of transmission
MOV A,BUFFER ; calculate threshold between short and
INC R0 ; length of first low-pulse
ADD A,BUFFER+1 ; plus length of first high-pulse
RRC A ; divided by two
RRC A ; plus half of the time
MOV R5,A ; yields threshold
MOV R1,#1 ; initial value
MOV R2,#13 ; Number of bits to decode
DECODE: MOV A,@R0
SUBB A,R5 ; compare length with threshold
CPL C ; short=1
INC R0 ; if short skip over next pulse
NOSKIP: XRL A,R1 ; new bit is calculated by XOR with
MOV R1,A ; Store new bit
MOV A,R3 ; Store new Bit in R3/R4 by rotating
ANL A,#00111111B ; extract command from R3
RLC A ; do some rotating to extract
RLC A ;device address
JNB ACC.5,TZ ; Check control bit
TZ: ANL A,#00011111B ; mask device address
POP ACC ; Restore old registerset
SETB NEW ; Set flag to indicate the new command
; 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
SETB EX0 ; Enable external Interrupt0
CLR IT0 ; triggered by a high to low transition
LOOP: JNB NEW,LOOP ; Wait until a command has been received
CALL C_OUT ; Ouput carriage return and linefeed
CALL BIN2BCD ; Output control Bit
CALL BIN2BCD ; Output subaddress
CALL BIN2BCD ; Output command
SUBB A,#0CH ; compare for standby command
JNZ LOOP ; go on receiving
CLR EX0 ; stop receiving
CLR EA ; and
JMP 8000H ; return to monitor which has its entry
; point at 8000H
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