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 0Notice 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 1Instead 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 0The 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.
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 (juergen.putzger@physik.uni-regensburg.de)
; ---------==========----------==========---------=========---------
$MOD52
INPUT EQU P3.2 ; Port3,Bit2 is used as input. The demodulated
signal
; with active low level is connected to this pin
LF EQU 0AH ; Linefeed
CR EQU 0DH ; Carriage return
SPC EQU 20H ; Space
RB0 EQU 000H ; Select Register Bank 0
RB1 EQU 008H ; Select Register Bank 1 ...poke to PSW to use
DSEG ; This is internal data memory
ORG 20H ; Bit adressable memory
FLAGS: DS 1
CONTROL BIT FLAGS.0 ; toggles with every new keystroke
NEW BIT FLAGS.1 ; Bit set when a new command has been received
COMMAND: DS 1 ; Received command byte
SUBAD: DS 1 ; Device subaddress
BUFFER: DS 30 ; Buffer to store length of transmitted pulses
STACK: DS 1 ; Stack begins here
CSEG ; Code begins here
;---------==========----------==========---------=========---------
; PROCESSOR INTERRUPT AND RESET VECTORS
;---------==========----------==========---------=========---------
ORG 00H ; Reset
JMP MAIN
ORG 0003H ; External Interrupt0
JMP RECEIVE
; ---------==========----------==========---------=========---------
; Output routines
; Don岐 forget to set up the serial port and Baud rate !
; ---------==========----------==========---------=========---------
N_OUT: ADD A,#30H ;Convert BCD number to ASCII
C_OUT: JNB TI,$ ;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.
; ---------==========----------==========---------=========---------
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