AVR410 RC5 IR Remote Control Receiver.pdf

AVR410: RC5 IR Remote Control Receiver

8-bit

Microcontroller

Features

• Low-cost

• Compact Design, Only One External Component

• Requires Only One Controller Pin, Any AVR Device Can be Used

• Size-efficient Code

Application

Note

Introduction

Most audio and video systems are equipped with an infrared remote control. This

application note describes a receiver for the frequently used Philips/Sony RC5 coding

scheme.

Figure 1. RC5 Receiver

DATA

The RC5 code is a 14-bit word bi-phase coded signal (See Figure 2). The two first bits

are start bits, always having the value one. The next bit is a control bit or toggle bit,

which is inverted every time a button is pressed on the remote control transmitter. Five

system bits hold the system address so that only the right system responds to the

code. Usually, TV sets have the system address 0, VCRs the address 5 and so on.

The command sequence is six bits long, allowing up to 64 different commands per

address.

The bits are transmitted in bi-phase code (also known as Manchester code) as shown

in Figure 3. An example where the command 0x35 is sent to system 5 is shown in Fig-

ure 4.

Rev. 1473B–AVR–05/02

Figure 2. RC5 Frame Format

Figure 3. Bi-phase Coding

Figure 4. Example of Transmission

Timing

The bit length is approximately 1.8 ms. The code is repeated every 114 ms. To improve

noise rejection, the pulses are modulated at 36 kHz. The easiest way to receive these

pulses is to use an integrated IR-receiver/demodulator like the Siemens SFH 506-36.

This is a 3-pin device that receives the infra-red burst and gives out the demodulated bit

stream at the output pin. Note that the data is inverted compared to the transmitted data

(i.e., the data is idle high).

The Software

The assembly code found in AVR410.ASM contains the RC5 decode routine. In addi-

tion, it contains an example program which initializes the resources, decodes the RC5

data and outputs the received command on port B.

The Detect

Subroutine

When the detect subroutine is called, it first waits for the data line to be idle high for

more than 3.5 ms. Then, a start bit can be detected. The length of the low part of the first

start bit is measured. If no start bit is detected within 131 ms, or if the low pulse is longer

than 1.1 ms, the routine returns indicating no command received.

Figure 5. Synchronizing and Sampling of the Data

The measurement of the start bit is used to calculate two reference times, ref1 and ref2,

which are used when sampling the data line. The program uses the edge in the middle

of every bit to synchronize the timing. 3/4 bit length after this edge, the line is sampled.

This is in the middle of the first half of the next bit (see Figure 5). The state is stored and

the routine waits for the middle edge. Then, the timer is synchronized again and every-

thing is repeated for the following bits. If the synchronizing edge is not detected within

5/4 bit times from the previous synchronizing edge, this is detected as a fault and the

routine terminates.

When all the bits are received, the command and system address are stored in the

“command” and “system” registers. The control bit is stored in bit 6 of “command”.

AVR410

1473B–AVR–05/02

AVR410

Ta ble 1. “Decode” Subroutine Performance Figures

Parameter

Value

Code Size

72 words

Execution Cycles

Low Registers Used: 3

High Registers Used: 6

Global Registers: 6

Pointers Used: None

Ta ble 2. “Detect” Register Usage

Internal

Output

“inttemp”–Used by TIM0_OVF

“ref1”–Holds Timing Information

“ref2”–Holds Timing Information

R16

“temp”–Temporary Register

R17

“timerL”–Timing Register

R18

“timerH”–Timing Register

R19

“system”– The System Address

R20

“command”–The Received Command

R21

“bitcnt”–Counts the Bits Received

Timer/Counter0

Overflow Interrupt

Handler

The function of the timer interrupt is to generate a clock base for the timing required.

The routine increments the “timerL” Register every 64 µs, and the “timerH” every

16,384 ms.

Ta ble 3. “TIM0_OVF” Interrupt Handler Performance Figures

Parameter

Value

Code Size

7 words

Execution Cycles

6 + reti

Low Registers Used: 2

High Registers Used: 2

Global Registers: 0

Pointers Used: None

Ta ble 4. “TIM0_OVF” Register Usage

Internal

Output

“S”–Temporary Storage of Sreg

“inttemp”–Used by TIM0_OVF

R17

“timerL”–Incremented every 64 µs

R18

“timerH”–Incremented every 16,384 ms

1473B–AVR–05/02

Example Program

The example program initializes the ports, sets up the timer and enables interrupts.

Then, the program enters an eternal loop, calling the detect routine. If the system

address is correct, the command is output on port B.

Ta ble 5. Overall Performance Figures

Parameter

Value

Code Size

79 words –“detect” and “TIM0_OVF

96 words – Complete Application Note

Low Registers: 4

High Registers: 6

Pointers: None

Interrupt Usage

Timer/Counter 0 Interrupt

Peripheral Usage

Timer/Counter

Port D, pin 2

Port B (example program only)

AVR410

1473B–AVR–05/02

AVR410

;***************************************************************************

;*APPLICATION NOTE FOR THE AVR FAMILY

;* Number

: AVR410

;* File Name

:"rc5.asm"

;* Title

:RC5 IR Remote Control Decoder

;* Date

:97.08.15

;* Version

:1.0

;* Support telephone

:+47 72 88 43 88 (ATMEL Norway)

;* Support fax

:+47 72 88 43 99 (ATMEL Norway)

;* Target MCU

:AT90S1200

;* DESCRIPTION

;* This Application note describes how to decode the frequently used

;* RC5 IR remote control protocol.

;* The timing is adapted for 4 MHz crystal

;***************************************************************************

.include "1200def.inc"

.device AT90S1200

.equ

INPUT

;PD2

.equ

SYS_ADDR =0

;The system address

.def

=R0

; Storage for the Status Register

.def

inttemp

=R1

; Temporary variable for ISR

.def

ref1

=R2

.def

ref2

=R3

; Reference for timing

.def

temp

=R16

; Temporary variable

.def

timerL

=R17

; Timing variable updated every 14 us

.def

timerH

=R18

; Timing variable updated every 16 ms

.def

system

=R19

; Address data received

.def

command

=R20

; Command received

.def

bitcnt

=R21

; Counter

.cseg

.org 0

rjmp

reset

;********************************************************************

;* "TIM0_OVF" – Timer/counter overflow interrupt handler

;* The overflow interrupt increments the "timerL" and "timerH"

;* every 64us and 16,384us.

1473B–AVR–05/02

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