Infrared communication, as the name suggests, involves transmitting data via infrared light. Back in the early stages of computer technology development, data was transferred through cables, which were cumbersome and required specific interfaces, making things quite inconvenient. Later, wireless data transfer methods like infrared, Bluetooth, and 802.11 emerged. In the initial stages of infrared communication technology, there were multiple standards, and devices from different standards couldn't communicate with each other. To allow various infrared devices to interconnect, in 1993, over 20 major manufacturers established the Infrared Data Association (IrDA) to unify the infrared communication standard. This led to the widely adopted IrDA infrared data communication protocol and specifications.
What are the advantages of infrared communication compared to wireless communication? Infrared communication uses infrared rays as a carrier to transmit data. As a form of wireless communication, it offers high cost-effectiveness, easy implementation, resistance to electromagnetic interference, high-speed applications, flexible spatial access, and economic benefits. These features make it widely used in indoor and outdoor point-to-point wireless communications, mobile computing devices, and military applications like infrared fuzes.
In scenarios where data exchange is needed but not excessively large, and real-time requirements aren't particularly high, infrared communication can be employed to enjoy the convenience of cordless communication while avoiding issues associated with high-frequency circuits. Examples include household appliance remote controls, computer remote keyboards and mice, and portable data collection devices like coal-water meter registers or tax collectors for data exchange.
Currently, wireless data communication using infrared is highly feasible in terms of miniaturization, lightweight design, and security. It's already being utilized in wireless multi-channel indoor voice systems, cordless phones, and keyboard-terminal connections, all of which involve short-range wireless links. The working bandwidth in these applications is significantly lower than that required by WLANs.
Now, let’s talk about the principle of remote infrared communication. In practical communication, the transmitted signal typically has a broad spectrum, distributing most of its energy in a relatively low-frequency range, known as a baseband signal. Such signals aren’t suitable for direct channel transmission. To facilitate transmission, enhance interference resistance, and efficiently utilize bandwidth, the signal is usually modulated into a frequency range appropriate for the channel and noise characteristics. This process is referred to as signal modulation. At the receiving end, the received signal is demodulated to recover the original baseband signal. While understanding this communication principle might seem complex, it’s essential for grasping how infrared communication works.
Infrared communication in common remote controls is often modulated using a 38K carrier. Let me explain the principle. First, consider the sending mechanism.
Modulation: This refers to the process of controlling the amplitude, phase, or frequency of a high-frequency signal using the signal to be transmitted, essentially loading one signal onto another. For instance, when transmitting an infrared remote control signal, it’s first modulated at 38K, as shown in Figure 1.
[Figure 1: Infrared Signal Modulation]
The original signal is either a data “0†bit or a data “1†bit we wish to send. The 38K carrier is a square wave signal with a frequency of 38K. The modulated signal is the final waveform we emit. Using the original signal, we control the 38K carrier. When the signal is a data “0,†the 38K carrier is transmitted without interruption. When the signal is a data “1,†no carrier signal is transmitted.
From a circuit perspective, how do we achieve this function? As shown in Figure 2:
[Figure 2: Infrared Emission Circuit Diagram]
For the 38K carrier, we can obtain 37.91K by dividing a 455K crystal oscillator by 12, generate it using the NE555 timing circuit, or produce it using the microcontroller’s PWM. When the signal output pin outputs a high level, Q2 turns off, and regardless of how the 38K carrier signal controls Q1, the right vertical branch won’t turn on, meaning the infrared LED L1 won’t send any information. When the signal output is low, the 38K carrier is released through Q1, generating a 38K carrier signal on L1. Note that the 38K duty cycle of most home appliance remote controllers is 1/3, though there are also 1/2 cases, albeit less frequently.
For normal communication, the receiver must first process the signal through a series of circuits including monitoring, amplification, filtering, demodulation, etc., and then output the baseband signal. However, the integrated receiving head HS0038B of infrared communication combines all these circuits. We only need to connect this circuit to directly output the desired baseband signal, as shown in Figure 3.
[Figure 3: Infrared Receiving Circuit Diagram]
Since the internal amplifier of the infrared receiving head has a very high gain, it’s prone to interference. Therefore, a filter capacitor must be added to the power supply pin of the receiving head. The official manual specifies a value of 4.7uF. A 10uF capacitor is also recommended in the manual, and a 100 ohm resistor is placed in series between the supply pin and the power supply to further reduce interference.
The circuit shown in Figure 3 is used to receive the waveform sent by the circuit in Figure 16-5. When HS0038 detects a 38K infrared signal, it outputs a low level on the OUT pin. When there’s no 38K signal, the OUT pin outputs a high level. By connecting the OUT pin to the microcontroller’s IO port and programming it, we can obtain the data sent by the infrared communication.
Consider this: Is the data output from the OUT pin restored to baseband signal data? And when we receive this baseband signal data, how do we determine what data is received, and what protocol should we follow? Communication protocols like UART, I2C, and SPI that we’ve learned before are baseband communication protocols, whereas the infrared 38K merely modulates and demodulates the baseband signal, making the signal more suitable for transmission.
Since our infrared modulation signal is half-duplex and only allows one source in the space, our infrared baseband signal isn’t suitable for I2C or SPI communication protocols. We mentioned that UART is a 2-line protocol, but during communication, actually one line suffices, so infrared can communicate using UART. Of course, this communication isn’t limitless. For example, in the datasheet of HS0038B, if HS0038B is to recognize the 38K infrared signal, the 38K carrier must be greater than 10 cycles, which limits our infrared communication. The bit rate of the baseband signal must not exceed 3800. If the signal output from the serial port is directly modulated by 38K, the baud rate cannot be higher than 3800.
Common infrared remote control protocols include:
**I. NEC Protocol**
Features:
- 8-bit address and 8-bit command length for improved reliability.
- Two-pass address (user code) and command (key value) for each transmission.
- The modulation of the signal is achieved by the time interval between the bursts.
- The 38KHz carrier has a period of 1.12ms per bit or 2.25ms.
Modulation:
[Figure: NEC Protocol Modulation]
Note: For the signal of the infrared receiver, the place where the pulse signal is high is high. That is, logic "1" is 0.56ms high level + 1.69ms low level, and logic "0" is 0.56ms high level + 0.56ms low level.
Protocol:
[Figure: NEC Protocol Transmission Format]
The figure above shows a typical NEC protocol transmission format. The start bit (boot code) is 9ms high + 4.5ms low. The valid data is address + address inverse code + command + command inverse code. The purpose of the inverse is to calibrate the previous address and command. If you’re not interested in reliability, you can also remove the inverted data or extend the address and command to 16 bits.
The address data transmitted in the above figure is 10011010. It should be noted that the low-order address is sent first and then the high-order address is sent. Therefore, the address of the waveform is 01011001 = 0X59. Similarly, the command is 00010110 = 0X16.
When the button is pressed, as shown in the figure below, it is sent once every 110ms, but the command is sent only once. The repeat is 9ms high + 2.25ms low + 0.56ms high + low.
[Figure: NEC Protocol Repeat Format]
Extended Protocol:
[Figure: NEC Extended Protocol]
The extension protocol simply changes the address to 16 bits, leaving the others unchanged.
Measured Waveform:
[Figure: NEC Measured Waveform]
Since the infrared receiving head reverses the waveform when receiving the signal (or when transmitting), the reverse function of the oscilloscope can be turned on while reading the data, and the valid data can be read.
The following example is the waveform captured by the known NEC type remote control:
Remote control ID is Address=0xDD20; one of the key values is Command=0x0E.
[Figure: NEC Remote Control Waveform]
**II. Philips RC5 Protocol**
Features:
- 5-bit address and 6-bit command length (7-bit extension protocol).
- Bidirectional encoding or Manchester encoding (i.e., level changes represent logic 0 and 1).
- 36KHz carrier.
- The period per bit is 1.778ms (64 cycles of 36 kHz).
Modulation:
[Figure: RC5 Modulation]
[Figure: RC5 Extended Modulation]
Protocol:
[Figure: RC5 Protocol]
A piece of data consists of 14 bits with a period length of 25ms.
The first two bits are the start bit S, which is usually a logic one.
In RC5 extended mode, the second bit S2 expands the 6-bit command code to the 7-bit code (as the MSB), which can be expanded from 64 key values to 128 key values.
The third bit is the control bit C. It flips after each key press, allowing distinction between repeated presses without release and single presses.
When the button is pressed, the data is sent repeatedly every 114ms, and the third bit doesn’t flip, meaning the signals sent repeatedly are completely consistent.
Measured Waveform:
[Figure: RC5 Measured Waveform]
When the same button is pressed twice in succession, only the third bit flips, and the other bits remain unchanged.
The value of the segment data can be read from the above waveform as 101 01010 010111. Since the protocol is the RC5 extension protocol, that is, the second bit is the seventh bit of the command, the address is 01010 = 0X0A, and the command is 0010111 = 0X17. (The actual remote controller manufacturer gives a command of 57, which may be the seventh bit of the command after inverting the second bit).
**III. Sony SIRC Protocol**
Features:
- Three modes of 12, 15, and 20 digits (12-bit mode described below).
- 5-bit address and 7-bit command length.
- Pulse width coding.
- 40KHz carrier.
- The period of each bit is 1.2ms or 1.8ms.
Modulation:
[Figure: SIRC Modulation]
Protocol:
[Figure: SIRC Protocol]
The start bit is 2.4ms high level + 0.6ms low level;
When the button is pressed, the data is sent repeatedly every 45ms.
Measured Waveform:
[Figure: SIRC Measured Waveform]
From the above waveform, the value of the segment data can be read as 1001000 10000 cmd:0001001 addr: 00001.
**IV. Others**
1. ITT
[Figure: ITT Remote Control]
2. JVC
[Figure: JVC Remote Control]
3. Nokia NRC17
[Figure: Nokia NRC17 Remote Control]
4. RCA
[Figure: RCA Remote Control]
5. Sharp
[Figure: Sharp Remote Control]
6. X-Sat
[Figure: X-Sat Remote Control]
In conclusion, infrared communication continues to play a significant role in many areas despite the advent of newer wireless technologies. Its simplicity and effectiveness make it a reliable choice for specific applications, especially where security and low bandwidth are critical.
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