Single chip duplex wireless serial transmission scheme

We know that the maximum effective distance of the RSR232 serial port transmission protocol is 10M, and it is wired transmission, which brings great inconvenience to some long-distance transmission control. According to the needs of the project, we designed a wireless transmission method based on ultra-high frequency (300M or more), which has good actual use effect and can meet general transmission and control. It can be applied to microcomputers, single-chip microcomputers and other control fields. For occasions with high bit error rate requirements, it is recommended to use CRC encoding to reduce the bit error rate.

1. Modulation and Transmission

Common analog modulation methods include amplitude modulation, frequency modulation and phase modulation, and common digital modulation methods include frequency shift keying, amplitude shift keying, etc. Although the OOK modulation method has poor performance, the circuit is simple and easy to implement, and the operation is stable. It is widely used in the field of wireless anti-theft and security. For example, this type of circuit is mostly used in car and motorcycle alarms, warehouse doors, and home security systems. See Figure 1

Early transmitters used LC oscillator circuits, but this circuit was greatly affected by distributed parameters, had serious frequency drift, poor stability, and complex debugging, and is now basically not used. Instead, new devices with high stability are waiting, such as SAW filters and ceramic resonators.
SAW filters are widely used in high-frequency electronic communications. This filter is small in size, light in weight, can have a very high center frequency, has a relatively wide bandwidth, and has the frequency selection characteristics of an ideal rectangular coefficient. In addition, this filter can use the same one-sided processing technology as the integrated circuit process, which is simple to manufacture, low in cost, high in remodelability and design flexibility, and can be mass-produced and processed. It is a filter with increasingly wide applications. Compared
with the crystal oscillator circuit, the SAW oscillator circuit is much simpler. The circuit in Figure 1 is a common transmitter circuit. Due to the use of SAW devices, the circuit works very stably. Even if the antenna, SAW device or other parts of the circuit are grasped by hand, the transmission frequency will not drift. The effective transmission distance of this circuit can reach more than 200 meters.
The filter formed by the piezoelectric effect of certain ceramic materials is called a ceramic filter. The commonly used ceramic filter is made of lead titanate [Pb (ZrTiO3)].
Compared with other filters, this filter has a simple manufacturing process and can be easily baked into various shapes, which is suitable for miniaturization of filters; it also has good heat and moisture resistance and is rarely affected by the outside world. Its QL value is usually several hundred, which is higher than that of LC filters, but lower than that of quartz crystal oscillation. Therefore, when making filters, the passband is not as narrow as that of quartz crystals, and the selectivity is also worse than that of quartz crystal filters. At present, ceramic filters are widely used in receivers and other instruments.

2. Reception and Demodulation

Common receiving circuits include super-regenerative circuits and super-heterodyne circuits. Super-regenerative circuits are low-cost and have power consumption as low as about 100uA. The sensitivity of a well-adjusted super-regenerative circuit is similar to that of a super-heterodyne receiver with one high-power amplifier, one oscillator, one mixer, and two intermediate amplifiers. However, the working stability and selectivity of super-regenerative circuits are relatively poor, so the anti-interference ability is also poor.
The sensitivity and selectivity of super-heterodyne circuits can be done very well. Now many companies have produced many receiving super-heterodyne monolithic circuits, which can demodulate the modulated signal with only a few external components. For example, the monolithic integrated circuit receiving and demodulation circuits MICRF001 and MICRF002 launched by Micrel Corporation of the United States are demodulation circuits designed for wireless digital communications. They can demodulate analog modulated high-frequency signals into digital signals with only a few external components, and are very simple to use.

4. Single-chip full-duplex wireless transmission scheme
The transmitting circuit adopts the circuit in Figure 1, and the receiving circuit adopts the circuit in Figure 2. The communication distance is not less than 200M. ICRF002 is a single-chip integrated circuit launched by Micrel, USA, which can complete reception and demodulation. MICRF002 is an improved version of MICRF001. Compared with MICRF001, it has lower power consumption and has a power shutdown control terminal, which can be easily interfaced with the single-chip microcomputer. MICRF002 has stable performance and is very simple to use. ICRF002 uses a ceramic resonator. By replacing a different resonator, the receiving frequency can cover 300-440MHz. MICRF002 has two working modes: scanning mode and fixed mode. The scanning mode accepts bandwidth of up to several hundred KHz. This mode is mainly used in conjunction with LC oscillator transmitters, because the frequency drift of LC transmitters is large, and the data communication rate in scanning mode is about 2.5KBS per second. The bandwidth of the fixed mode is only tens of KHz. This mode is used to match the transmitter using crystal oscillator frequency stabilization, and the data rate can reach 10KBytes per second. The working mode selection is realized through the 16th pin (SWEN) of MICRF002. In addition, the wake-up function can be used to wake up the decoder or CPU to minimize power consumption.
Figure 3 is the main schematic diagram of the 256-channel drip data acquisition system composed of AT89C52. The host and the slave can communicate wirelessly in full-duplex serial mode. The serial port transmitter of the microcontroller is connected to the data input end of the circuit in Figure 1, and the data receiver is connected to the data output end of the circuit in Figure 2. The communication dial rate of AT89C52 is set to 9.8KB, and T2 works in mode 2 and is used as a dial rate generator. The HD7279 chip is used for display and keyboard management. This chip belongs to the I2C bus device and can manage 64 keyboards and 16-bit digital tubes, or 64-bit LEDs. It is very convenient to use.