Design of wireless serial interface circuit for single chip microcomputer-EEWORLD

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Abstract: This paper introduces a single-chip wireless serial interface circuit composed of MICRF102 single-chip transmitter chip and MICRF007 single-chip receiver chip.

Keywords: MCU serial interface RF transceiver

1 Overview

The single-chip wireless serial interface circuit is composed of the MICRF102 single-chip transmitter chip and the MICRF007 single-chip receiver chip, and operates in the 300~440 MHz ISM frequency band; it has ASK modulation and demodulation capabilities, strong anti-interference ability, and is suitable for industrial control applications; it adopts PLL frequency synthesis technology and has good frequency stability; the receiving sensitivity is as high as -96dBm, and the maximum transmitting power is -2.5dBm; the data rate can reach 2 Kb/s; low operating voltage: 4.75~5.5 V; low power consumption, the current is 3mA when receiving, the current is 7.75 mA when transmitting, the receiving standby state is only 0.5uA, and the transmitting standby state is only 1.0uA; it can be used for wireless transmission of serial data between single-chip microcomputers, and can also be used in single-chip microcomputer data acquisition, telemetry and remote control systems.

2 Circuit composition and working principle

2.1 Wireless Transmitter Circuit

The wireless transmitting circuit is shown in Figure 1. The circuit is based on MICRF102. MICRF102 is a monolithic UHF ASK transmitter launched by Micrel. It adopts SOP(M)-8 package. The chip contains: a synthesizer composed of a reference oscillator, a phase detector, a frequency divider, a bandpass filter, a voltage-controlled oscillator, a transmitting bias control, an RF power amplifier, an antenna tuning control and a varactor diode. It is a true "data input-wireless output" monolithic wireless transmitting device. The UHF synthesizer generates carrier frequency and orthogonal signal output. The input phase signal (I) is used to drive the RF power amplifier. The antenna tuning orthogonal signal (Q) is used to compare the antenna signal phase. The antenna tuning control part detects the phase of the transmitting signal in the antenna channel and controls the capacitance of the varactor diode to tune the antenna and realize automatic antenna tuning. The power amplifier output is controlled by the transmitting bias control unit. ASK/OOK modulation provides a low power mode with a data transmission rate of 20 kb/s.

The following points should be noted during use: (1) REFOSC (pin 4) is the reference oscillator terminal. Connect the crystal oscillator to the ground, or use AC coupling to input a clock pulse with a peak-to-peak value of 0.5 V. The transmission frequency is 32 times the reference oscillator frequency: reference oscillator frequency x32 = transmission frequency. If an external clock signal is used, AC coupling must be used, and the input signal amplitude is 200~500 mV peak-to-peak. (2) MICRF102 uses differential output to drive the antenna load. The power amplifier output stage contains a varactor diode, which automatically tunes with the inductance of the antenna to ensure resonance at the transmission frequency. The inductance of a typical PCB wire antenna is related to the size of the loop, the width of the antenna wire, the thickness of the PCB copper foil, and the position of the ground plane. When designing, the capacitance value of the varactor diode is generally selected to be 6.5 pF. The antenna inductance L is calculated by the formula L=1/(4π2f2C). (3) The output power of the power amplifier is related to the voltage on the PC terminal (pin 1). During normal operation, the voltage on this pin is set between 0.2 and 0.4 V. As the voltage on the PC increases, the output power increases; however, if the voltage on the PC exceeds 0.4 V, the power amplifier is current limited and the output power no longer increases. Reducing the voltage on the PC reduces power consumption and also reduces RF output power. (4) The STBY terminal (pin 5) is the standby mode control. Connecting to VDD is for transmission mode and connecting to VSS is for standby mode. (5) The MICRF102 chip is sensitive to power supply ripple, so proper power supply bypassing is required. Generally, three capacitors of 4.7uF, 0.1uF, and 100pF are connected in parallel between VDD and VSS.

2.2 Wireless receiving circuit

The wireless receiving circuit is shown in Figure 2. The circuit is based on MICRF007. MICRF007 is a single-chip UHF ASK/OOK (on-off keying) superheterodyne radio receiving chip launched by Micrel. MICRF007 uses SOP(M)-8 package. The circuit inside the chip can be divided into three parts: UHF down-converter, OOK demodulator and reference control. The UHF down-converter includes RF amplifier, mixer, intermediate frequency amplifier, bandpass filter, peak detector, synthesizer, AGC control circuit; OOK demodulator includes low-pass filter, comparator; reference control circuit includes reference oscillator and control logic circuit. Only two external capacitors CAGC and CTH, a crystal oscillator and power supply decoupling capacitor are needed to form a UHF ASK receiver. All RF and IF tuning are automatically completed in the chip. It is a true "wireless input-data output" single-chip device.

MICRF007 is a standard narrow RF bandwidth superheterodyne receiver. Narrow bandwidth receiver is insensitive to RF interference signals. The RF center frequency is controlled by a fully integrated PLL/VCO frequency synthesizer, which is related to the reference oscillator external crystal. The bandwidth of the intermediate frequency bandpass filter is 430 kHz, and the bandwidth of the low-pass filter of the baseband demodulator is 2.1 kHz. Receive digital ASK signals, the receiver data transmission rate is 2 Kb/s.

The following should be noted during use: (1) MICRF007 is a narrow bandwidth receiver, requiring the transmitter circuit to use SAW or crystal oscillator for frequency stabilization. (2) If the receiver is in a high noise environment, a fixed value bandpass network can be connected between the antenna ANT terminal and VSS to provide reception selectivity and input overload protection. (3) The reference oscillator can be connected to an external crystal oscillator or input clock signal through the REFOSC terminal (pin 8). The frequency fT of the reference oscillator is 64.5 times the frequency of the external crystal oscillator. For a superheterodyne receiver, the difference between the local oscillator frequency fLO and the transmission frequency fTX must be equal to the center frequency of the intermediate frequency. Therefore, the relationship between the transmitter frequency fTX (i.e., the receiver receiving frequency), the reference oscillator frequency fT, and the local oscillator frequency fLO is: fT = fLO/64.5, fLO = fTX±(1.064fTX/390). (4) The SHUT terminal (pin 6) controls the receiver enable. When the SHUT terminal voltage VSHUT is high, the chip enters a low-power standby mode with a current consumption of only 0.5uA; when VSHUT is low (pulled down to ground), the chip is enabled and in receiving state. (5) The DC value of the demodulated signal on the CTH terminal (pin 4) is used as the reference threshold of the comparator. The external capacitor C2 on the CAGC terminal (pin 7) can increase the input dynamic range. (6) The MICRF007 chip is sensitive to power supply ripple, and proper power supply bypassing is required. Generally, three capacitors of 4.7uF, 0.1uF, and 100 pF are connected in parallel between VDD and VSS.

2.3 MCU serial interface circuit

The wireless transceiver circuit can be directly connected to commonly used single-chip microcomputers such as 8051, 68HC05, PIC16C5X, etc. to realize wireless transmission of serial data between single-chip microcomputers. The connection circuit is shown in Figure 3.

Conclusion

Experiments show that the designed single-chip serial interface wireless transceiver circuit has a simple structure and reliable operation, and can easily form a point-to-point or point-to-multipoint wireless serial data transmission channel between single-chip microcomputers.

The following points should be noted during use: ① In the transmission mode, the communication rate is up to 2 Kb/s; the circuit must be placed in the transmission mode before sending data (STBY = 1 on the 5th pin of MICRF102); the conversion time from the receiving mode to the transmitting mode is at least 5ms; data of any length can be sent; after the transmission is completed, the circuit should be placed in the receiving mode (SHUT = 0 on the 6th pin of MICRF007); the conversion time from the transmitting mode to the receiving mode is at least 5 ms. ② In the standby mode (STBY = 0 of MICRF102, SHUT = 1 of MICRF007), the circuit does not transmit/receive data. When designing a serial communication program, the following should be considered: the protocol of the communication between the two parties, the valid data identification flag, and the error detection, correction and verification of the data.

References

1 Micrel Inc. QwikRadio TM UHF ASK Transmitter www.micrel.com. 2001.8

2 Micrel Inc. QwikRadio TM Low Power UHF Receiver www.micrel.com. 2001.8

Reference address：Design of wireless serial interface circuit for single chip microcomputer

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