Design of wireless data transmission module based on RF chip nRF401

The designed wireless data transmission module consists of a single-chip RF transceiver chip NRF401, an AT89C52 microcontroller and a MAX3316 interface chip, and operates in the 433.92/434.33MHz frequency band. It can be easily embedded in various measurement and control systems for wireless data transmission, and is used in vehicle monitoring, wireless meter reading, wireless 232 data communication, and computer remote control and telemetry systems.

nRF401 is a product of Nordic Integrated Circuit Company (NORDIC). It is a real monolithic UHF wireless transceiver chip designed for the 433MHz ISM band, meeting the European Telecommunications Industry Standard (ETSI) EN300 200-1 V1.2.1. It uses FSK modulation and demodulation technology, with a maximum operating rate of 20K, adjustable transmission power, and a maximum transmission power of +10dBm. The antenna interface of nRF401 is designed as a differential antenna to facilitate the use of low-cost PCB antennas. It requires very few peripheral components (about 10), no expensive components such as SAW filters and varactors, only cheap and easily available 4MHz crystals, and a combined transceiver antenna. No initialization and configuration are required, no Manchester encoding of data is required, there are two operating bandwidths (433.92/434.33MHz), the operating voltage range can be from 2.7-5V, and it also has a standby mode, which can be more power-saving and efficient.

The block diagram of the nRF401 wireless transceiver chip is shown in Figure 1: The internal structure can be divided into the transmitting circuit, receiving circuit, mode and low-power control logic circuit and serial interface. The transmitting circuit includes: RF power amplifier, phase-locked loop (PLL), voltage-controlled oscillator (VCO), frequency synthesizer and other circuits. The reference oscillator uses an external crystal oscillator to generate the reference frequency required by the circuit.

Its main features are as follows: the working frequency is FSK modulation

of the internationally common digital transmission frequency band , with strong anti-interference ability, especially suitable for industrial control occasions; it adopts PLL frequency synthesis technology, with excellent frequency stability; high sensitivity, reaching -105dBm (nRF401); low power consumption, 250 A in receiving state, and only 8 A in standby state (nRF401); the maximum transmission power reaches +10dBm; low working voltage (2.7V), which can meet the requirements of low-power devices; it has multiple channels and can easily switch the working frequency; the working rate can reach up to 20Kbit/s (RF401); only one crystal and several resistors, capacitors and inductors are connected externally, and basically no debugging is required; due to the design of low transmission power and high receiving sensitivity, no license is required for use, and the maximum use distance in open areas can reach 1000 meters (related to the specific use environment and component parameters).

Pinout and Function

The nRF401 wireless transceiver chip has 20 pins.

Important Timing Parameters

Switching between TX and RX

When switching from RX to TX mode, the data input pin (DIN) must be kept high for at least 1ms to send and receive data. When switching from TX to RX, the data output pin (DOUT) must output data after at least 3ms.

Switching between Standby and RX

From standby mode to receiving mode, when the PWR_UP input is set to 1, the DOUT pin output data is valid after tSR time. For nRF401, the longest tST time is 3ms.

From standby mode to transmit mode, the maximum time required for stabilization is tST.

Switch between Power Up and TX

In order to avoid interference and radiation during startup, the TXEN input pin must be kept low during the power-on process so that the frequency synthesizer can enter a stable working state. When entering the transmission mode from power-on, TXEN must be kept low for 1ms before sending data to DIN.

From power-on to receiving mode, the chip will not receive data and DOUT will not output data until the voltage reaches 2.7V or above and remains stable for at least 5ms. If an external oscillator is used, this time can be shortened to 3ms.

Application circuit and design issues that should be paid attention to

In actual application, the microcontroller uses Atmel's AT89C52, and the five pins of nRF401, DIN, DOUT, TXEN, PWRUP, and CS, can be controlled by the pins of the P1 port of the microcontroller respectively.

The interface chip uses the RS232 conversion chip MAX3316 from Maxim, which completes the level conversion and data sending, receiving, requesting and clearing functions of the RS232 interface between the microcontroller and the computer. For the use of this chip, please refer to its manual.

When the nRF401 chip is used, the operating frequency is set and after entering the normal working state, the microcontroller performs the transceiver conversion control as needed, sends/receives data or performs state conversion. In actual design applications, the following issues need to be noted:

1) Antenna access

ANT1 and ANT2 are the input of LNA when receiving and the output of power amplifier when transmitting. The antenna connected to nRF401 is connected to nRF401 in differential mode. The recommended load impedance at the antenna end is 400 ohms. The output of the RF power amplifier is two open-circuit output transistors, configured in a differential pairing mode. The VDD of the power amplifier must pass through the collector load. When a differential loop antenna is used, the VDD must be input through the center of the loop antenna.

2) Share a crystal oscillator with the microcontroller

nRF401 can share a crystal oscillator with the microcontroller. It should be noted that the crystal trace introduced from the microcontroller should not be too close to the data line or control line.

PCB layout and decoupling design The design of the printed circuit board (PCB) is directly related to the RF performance. In order to obtain better RF performance, the PCB design requires at least two layers of boards to achieve. The PCB is divided into two parts: the RF circuit and the control circuit. nRF401 uses a PCB antenna, and there is no ground plane under the antenna. The power supply of the RF part is separated from the power supply of the digital circuit part.

In order to reduce the influence of distributed parameters, long power supply traces should be avoided on PCB. All component ground wires, VDD connection wires, and VDD decoupling capacitors must be as close to nRF401 as possible. The power supply of nRF401 must be well filtered and separated from the power supply of digital circuits. Use high-performance capacitors for decoupling as close to the power supply pin VDD as possible, preferably a small capacitor and a large capacitor in parallel. It is best to ground the top and bottom layers of the PCB with copper, connect the two layers of copper closely with more vias, and then connect the VSS pin to the copper surface. All switching signals and control signals cannot pass near the PLL loop filter components and VCO inductors.

The location of the VCO inductor is very important for the PCB layout of nRF401. The optimal design of the nRF401 VCO inductor location is to ensure that a PLL loop filter voltage of 1.1 0.2V is generated, which can be measured from FILT1 (pin4).

Design of communication protocol

nRF401 is often used in portable and mobile devices. In such applications, it is necessary to work as long as possible. Considering the energy consumption of the battery, it is often necessary to consider energy saving and low power consumption design. In order to save energy, nRF401 should be turned off in most cases. Since the wireless hardware does not have an automatic wake-up function, in order to achieve the purpose of energy saving, a reasonable communication protocol must be adopted through software to ensure energy saving without losing data.

1) First, each transmission should have a preamble code, usually 101010101010..., which lasts for a given period (such as 1 second). This preamble code is the basis for energy saving.

2) The receiving end can open the receiving end for a few milliseconds. If the prescribed preamble 101010101010... is not received, it will be closed for about 1 second and synchronization is obtained by detecting the preamble. The switching time ratio is the duty cycle of the work. Increasing the preamble cycle can reduce the working time and thus reduce the average working current. It should be noted that although increasing the length of the preamble can reduce power consumption, it will reduce the response speed of the system and needs to be determined according to the requirements of the system.

Software Design

When designing a program, pay attention to the delay of each state transition. The maximum communication rate of nRF401 is 20kbit/s. Before sending data, the circuit must be placed in the transmission mode; the conversion time from the receiving mode to the transmission mode is at least 1ms; data of any length can be sent; the conversion time from the transmission mode to the receiving mode is at least 3ms. In standby mode, the circuit enters the standby state, and the circuit does not receive or transmit data. The conversion time from the standby mode to the transmission mode is at least 4ms; the conversion time from the standby mode to the receiving mode is at least 5.0ms.