Design of wireless data transmission module based on ultra-low power consumption wireless chip SX1212

SX1212 is an ultra-low power single-chip wireless chip launched by SEMTECH, with a frequency range from 300MHz to 510MHz. SX1212 has been optimized to have very low receiving power consumption, with a typical receiving current of 2.6mA, which is much lower than the receiving current of similar transceivers. The operating voltage is 2.1-3.6V, and the maximum transmit power is +12.5dBm. SX1212 is highly integrated, including integrated circuits of RF functions and logic control functions, internal integrated voltage-controlled oscillator, phase-locked loop circuit, power amplifier circuit, low-noise amplifier circuit, modulation and demodulation circuit, inverter, intermediate amplifier circuit, etc. In addition, it integrates the data transmission rate of the baseband modem up to 150Kbps. The data processing functions include a 64-byte FIFO, packet processing, automatic CRC generation and data whitening. Its highly integrated architecture allows the minimum number of external components while maintaining design flexibility. All major RF communication parameters are programmable, and most of them can be set dynamically. It complies with European (ETSI EN 300-220 V2.1.1) and North American (FCC part 15.247 and 15.249) standards.

This article introduces the design of a wireless module based on the wireless chip SX1212, which has many advantages such as relatively long transmission distance, high receiving sensitivity, and low working power consumption. Therefore, it is suitable for wireless water and gas meter reading, wireless remote control system, wireless sensor network, wireless temperature and pressure data acquisition, robot control and other fields that require long-term operation with batteries .

System circuit design

The system mainly consists of an MCU and SX1212 as shown in Figure 1. The MCU uses ST's low-power single-chip STM8L101F3. The SX1212 communicates with the single-chip using the SPI interface and communicates with the external terminal using the UART interface. Since the highly integrated SX1212 has very few peripheral parts, the key to the design is the design of the matching circuit of the RF front end. In addition, the high-frequency part of the routing should be as short and thick as possible, and the component parameters should be appropriately adjusted according to the actual situation of the circuit board to offset the influence of the distributed parameters. The impedance of the transmitting and receiving ports of the general RF chip is not the standard 50Ω impedance. To achieve the best reception effect, the input impedance must be compensated by peripheral devices to match it with the 50Ω antenna. The circuit is shown in Figure 1. In the figure, Y2 is a surface acoustic wave filter used to attenuate signals outside the specified frequency band. L2, C3, and C22 are impedance matching networks. L4, L5, voltage-controlled oscillator inductors. Adjusting the voltage-controlled oscillator inductors can make the module work at different frequencies. C7, R3, and C8 are phase-locked loop circuits.

Figure 1 System circuit design of SX1212

Working mode design

Typical wireless transceiver encoding is shown below.

The preamble is an alternating code of "1010", which is used to synchronize the clock of the destination receiver with the transmitter. In normal mode, the length of the preamble is generally 32 bits. If it works in the power saving mode, the preamble also has the function of waking up the receiver. At this time, the transmitter must send a longer preamble to wake up the receiver in the power saving mode and enter the normal working state. If the receiver is set to wake up once every 1 second, then the receiver wakes up once every 1 second to search for the preamble (tw), and the continuous length is generally 16 bits. The transmitter first transmits a preamble of more than 1 second and then transmits the subsequent synchronization code, etc. This means that in the wake-up cycle, as long as the preamble is found in the channel, the receiver can successfully detect and wake up the receiver under normal circumstances. See Figure 2 for a schematic diagram.

Here we have designed four working modes, see Table 1. These four working modes are switched using the SET_A and SET_B pins of the MCU, and the four modes can be switched to each other.

Table 1: Description of the four working modes

Figure 2: Transmitter in Mode 2, Receiver in Mode 3

[page]

The sleep mode is implemented by software, so that the system interfaces maintain the corresponding level when in sleep mode, and can quickly switch between various states. Since the MCU main clock is generated by an RC oscillator, the start-up time only takes 4uS. The measured time from sleep to wake-up plus wake-up initialization only takes 20uS, which means that when the module is in sleep mode, 20uS after setting the SET_A pin low, data can be input to the module through the UART port. Here we have designed the system to complete the receiving or sending process before entering the power saving mode or sleep mode even if it is set to work in mode 3 or 4. The AUX pin will be set low during the receiving or sending process. Using this feature, when the module is in mode 3 or module 4, the lower computer user sets the SET_A pin low to wake up the module and input data. If you need to sleep, you can immediately set the SET_A pin high without waiting for the module to complete the wireless data transmission. The module will automatically detect the SET_A pin after the data transmission is completed. If it is high, it will enter sleep mode. The user can obtain whether the data transmission is completed by querying the AUX pin.

Figure 3: Connection diagram between module and lower computer

In a battery-powered circuit, the slave module (such as a water meter) can be set to mode 3 normally. When the master module (such as a collector or a copy machine) sends data in mode 2, the slave module wakes up and receives the data. After completion, the AUX pin is used to wake up the lower computer MCU, and then the data is output. After the MCU receives the data, the slave module can be switched to mode 1 to respond to the master module. If the master module receives the response, it can also be switched to mode 1. At this time, both the master and slave modules are in normal mode, and high-speed data transmission can be achieved. If the master module receives the response, there is no subsequent data exchange, and the slave can be switched to mode 3 again in power saving mode, waiting for the next wake-up, and the master module can be switched to mode 4 sleep state.

Because power saving is achieved by periodically waking up from sleep and then waking up again, the power consumption in power saving mode is related to the wake-up cycle and the time (tw) for searching the preamble code each time, as well as the static power consumption of sleep. The wake-up cycle can be set online by the user from 50ms to 5s. The time for searching the preamble code each time is related to the rate of RF transmission, which can also be set. At a rate of 10Kbps, the average wake-up search preamble code time is about 4.5ms.

In power saving mode, the battery life can be calculated using the following formula:

Service life =

For example: the battery is a 3.6V/3.6A ER18505 lithium-ion battery, the module includes an MCU receiving current of 3.2mA, and a sleep current of 1.5uA. The RF transmission rate is 10Kbps, and the wake-up cycle is 1SEC, then the battery life is:

Taking into account the battery's self-discharge, capacity differences at different currents, temperature, client MCU sleep power consumption, and several times of use per month, a 3.6V/3.6A ER18505 lithium-ion battery normally has a service life of more than 10 years.

The power saving mode is very suitable for water, gas and heat meters, container information management, data acquisition systems and other occasions where the use is not too frequent but long-term battery operation is required.

Summarize

In some wireless transceiver applications, if battery power is required and the receiving state is required for a long time to ensure real-time response, the only way is to use the timed wake-up reception method. If the battery capacity remains unchanged, if you want to extend the working time, you can only reduce the duty cycle and increase the wireless transceiver rate. However, reducing the duty cycle will directly affect the real-time response, and the sensitivity of wireless reception will generally decrease by 2-3dBm for each doubling of the rate. This directly affects the communication distance. The common single-chip wireless IC receiving current on the market is generally 15-20mA. For example, in systems such as wireless water vapor meters that require batteries to work for 6-10 years, various parameters such as distance, working life, and response time are difficult to choose. The SX1212 launched by SEMTECH innovatively reduces the current to 2.6mA, while the receiving sensitivity, anti-interference, adjacent channel selectivity and other indicators are still high.

At present, the wireless module based on SX1212 has been developed by Shenzhen Anmeitong Co., Ltd. and successfully applied to the collection of wireless water and gas meters. The actual measurement shows that the distance is about 450 meters at 10Kbps and about 600 meters at 2Kbps in an open area. When it is opened once with 1SEC, the water and gas meter data can be collected once with a hand-copied machine in less than 2SEC. If it is used in conjunction with a collector, it can realize fully automatic meter reading. If the hand-copied machine is equipped with GPS, it can realize automatic discovery of surrounding water and gas meters without manual input commands, thereby further improving the meter reading speed. Compared with the traditional manual meter reading, each person can only read 3-4K households per month, and most gas meters have to be read at home at night, which greatly improves efficiency.