"Lora is a spread spectrum modulation technology of Semtech company. Compared with the common FSK modulation mode, Lora's spread spectrum modulation greatly improves the sensitivity of the system and can transmit further under the same transmission power. In recent years, with the development of the Internet of things industry, Lora has attracted more and more attention. Lorawan is a set of communication protocol and system architecture based on Lora communication to realize the design of long-distance communication network.
Sx1301 gateway lorawan evaluation development kit
Price ￥ 2897.10
It can evaluate and test the data communication format, communication distance and signal quality evaluation under lorawan protocol.
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This evaluation is the lorawan industrial evaluation development kit launched by Xueli technology. The suite complies with the standard lorawan protocol and can be used to quickly learn lorawan's technical framework, use process and direct secondary development.
Lorawan industrial assessment development kit
Unpacking family photos
The kit adopts the design of core board + backplane + module. It can be seen that the gateway kit uses raspberry Pie 3 computing module as the core board, and the Lora RF part of the gateway is made into a separate module. The module uses standard sodimm and mini PCIe socket to facilitate secondary development and scheme verification.
Main resources of the suite:
Hardware analysis
Gateway motherboard
From left to right on the backplane of the gateway evaluation board, there are power interface, Poe Ethernet interface, USB, HDMI, 4G antenna (4G module required, optional), Lora antenna, GPS antenna interface, and a 40pin 2.54 pin on the other side, which basically covers the interfaces needed in the industrial site. The three USB interfaces and 40pin extended IO also leave expandable space for the Evaluation Kit.
Backplane interface
Raspberry compute module 3 is used in the gateway motherboard. The computing module includes an arm processor, ram and flash memory chip. USB, SPI, IIC, HDMI and GPIO interfaces are led out through the standard sodimm 200pin pin. The 64 bit 4-core 1.2g main frequency armcortex-a53 processor of Broadcom Corporation is used for processing, The module simplifies the process for engineers to develop application products. They do not need to pay attention to the complex interface design of the processor and the wiring of memory high-speed signals. They only need to focus on their own system to design the required interfaces and application software, which will greatly shorten the hardware development cycle and reduce the design difficulty and risk.
Raspberry cm3 module
The system used by raspberry school is Linux based on Debian distribution and supports a large number of open source software. The familiar Ubuntu system is also based on Linux based on Debian distribution. To install most open source software, we can use the apt get command as in Ubuntu. For example, if we want to install MySQL database on the gateway, all we need to do is enter the command sudo apt get install MySQL server. The rest is to wait for the installation to complete.
There are two versions of cm3 module, one is the standard version and the other is the lite version. The difference between the two versions is that the standard board is equipped with 4G EMMC flash without additional memory, while the lite version does not have memory and requires external TF card storage system firmware. The lorawan gateway Evaluation Kit of Xueli technology is equipped with a standard cm3 module, but a non welded TF card socket can be seen on the edge of the module slot, The lite version module can also be used if necessary.
The gateway part on the gateway evaluation board is independently made into a module, using the standard mini-pcie interface. The module includes one sx1301 baseband chip and two sx1255 RF front-end chips. An acoustic meter bandpass filter and a power amplifier chip can be seen in the RF transmission path. After the transmitted signal comes out of the chip, it first filters the signal outside the working bandwidth through the filter, and then sends it to the power amplifier for amplification and transmission. This structure can reduce the out of band emission of the RF part and reduce the interference to the nearby frequency band equipment. The receiving path has a low noise amplifier and two sound meter filters, which are distributed in front of and behind the low noise amplifier. The received signal first passes through the filter to filter out the signals of irrelevant frequency bands, then passes through the low noise amplifier to amplify the received signal once, and the amplified signal passes through the filter again, Finally, the useful signals arrive at the RF front-end chip for sampling and processing for many times, which improves the signal-to-noise ratio of the receiver and makes it have excellent receiving ability in the environment with certain interference.
Gateway module
Sx1301 is the focus of lorawan evaluation suite. It has sensitivity up to - 142.5dbm, adaptive rate, 49 virtual channels, 10 parallel demodulation paths, and 10 parallel demodulation paths. These 10 channels can set frequencies independently. Eight demodulators have a fixed 125kHz band width, and one channel supports 125 kHz, 250 kHz and 500KHz. One channel uses FSK to send and receive signals. This design enables the gateway to receive data from 8 nodes at the same time, and use a high-speed channel to communicate with nearby gateways, which is the basis for providing access servers for many wireless nodes.
The evaluation board power can be provided by the power interface or Poe, which is also a highlight of the kit. In the actual production environment, Internet of things gateways are placed at the height of iron towers or roofs, and it is often difficult to obtain 220V power supply in these places. If low-voltage DC is pulled to the height through long lines, on the one hand, it is bound to increase the interference introduced into the system and reduce the system stability. On the other hand, the additional power lines need lightning protection measures, which increases the complexity and stability of installation. Poe power supply can get 48V power from the standard Poe switch through the network cable. At present, commercial Poe switches on the market generally have lightning protection measures, which reduces the difficulty of installation and wiring.
Gas discharge tube and varistor are configured at the power input and network interface. Gas discharge tube is widely used in lightning protection of communication equipment, which can protect the equipment from discharging the instantaneous high voltage and large current through gas discharge tube when lightning strikes; Varistor is a nonlinear device. When the voltage at both ends of the varistor exceeds the nominal value, the resistance value of the varistor will decrease rapidly. When the circuit is impacted by exceeding the rated voltage, the varistor can protect the subsequent circuit by reducing its own resistance to provide a current path.
Poe and primary step-down power supply
A Texas Instruments DCDC step-down chip tps54360 is used at the power input. According to the datasheet description, the maximum input voltage of the chip is 60V, and the nominal voltage of Poe is 48V. It can be guessed that the output of Poe power supply module is 48V, and then step-down to 5V through tps54360. The supporting switching power supply of the kit is 12V. A large current diode can be seen behind the power input port and near the POE module, which can prevent the current from flowing back and burning the equipment after inserting the POE switch and external power supply at the same time.
After the first DCDC step-down, a power management chip pam2306ke supplies power to the core board, and the maximum conversion efficiency of the chip can reach 96%.
Power management chip
The USB interface and Ethernet are extended through the USB interface of raspberry pie using a lan9514 (under the heat sink). Lan9514 is a highly integrated hub chip, which can work with only a small number of external devices. The driver of the chip has been integrated in the Linux of raspberry pie, so the USB interface and Ethernet interface can be easily extended. The hub chip can provide four USB 2.0. Port and a 10 / 100M adaptive Ethernet interface.
There are two gas discharge tubes and two varistors behind the Ethernet interface to provide perfect electrical protection for the later stage circuit working in the harsh industrial field environment.
USB interface and Ethernet interface
MAX485 is the most widely used bus system in the industrial field at present. The physical layer of commonly used protocols such as MODBUS and PROFIBUS in the industry is 485 bus. 485 bus runs through the control and data acquisition of the industrial field and is the bus with the highest appearance rate in the industrial field. As an industrial product, this development kit should have industrial level interface protection measures, It also needs to have the interface function required by the industrial field.
In this kit, MAX485 interface chip is provided with RS485 interface. MAX485 is an RS485 interface chip positioned at the industrial site launched by Meixin company. Its maximum interface rate is 2.5mbps. As an industrial chip, MAX485 can provide 15kV electrostatic protection ability to avoid cloth on the production line, Damage to equipment caused by products such as plastic film or electrostatic discharge caused by human touch. MAX485 also has fault voltage tolerance up to 80V. In the interface circuit of the kit, three high current TVs transient diodes are additionally used to provide a release path for instantaneous high voltage pulses.
RS485 interface
The three SMA interfaces on the rightmost side of the gateway evaluation board are 4G antenna, Lora gateway antenna and GPS antenna respectively. The 4G module is optional and can provide the gateway with the ability to access the network where there is no wired network and WiFi; The GPS antenna is connected to the GPS module on the board. There is a ublox GPS module on the base plate of the kit to provide an accurate clock reference for the gateway. In lorawan's ClassB node, the node has a sending time window and a receiving time window. Outside the time window, the devices are in a low-power state to prolong the battery life. If you want to send information to a ClassB node, the gateway needs to know exactly when the device will wake up. This accurate clock reference comes from the GPS PPS signal, The PPS signal of GPS comes from the high-precision clock on GPS satellite, so the time accuracy is very high.
SMA antenna interface
GPS module
There is also a 40pin expansion interface on the gateway evaluation board. Its power supply and some pins are compatible with raspberry second-generation and third-generation interfaces. The pin numbers are shown in the figure.
40pin extension interface
Node evaluation module with backplane
The kit comes with three node modules. One module has been welded on the test base plate, and the other two are stamp packaging modules. Stm32l series low-power processor and sx1278lora RF chip of Italian French semiconductor company are used on the module, and the nominal standby current can be as low as 1.8ua.
The lpmd003 encapsulated by the other two stamps in the kit has a very small size, with a length and width of only 2.8cm * 1.5cm. The pins of the MCU on the board are led out through the stamp hole, which is very suitable for system integration and secondary development.
Lpmd003 module size
Hardware module group photo
Software and resources
Mature and stable modules can speed up the progress of hardware research and development. If there is another set of perfect and detailed reference routines, it can greatly speed up the progress of application software research and development. In addition to the manual and user manual, this industrial evaluation and development kit also provides complete procedures for the gateway part and node part.
Power on and parameter test
There is no suspense about starting the 12V external power supply. This time, the switch supporting Poe function is directly used to supply power to the evaluation board. After the HDMI interface is plugged into the display and Poe Ethernet is connected, the evaluation board starts to take power from the switch, and the startup interface of raspberry pie appears.
It should be noted here that the antenna must be connected before the RF equipment is powered on, otherwise it is easy to damage the RF front-end circuit. The kit is equipped with the FRP antenna of the gateway and the spring antenna of the node. Here, in order to facilitate the next test, the author directly connects the gateway antenna interface to the test instrument.
Startup interface
In the application scenario of IOT gateway, the display is generally not required. The purpose of the display is to see the startup information during debugging. After confirming that the system is started, find the IP address of the evaluation board in the client list of the router and log in with the terminal software. According to the steps, user name and password given in the manual, successfully log in to the Linux system, check the two installed lorawan related services, and confirm that the services are normal.
SSH login evaluation board
After the login and service status check are completed in the above operation, you can use the browser to access http://192.168.1.100:8080 Access the management page, user name admin and password admin. Here 192.168.1.100 is the author's IP address. If you see the following interface, it indicates that the gateway works normally
Gateway management page
Next, connect the node and test the network communication. Connect the node module with backplane in the kit to the computer through USB cable. After installing the serial port driver, there will be an additional serial port in the device manager. Write down the serial port number, open the serial port debugging tool in the attached data, and press the reset key on the board to see the basic information of the node, The meaning of these configuration information will be explained in detail when introducing the lorawan development process next time.
Node startup information
Select nodes from the menu on the left of the gateway management page just now to enter the node management page. Find the Create button on the top right to create a node. The node information is filled in and submitted according to the information printed during startup.
Create node
After the node is added, you can send data through the at command on the serial port debugging assistant just opened. After the data is sent to the gateway and replied, you will see the following display
And the received frame record can be seen in the received frames menu of the web management page. Here, the data of nodes and gateways are even through.
RF circuit test
RF circuit is a difficulty in hardware design. The focus of this hardware test is RF parameters. The author has purchased many sx1278 modules on Taobao, with uneven RF performance. Some even directly copy the parameters of the official reference circuit. Some of the final power is several times worse than the nominal of the chip, and some RF stray emission is obviously too high.
Firstly, the transmission of the node module is tested. The spectrum test uses the spectrum test function of Rohde & Schwarz's wireless communication comprehensive tester. The coaxial cable is used to connect the SMA antenna end of the module with the test input end of the instrument. The transmission spectrum is obtained by sending data through at command.
The spectrum is displayed in peak hold mode, with a center frequency of 470MHz and a bandwidth of 100MHz. At first glance, the spectrum is OK. However, by increasing the display spectrum width and displaying the frequency from 10mhz-2.7g, the problem appears. There is a large stray emission at the position of the first, second and third harmonics. The author speculates that the reason for the harmonics is that the matching circuit is not well done, because the schematic diagram of the RF part is not provided in the data, which is inconvenient for further analysis.
At the same time, through the above test, it can be seen that the transmission power of the module is 14.6d
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