“With the rapid development of modern technology, wireless communication technology has become one of the issues that people pay more and more attention to. in industrial system control. The application of short-range wireless communication technology is becoming more and more extensive. Water resistance test is one of the main methods of constant power load test for diesel locomotives. This paper discusses the improvement of the data acquisition part of the intelligent water resistance test system for diesel locomotives, changing the original wired data acquisition to wireless data acquisition and transmission. Wireless module design.
Authors: Liu Jiangsha, Lei Wei, Yin You
With the rapid development of modern technology, wireless communication technology has become one of the issues that people pay more and more attention to. in industrial system control. The application of short-range wireless communication technology is becoming more and more extensive. Water resistance test is one of the main methods of constant power load test for diesel locomotives. This paper discusses the improvement of the data acquisition part of the intelligent water resistance test system for diesel locomotives, changing the original wired data acquisition to wireless data acquisition and transmission. Wireless module design.
2 Overall Design
The basic circuit diagram of the wireless module is shown in Figure 1, which is mainly composed of a power supply, a reset circuit, a serial port connection circuit and a wireless transceiver circuit. It can realize wireless transmission and reception of serial data, that is: when sending data, the RS-232 serial data is converted into TTL level through MAX232, and then sent wirelessly through CC2430. Receiving data is the reverse process of sending data. CC2430 first receives the data signal, then converts the TTL level to the standard level of RS-232 through MAX232, and then inputs data to the upper computer through RS-232.
3 Hardware Design
The wireless transceiver used in the wireless module is CC2430. CC2430 comes from Norway’s Chipcon Company, and it is a ZigBee product on chip that truly complies with the IEEE802.15.4 standard. CC2430 adopts Chipcon’s latest SmartRF 03 technology and 0.18μm CMOS process, and adopts 7 mmx7 mm QLP48 package; in addition to including RF transceiver, it also integrates enhanced 8051MCU, 32/64/128 KB Flash memory, 8 KB RAM , ADC, DMA, watchdog, etc. CC2430 works in the 2.4GHz frequency band, uses low voltage (2.0 V ~ 3.6V) for power supply, low power consumption (27 mA when receiving data, 25 mA when sending data), high sensitivity (-97 dBm), and a maximum output of 24 mA dBm, the maximum transmission rate is 250 kb/s.
The number of peripheral components of CC2430 is small, it uses an unbalanced antenna, and the unbalanced transformer is connected to make the antenna performance better. The balun in the circuit consists of capacitor C12, inductors L1, L2, L3 and a PCB microwave transmission line. The whole structure meets the requirement of RF input/output matching resistance (50Ω). The internal T/R exchange circuit completes the exchange between the LNA and the PA. R4 and R5 are bias resistors, where R4 is mainly used to provide a suitable operating current for the 32 MHz crystal oscillator. Use a 32 MHz quartz resonator and two capacitors (C13, C14) to form a 32 MHz crystal oscillator circuit. The voltage regulator supplies all 1.8 V pins and the internal power supply, and C11, C15 are decoupling capacitors for power supply filtering.
Since CC2430 has the characteristics of low power consumption, two dry batteries are selected to supply power for the module. In addition, the AH805 boost voltage regulator is also selected, which can boost the 3 V voltage to 5 V, so the power supply part can provide two voltages of 3 V and 5 V, of which the 3 V voltage is for the CC2430 power supply, and the 5 V voltage is for the MAX232 and reset circuit power.
The reset circuit adopts the typical RC reset method, which has two reset functions: power-on reset and manual reset, and the low level is active. It is worth noting that, in order to make the system achieve a valid reset, the RESET-N terminal should be kept low for 10 ms. Errors can easily occur when the device is initialized immediately after reset. Therefore, after entering the 0000H address, the software delay of 1 ms to 10 ms should be executed first, and then initialized.
Since the module uses RS232 to connect with the host computer, it is necessary to use MAX232 to convert the RS232 level to TTL level.
The design of wireless module includes hardware connection and CC2430 software design. To write a software program, we must first figure out the transmission path and process of data in the module, as shown in Figure 2.
When sending data, the data enters the DATA memory area of CC2430 through the serial port USART. For the water resistance test bench, the upper 128 bytes of the DATA memory area can meet the storage capacity requirements, so the data is stored in the upper 128 bytes of the DATA memory area. In order to improve the transmission speed of data, the data in the memory area is transferred to the FIFO buffer of Radio using DMA transmission method. TXFIFO. After the data enters the Radio, it goes through a series of hardware processing, and finally transmits wireless signals through the antenna.
Receiving data is the reverse process of sending data. Radio receives the wireless signal from the antenna, converts the signal into data through a series of hardware processing, stores it in the RXFIFO, and then sends it into the lower 128 bytes of the DATA memory area through DMA, and then sends the received data through the USART serial port. data is sent.
It is not difficult to see that most of the entire data transmission process is completed inside the CC2430. This is due to the high integration level of CC2430, which is a system-on-chip, which can provide high system stability and reliability. CC2430 includes more custom SFR registers, how to use these SFR registers is more critical. For example, in the control operation of the USART, the control register and status register of the USART integrated in the CC2430 are UxUCR and UxCSR respectively (where x can be 0 and 1, which means one of the two USARTs integrated by the CC2430). The UxCSR status word format and the UxUCR control word format are shown in Table 1. Use these two registers to complete the basic settings of the USART, such as:
It is worth noting that the setting of the serial port baud rate of CC2430 is different from that of the general 8051, because it integrates a baud rate generator. Therefore, it is not necessary to use a timer and only need to set the relevant SFR register UxBAUD.BAUD_M[7: 0]and UxGCR.BAUD[4:0]the baud rate required by the system can be obtained, and the relationship is as follows:
Likewise, there are such full-featured SFR registers for DMA and Radio operations. The programming of CC2430 is actually the understanding and application process of its SRF register, which will not be repeated here due to space limitations.
5 System anti-interference
Due to the strong electromagnetic field interference in the environment where the system is located, electromagnetic shielding is required. The specific method is as follows: the module is placed in an aluminum casing, and the casing is connected to the upper computer casing through the RS232 interface to realize grounding. The aluminum casing is equivalent to a hollow conductor, which is placed in an electric field, and the power line terminates on the surface of the conductor, thereby shielding the electric field. The aluminum casing is also equivalent to a shield, which generates eddy current under the action of the high-frequency magnetic field, and the magnetic field of the eddy current is opposite to the original magnetic field, so that the high-frequency magnetic field can be shielded.
Although the wireless module mentioned in this article is modified for the data acquisition part in the water resistance test system, because the connection with the host computer adopts RS232, this wireless module can also be applied to the general data acquisition system with RS232 serial port .