Analysis of LPC2119USB-CAN connector circuit design

　　At present, in the application field where the computer is the host computer, the USB port is the preferred interface between the computer and the peripherals. Due to the advantages of high reliability, complete functions, reasonable cost, and real-time performance of the CAN bus, it is widely used in various automation control systems. The CAN bus is one of the most widely used field buses in the world. In order to better combine the versatility of USB with the professionalism of CAN, the computer's USB interface is connected to the CAN professional network to achieve the convenience of system control and the efficiency of application. This article describes an example of realizing the USB interface and CAN bus based on the ARM7 processor, through which the PC can monitor the devices on the CAN bus.

　　Introduction to processor and its peripheral circuits

　　LPC2119 is based on a 16/32-bit ARM7TDMI-STM CPU that supports real-time simulation and tracing, and has 128 KB embedded high-speed FLASH(＄44.9500) memory. The 128-bit wide memory interface and unique acceleration structure enable 32-bit code to run at the maximum clock rate. Applications that have strict control over code size can use 16-bit Thumb(＄66.9600) mode to reduce code size by more than 30%, while performance loss is very small. It implements pipeline operation, provides Embedded(＄474.0000) ICE logic, supports on-chip breakpoints and debug points, and has an advanced software development and debugging environment. The LPC2119 features a very small 64-pin package, extremely low power consumption, multiple 32-bit timers, 4 10-bit ADCs, 2 CAN, PWM channels, multiple serial interfaces including 2 16C550 industrial standard UARTs, a high-speed I2C interface and 2 SPI interfaces, 46 GPIOs and up to 9 external interrupts, making it particularly suitable for automotive, industrial control applications, medical systems and fault-tolerant maintenance buses.

　　

　　Figure 1 LPC2119 peripheral circuit

　　LPC2119 integrates two CAN controllers, each of which has a similar register structure to the independent CAN controller SJA1000 (＄2.8080). Its main features are: data transmission rate on a single bus up to 1 Mb/s; 32-bit register and RAM access; compatible with CAN2.0B, ISO11898-1 specifications; global acceptance filter can recognize all 11-bit and 29-bit identifiers; acceptance filter provides Full CAN-style automatic reception for selected standard identifiers. Figure 1 shows the LPC2119 peripheral circuit. To ensure reliable reset, an external reset circuit STM809 (＄0.1276) is used.

　　USB interface circuit design

　　The USB interface uses CH375 from Qinheng Electronics. CH375 is a universal interface chip for USB bus, supporting USB-HOST host mode and USB-DEVICE/SLAVE device mode. On the local side, CH375 has an 8-bit data bus and read, write, chip select control lines and interrupt output, which can be easily connected to the system bus of microcontroller/DSP/MCU/MPU and other controllers. CH375 provides serial communication mode, and is connected to microcontroller/DSP/MCU/MPU and other controllers through serial input, serial output and interrupt output. Figure 2 shows the interface circuit of CH375.

　　

　　Figure 2 USB interface circuit

 

　　CAN bus interface circuit design

　　The CAN bus transceiver uses 82C250, and 6N137 (＄0.2160) is selected for isolation. The TD and RD pins of LPC2119 are not directly connected to the TX and RX pins of 82C250, but are connected to 82C250 through high-speed optical coupler 6N137 (＄0.2160), which can enhance the anti-interference ability of the CAN bus node, thereby realizing electrical isolation between the bus nodes. The high-speed optical coupler 6N137 (＄0.2160) is used to protect the CAN bus controller inside LPC2119. The 5 V DC-DC power supply is used on both sides of the optical coupler, which can completely isolate the VCC and VCC1 of the device, improve the anti-interference ability of the system and the stability and safety of the node. Figure 3 shows the connection circuit between LPC2119 and CAN driver 82C250. The DC-DC power supply module uses B0505LS-2W, and the circuit is shown in Figure 4.

　　

　　Figure 3 Connection circuit of CAN driver 82C250

　　

　　Figure 4 DC-DC isolation circuit

　　This system design uses LPC2119 with built-in CAN controller as the main controller and CH375 as the USB interface chip to realize USB-CAN converter. It discusses the peripheral circuit of LPC2119, CAN bus driver circuit and the interface connection between LPC2119 and CH375, and gives the method of using general I/O to simulate parallel port read and write timing in software. It has certain reference value for the practical application of LPC2119, CH375 and CAN bus.