Design and implementation of network interface based on ARM-LPC2368

1. Introduction

With the rapid development of Internet technology, people's requirements for information sharing are also increasing. At present, embedded systems have penetrated into every corner of our lives. Its perfect combination with the network provides great convenience for us to share information. PHILIPS's LPC2368 is an excellent microprocessor. If the embedded system based on it does not have an Ethernet interface, its application value will be greatly reduced. Therefore, for the entire system, the Ethernet interface circuit should be indispensable, but it is also relatively complex.

The Ethernet interface circuit is mainly composed of two parts: the MAC controller and the physical layer interface (PHY). LPC2368 has an embedded Ethernet controller that supports the Reduced Media Independent Interface (RMII) and the Buffered DMA Interface (BDI), and can provide 10M/100Mbps Ethernet access in half-duplex and full-duplex modes. Therefore, the LPC2368 actually contains Ethernet MAC control, but does not provide a physical layer interface, so an external physical layer chip is required to provide an Ethernet access channel. Here, National Semiconductor's DP83848C is selected as the Ethernet physical layer interface chip, which provides MII/RMII/SNI interfaces and can be easily connected to the LPC2368.

2 Introduction of LPC2368 and DP83848C

2.1 Microprocessor LPC2368

The LPC2368 is a 32-bit microcontroller based on the ARM7TDMI-S core. It can operate at frequencies up to 72MHz. It is powerful and cost-effective, supporting 10/100 Ethernet, full-speed (12Mbps) USB 2.0 and CAN 2.0B. It has up to 512KB of on-chip Flash, 58KB of SRAM, 10-bit A/D and D/A converters and an IRC oscillator. It also has an optional SD memory card interface and a 100-pin LQFP package (14×14×1.4mm). It is widely used in industrial control, POS systems, protocol conversion, encryption systems and other fields ^[1] .

2.2 Physical layer chip DP83848C

The DP83848C is a 10/100Mb/s single-ended low-power physical layer device with several intelligent power-saving modes, including a 25MHz clock output that is easily accessible via an external transformer and twisted pair media interface; support for both IEEE 802.3u MII and RMII Rev 1.2 for easy design; integrated sublayer support for 10BASE-T and 100BASE-TX Ethernet protocols; low power consumption less than 270mW, 3.3V MAC interface; configurable SNI interface; 48-pin LQFP package (7x7mm). As an Ethernet physical layer transceiver, the DP83848C is widely used in high-end peripherals, industrial control and factory automation operations, and general embedded applications ^[2] .

3. Hardware design

3.1 Circuit diagram

The connection between LPC2368 and DP83848C is relatively simple, and can be directly connected through the RMII interface. After the connection is completed, DP83848C is connected to the transmission media through the network isolation transformer and RJ45 interface. The circuit block diagram is shown in Figure 1.

Figure 1 Block diagram of Ethernet interface circuit between microcontroller LPC2368 and PHY chip DP83848C

3.2 Ethernet interface actual circuit schematic design

DP83848C supports several MAC interface modes: (1) MII; (2) RMII (Reduced MII); (3) 10 Mb Serial Network Interface (SNI). Here we use the RMII interface mode, which is determined by setting pins pin39 and pin6, as shown in Table 1.

Table 1 : MII mode selection

As can be seen from Table 1, pin39 should be connected to a high level and pin6 should be connected to a low level. At the same time, because pin6 is pulled down inside the chip and defaults to 0, it is sufficient to set pin39 to a high level, so that the microprocessor LPC2368 can be connected to the PHY chip DP83848C through the RMII interface. They are connected through the RMII interface, and the number of pins used to connect the chip and the controller will be relatively small, and the data transmission rate is 2 bits each time, that is, the frequency is 50MHz, so a 50MHz crystal oscillator needs to be connected to pin34 X1.

In RMII mode, the main pins used are: 1 Serial management: MDC (pin31), MDIO (pin30); 2 MAC data: TX_EN (pin2), TXD[1:0] (pin4 pin3), RX_ER (pin41), CRS_DV (pin40), RXD[1:0] (pin44 pin43); 3 Clock: X1 (pin34, RMII reference clock is 50MHz), X2 (pin33).

Pin27 is set high to make DP83848C work at a rate of 100Mbps, and the LED displays the working status of DP83848C. 16ST8515 is a network isolation transformer, which mainly plays the role of signal transmission, impedance matching, waveform repair, clutter suppression and high voltage isolation to protect the safety of the system. The circuit schematic diagram drawn by protel 99 SE is shown in Figure 2.

Figure 2: Schematic diagram of the actual circuit of the Ethernet interface