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THE ETHERNET DEVELOPMENT BOARD

Part 1: Putting it all Together
by Fred Eady

Start ý Overview ý Elmerýs Silicon Glue ý Time to Kill ý The CS8900A-CQ ý Software and Firmware ý ICSP and LVP ý Assemble the Ethernet Electronics ý No Floating Allowed ý Installing the Bypass Capacitors ý Pin Point ý Finishing Installation ý Assemble the PIC16F877 Electronics ý The Rest of the Project ý Winding Down ý Finishing Up ý Sources and PDF

THE CS8900A-CQ

While the Elmerýs is hardening, letýs take this opportunity to talk about the IC you just glued to the printed circuit board. The CS8900A-CQ is a fully functional Ethernet IC capable of encoding and decoding Ethernet frames. Originally, the CS8900A-CQ was designed for standard PC-based Ethernet adapters. Unlike some of the cheaper NE2000 clone ICs, the CS8900A-CQ comes equipped with on-chip RAM and 10BaseT transmit and receive filters. The on-chip RAM eliminates the need for an external static RAM IC. Obviously, onboard analog circuitry complexity is slashed as well. With a little help from its friends, the CS8900A-CQ does everything necessary to build and propagate Ethernet frames. All you have to do is feed it data and turn it loose.

Because of a unique I/O port access scheme, the CS8900A-CQ can be used with any 8- or 16-bit micro. This I/O port scheme is called PacketPage. PacketPage technology consists of a set of eight 16-bit I/O ports that are mapped into the CS8900A-CQ I/O space. As you can see in Table 1, the PacketPage ports are located between offset 0000 and 000F hexadecimal. To access the PacketPage I/O ports, the Ethernet adapter base address is added to the offset.

If youýve ever worked with commercial Ethernet adapters you know that the base address for the PC Ethernet card is usually programmable either by jumpers or an address setup application. And usually, youýll find an EEPROM dedicated to setup and addressing information that is loaded into the Ethernet IC during boot up. To keep it simple, the base address of the Ethernet Development Board will be hardwired for 0300 hex. This is done using the CS8900A-CQ address lines. The CS8900A-CQ internal I/O base address register defaults to 0x300 on powerup. This combination of internal address register contents and external address lines results in our PacketPage I/O port addresses being defined between 0300 to 030F hex without the need for a dedicated boot EEPROM.

Looking at Figure 1 and assuming the least significant nibble is zero, you see 0x0300 hardcoded on the CS8900A-CQ address pins. To navigate the entire PacketPage port structure, itýs only necessary to manipulate the least significant nibble of the address. Thus, you only need four of the PIC16F877ýs I/O lines to serve as the PacketPage address lines. The ý877 is on the other side of the Ethernet Development Board and will be installed a little later.

Figure 1ýAs you can see, just a thumbnail of extended components helps the CS8900A-CQ transfer data via the Ethernet.

In addition to four address lines, the PIC16F877 will provide 8 bits of data bus, three bits of CS8900A-CQ I/O control, a CS8900A-CQ RESET line, and a CS8900A-CQ polling input line. That still leaves you with plenty of ý877 I/O and the analog-to-digital pins.

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