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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

PIN POINT

Stepping through the CS8900A-CQ pin set, you find some power pins designated strictly for the analog circuitry found within the CS8900A-CQ. To properly bias these internal analog modules, a 4.99-kilohm, 1% resistor (R1) is installed as close as possible to the RES pin (93) and a neighboring ground pin (94). Resistor R2 (4.7-kilohm, 1%) is used to pull up the CS8900A-CQ Sleep line at pin 77. R2 really doesnýt have to be a precision part because itýs only a logic level pull-up device.

The same non-precision definition applies for LED current limiting resistors R3 and R4, but like R2, they too are provided as 1% devices when you purchase a kit from me. R5, a 100-ohm, 1% part, provides receive side termination for a communications cable segment with impedance of 100 ohms. By simply changing the resistance of R6 and R7 along with the capacitance of C10, the CS8900A-CQ can be programmed to support communications cable impedance of 100, 120, and 150 ohms.

The receive magnetics (T1, the HALO isolation transformer) have a primary-to secondary turns ratio of 1:1. Thus, for optimal transfer characteristics, the termination resistor value follows the communications cableýs impedance. As I stated earlier, transmit side impedance matching is performed by resistors R6 and R7 with some help from C10 and the windings ratio of the isolation transformer. On the transmit side, the ratio of primary to secondary windings is 1:1.414. To make things work for a 100-ohm cable impedance, you must have a source impedance of 50 ohms. Using a couple of 24.3-ohm resistors splits this evenly for balance.

The LINKLED is a green LED driven by pin 99 on the CS8900A-CQ. The HCE0 bit of the self-control register is set to force the pin low when valid link pulses are detected on the communications segment.

The CS8900A-CQýs pin 100 drives the red LANLED. This pin is driven active low for 6 ms whenever a collision or a transmit/receive operation occurs. Iýve gone to the trouble of pointing out which color LED goes where because the cathode marking for the green LINKLED is the anode mark for the red LANLED.

A 20-MHz crystal supplies clocking for the CS8900A-CQ internals. If youýre building from scratch, donýt try to use a 20-MHz jobber from your stash until you make sure it meets the specifications for the CS8900A-CQ. This bird is parallel resonant with a maximum resonant frequency error rate of 50 ppm. The crystalýs capacitance should be 18 pF with a series resistance of 50 ohms. There are plenty of crystals that meet these specs in the Digi-Key catalog alone. It all depends on how much youýre willing to pay for your favorite brand.

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