DM9008 datasheet(61/68 Pages) ETC

61 / 68 page

DM9008

ISA/Plug & Play Super Ethernet Contoller

Final

Version :DM9008-DS-F02

June 14, 2000

2. Oscillator

The oscillator is controlled by a 20 Mhz parallel resonant

crystal connected between X1 and X2 or by an external clock

on X2. The 20 Mhz output of the oscillator is divided by 2 to

generate the 10 MHz transmit clock for the controller. The

oscillator also provides internal clock signals to the encoding

and decoding circuits.

Note:When X2 is being driven by an external oscillator, X1

MUST be grounded.

Crystal Specifications

Resonant Frequency

20 MHz

Tolerance

±0.001% at 25°C

Stability

±0.0005% at 0°C - 70°C

Type

AT Cut

Circuit

Parallel Resonance

Max. ESR

Ω

Crystal Load Capacitor

20 pF

The 20 Mhz crystal connection to DM9008 requires special

care. The IEEE 802.3 standard requires the transmitted signal

frequency to be accurate within

±0.01%. Stray capacitance

can shift the crystal's frequency out of range and cause

transmitted frequency to exceed its 0.01% tolerance. The

frequency marked on the crystal is usually measured with a

fixed load capacitance specified in the crystal's data sheet,

typically 20 pF.

In order to prevent distortion on the transmitted frequency, the

total capacitance seen by the crystal should equal the total

load capacitance. For a standard parallel setup, as shown in

the diagram below, the 2 load caps C1 and C2 should equal

2(C1) minus any stay capacitances. 2(C1) is equal to the

specific load capacity acting in series. Thus the trim capacitors

required can be calculated as follows:

C1 = 2XC1 - (Cb1 + Cd1), where Cb1 = Board cap on X1

and Cd1 = X1 dev cap

C2 = 2XC1 - (Cb2 + Cd2), where Cb2 = Board cap on X2

and Cd2 = X2 dev cap

The values of STNIC pins X1 and X2are the region of 5 pF.

Figure 1.

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