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VITESSE
SEMICONDUCTOR CORPORATION
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
Gigabit Interconnect Chip
VSC7212
Notation
Differential signals (i.e., PTX+ and PTX-) may be referred to as a single signal (i.e., PTX) by dropping
reference to the “+” and “- ”. REFCLK refers to the single-ended TTL or differential PECL input pair
REFCLKP/REFCLKN, whichever is used.
Clock Synthesizer
Depending on the state of the DUAL input, the VSC7212 clock synthesizer multiplies the reference
frequency provided on the REFCLK input by 10 (DUAL is LOW) or 20 (DUAL is HIGH) to achieve a baud
rate clock between 0.98GHz and 1.36GHz. The on-chip PLL uses a single external 0.1µF capacitor, connected
between CAP0 and CAP1, to control the Loop Filter. This capacitor should be a multilayer ceramic dielectric,
or better, with at least a 5V working voltage rating and a good temperature coefficient; NPO is preferred but
X7R may be acceptable. These capacitors are used to minimize the impact of common-mode noise on the Clock
Multiplier Unit, especially power supply noise. Higher value capacitors provide better robustness in systems.
NPO is preferred because if an X7R capacitor is used, the power supply noise sensitivity will vary with
temperature. For best noise immunity, the designer may use a three capacitor circuit with one differential
capacitor between CAP0 and CAP1, C1, a capacitor from CAP0 to ground, C2, and a capacitor from CAP1 to
ground, C3. Larger values are better but 0.1µF is adequate. However, if the designer cannot use a three capacitor
circuit, a single differential capacitor, C1, is adequate. These components should be isolated from noisy traces.
Figure 1: Loop Filter Capacitors (Best Circuit)
C1=C2=C3= >0.1µF
MultiLayer Ceramic
Surface Mount
NPO (Preferred) or X7R
5V Working Voltage Rating
C2
C3
CAP0
C1
VSC7216
CAP1
The REFCLK signal can be either single-ended TTL or differential LVPECL. If TTL, connect the TTL
input to REFCLKP but leave REFCLKN open. If LVPECL, connect the inputs to REFCLKP and REFCLKN.
Internal biasing resistors sets the proper DC Level to V /2.
DD
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Page 2
G52268-0, Rev 3.3
04/10/01
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