VITESSE
SEMICONDUCTOR CORPORATION
2.48832Gb/s 16:1 SONET/SDH
Transceiver with Integrated Clock Generator
Data Sheet
VSC8140
Figure 5: AC Termination of Low-Speed LVPECL RXCLK16O, RXCLK16_32O, TXCLK16O Outputs
VSC8140
Z
o
Z
o
50Ω
50Ω
100nF
downstream
bias point
generated
internally
100nF
V
CC
-2V
Receiver Low-Speed Interface
The demultiplexed serial stream is made available by a 16-bit single-ended LVPECL interface
RXOUT[15:0] with accompanying differential LVPECL divide-by-16 clock RXCLK16O
±
and selectable
LVPECL divide-by-16 or -32 clock RXCLK16_32O
±
.
RXCLKO_FREQSEL is used to select RXCLK16_32O
±
. RXCLKO_FREQSEL = “0” designates
RXCLK16_32O± output as 77.76MHz, RXCLKO_FREQSEL = “1” designates RXCLK16_32O± output as
155.52MHz.
The RXCLK16O and RXCLK16_32O output drivers are designed to drive a 50Ω transmission line. The
transmission line can be DC terminated with a split-end termination scheme (see Figure 3), or DC terminated by
50Ω to V
CC
-2V on each line (see Figure 4). AC-coupling can be achieved by a number of methods. Figure 5
illustrates an AC-coupling method for the occasion when the downstream device provides the bias point for
AC-coupling. The divide-by-16 output (RXCLK16O) or the divide-by-16 or -32 output (RXCLK16_32O) can
be used to provide an external looptiming reference clock (after external filtering with a 1x REFCLK PLL) for
the clock multiplication unit on the VSC8140.
The RXOUT[15:0] output drivers are designed to drive a 50Ω transmission line which can be DC termi-
nated with a split-end termination scheme (see Figure 6), or a traditional termination scheme (see Figure 7).
Figure 6: Split-end DC Termination of Low-Speed LVPECL RXOUT[15:0] Outputs
Split-end equivalent termination is Z
O
to V
TERM
R1 = 125Ω R2 = 83Ω, Z
O
=50Ω, V
TERM
= V
CC
-2V
V
CC
R1 = 125Ω
VSC8140
Z
o
R1||R2 = Z
o
V
CC
R2 + V
EE
R1
R1+R2
= V
TERM
R2 = 83Ω
V
EE
Page 4
©
VITESSE
SEMICONDUCTOR CORPORATION
741 Calle Plano, Camarillo, CA 93012 • 805/388-3700 • FAX: 805/987-5896
G52251-0, Rev. 4.0
9/6/00
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
