STPCI2 STMicroelectronics, STPCI2 Datasheet - Page 78

• Manufacturer Part Number: STPCI2
• Description: STPC ATLAS DATASHEET - X86 CORE PC COMPATIBLE SYS
• Manufacturer: STMicroelectronics

DESIGN GUIDELINES
6.3.1. POWER DECOUPLING
An appropriate decoupling of the various STPC
power pins is mandatory for optimum behaviour.
When insufficient, the integrity of the signals is
deteriorated, the stability of the system is reduced
and EMC is increased.
6.3.1.1. PLL decoupling
This is the most important as the STPC clocks are
generated from a single 14MHz stage using
multiple PLLs which are highly sensitive analog
cells. The frequencies to filter are the 25-50 KHz
range which correspond to the internal loop
bandwidth of the PLL and the 10 to 100 MHz
frequency of the output. PLL power pins can be
tied together to simplify the board layout.
6.3.1.2. Decoupling of 3.3V and Vcore
A power plane for each of these supplies with one
decoupling capacitance for each power pin is the
78/111
VDD_PLL
VSS_PLL
Connections must be as short as possible
Figure 6-3. PLL decoupling
15pF
XTALI
100nF 47uF
XTALO
Figure 6-4. 14.31818 MHz stage
GND
PWR
15pF
Issue 1.0 - July 24, 2002
minimum. The use of multiple capacitances with
values in decade is the best (for example: 10pF,
1nF, 100nF, 10uF), the smallest value, the closest
to the power pin. Connecting the various digital
power planes through capacitances will reduce
furthermore the overall impedance and electrical
noise.
6.3.2. 14MHZ OSCILLATOR STAGE
The 14.31818 MHz oscillator stage can be
implemented using a quartz, which is the
preferred and cheaper solution, or using an
external 3.3V oscillator.
The crystal must be used in its series-cut
fundamental mode and not in overtone mode. It
must have an Equivalent Series Resistance (ESR,
sometimes referred to as Rm) of less than 50
Ohms (typically 8 Ohms) and a shunt capacitance
(Co) of less than 7 pF. The balance capacitors of
16 pF must be added, one connected to each pin,
as described in
In the event of an external oscillator providing the
master clock signal to the STPC Atlas device, the
LVTTL signal should be connected to XTALI, as
described in
As this clock is the reference for all the other on-
chip
recommended to shield this stage, including
the 2 wires going to the STPC balls, in order to
reduce the jitter to the minimum and reach the
optimum system stability.
generated
XTALI
Figure
Figure
6-4.
clocks,
6-4.
XTALO
3.3V
it
is
strongly