EP2C20F256C8N Datasheet, PDF (215/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

PLLs in Cyclone II Devices
VCCA & GNDA
Each Cyclone II PLL uses separate VCC and ground pin pairs for their
analog circuitry. The analog circuit power and ground pin for each PLL is
called VCCA_PLL<PLL number> and GNDA_ PLL<PLL number>. Connect
the VCCA power pin to a 1.2-V power supply, even if you do not use the
PLL. Isolate the power connected to VCCA from the power to the rest of
the Cyclone II device or any other digital device on the board. You can use
one of three different methods of isolating the VCCA pin:
■ Use separate VCCA power planes
■ Use a partitioned VCCA island within the VCCINT plane
■ Use thick VCCA traces
Separate VCCA Power Plane
A mixed signal system is already partitioned into analog and digital
sections, each with its own power planes on the board. To isolate the VCCA
pin using a separate VCCA power plane, connect the VCCA pin to the
analog 1.2-V power plane.
Partitioned VCCA Island Within the VCCINT Plane
Fully digital systems do not have a separate analog power plane on the
board. Since it is expensive to add new planes to the board, you can create
islands for VCCA_PLL. Figure 7–16 shows an example board layout with
an analog power island. The dielectric boundary that creates the island
should be 25 mils thick. Figure 7–16 shows a partitioned plane within
V_CCINT for VCCA.
Altera Corporation
February 2007
7–31
Cyclone II Device Handbook, Volume 1