EP2S90F1020C5 Datasheet, PDF (761/768 Pages) Altera Corporation – Stratix II Device Handbook, Volume 1

High-Speed Board Layout Guidelines
Altera recommends that you take the following action to reduce ground
bounce and V_CC sag:
■ Configure unused I/O pins as output pins, and drive the output low
to reduce ground bounce. This configuration will act as a virtual
ground. Connect the output pin to GND on your board.
■ Configure the unused I/O pins as output, and drive high to prevent
V_CC sag. Connect the output pin to V_CCIO of that I/O bank.
■ Create a programmable ground or V_CC next to switching pins.
■ Reduce the number of outputs that can switch simultaneously and
distribute them evenly throughout the device.
■ Manually assign ground pins in between I/O pins. (Separating I/O
pins with ground pins prevents ground bounce.)
■ Set the programmable drive strength feature with a weaker drive
strength setting to slow down the edge rate.
■ Eliminate sockets whenever possible. Sockets have inductance
associated with them.
■ Depending on the problem, move switching outputs close to either a
package ground or VCC pin. Eliminate pull-up resistors, or use
pull-down resistors.
■ Use multi-layer PCBs that provide separate V_CC and ground planes
to utilize the intrinsic capacitance of the V_CC /GND plane.
■ Create synchronous designs that are not affected by momentarily
switching pins.
■ Add the recommended decoupling capacitors to V_CC/GND pairs.
■ Place the decoupling capacitors as close as possible to the power and
ground pins of the device.
■ Connect the capacitor pad to the power and ground plane with
larger vias to minimize the inductance in decoupling capacitors and
allow for maximum current flow.
■ Use wide, short traces between the vias and capacitor pads, or place
the via adjacent to the capacitor pad (see Figure 11–33).
Figure 11–33. Suggested Via Location that Connects to Capacitor Pad
Via Adjacent
to Capacitor Pad
Wide and
Short Trace
Capacitor
Pads
Via
Altera Corporation
May 2007
11–25
Stratix II Device Handbook, Volume 2