EP2C20F256C8N Datasheet, PDF (317/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

Selectable I/O Standards in Cyclone II Devices
Figure 10–21. Driving a Cyclone II Device with a 5.0-Volt Device
5.0 V Device
5.0 V ± 0.25 V
V_CC
Cyclone II Device
V_CCIO
3.0 - 3.4 V ± 0.25 V
PCI Clamp
V_CCIO
Model as R₁
I
R₂
I
B
If V_CCIO is between 3.0 V and 3.6 V and the PCI clamping diode is enabled,
the voltage at point B in Figure 10–21 is 4.3 V or less. To limit large current
draw from the 5.0-V device, R₂ should be small enough for a fast signal
rise time and large enough so that it does not violate the high-level output
current (I_OH) specifications of the devices driving the trace. The PCI
clamping diode in the Cyclone II device can support 25 mA of current.
To compute the required value of R₂, first calculate the model of the
pull-up transistors on the 5.0-V device. This output resistor (R₁) can be
modeled by dividing the 5.0-V device supply voltage (V_CC) by the
I_OH: R₁ = V_CC/I_OH.
Figure 10–22 shows an example of typical output drive characteristics of
a 5.0-V device.
Altera Corporation
February 2008
10–35
Cyclone II Device Handbook, Volume 1