Selectable I/O Standards in Cyclone II Devices
Figure 10–21. Driving a Cyclone II Device with a 5.0-Volt Device
Cyclone II Device
5.0 V Device
3.0 - 3.4 V 0.25 V
V
CCIO
5.0 V 0.25 V
V
V
PCI Clamp
CC
CCIO
I
I
R
2
Model as R
1
B
If VCCIO 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 (IOH) 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 (VCC) by the
IOH: R1 = VCC/IOH
.
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
ALTERA [ ALTERA CORPORATION ]
