EP2C5T144I8N Datasheet, PDF (318/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

Conclusion
Figure 10–22. Output Drive Characteristics of a 5.0-V Device
150
135
120
90
60
Typical I_o
Output
30
Current (mA)
0
I_OL
V_CCINT = 5.0 V
V_CCIO = 5.0 V
I_OH
1 23 45
V_o Output Voltage (V)
Conclusion
As shown above, R₁ = 5.0-V/135 mA.
1 The values shown in data sheets usually reflect typical operating
conditions. Subtract 20% from the data sheet value for guard
band. This subtraction when applied in the example in
Figure 10–22 gives R₁ a value of 30 Ω.
R₂ should be selected so that it does not violate the driving device’s I_OH
specification. For example, if the device has a maximum I_OH of 8 mA,
given that the PCI clamping diode, V_IN = V_CCIO + 0.7-V = 3.7-V, and the
maximum supply load of a 5.0-V device (V_CC) is 5.25-V, the value of R₂ can
be calculated as follows:
R₂ = (5.25 V – 3.7 V) – (8 mA × 30 Ω) = 164 Ω
8 mA
This analysis assumes worst case conditions. If your system does not have
a wide variation in voltage-supply levels, you can adjust these
calculations accordingly.
1
Because 5.0-V device tolerance in Cyclone II devices requires use
of the PCI clamp, and this clamp is activated during
configuration, 5.0-V signals may not be driven into the device
until it is configured.
Cyclone II device I/O capabilities enable you to keep pace with
increasing design complexity utilizing a low-cost FPGA device family.
Support for I/O standards including SSTL and LVDS compatibility allow
Cyclone II devices to fit into a wide variety of applications. The Quartus II
10–36
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2008