8–6
Chapter 8: Using MAX II Devices in Multi-Voltage Systems
5.0-V Device Compatibility
Figure 8–4. Driving a MAX II PCI-Compliant Device with a 5.0-V Device
MAX II Device
5.0-V Device
3.3 V
5.0 V 0.5 V
V
CCIO
V
V
CC
CCIO
PCI Clamp
(1)
I
I
R
2
Model as R
1
B
Note to Figure 8–4:
(1) This diode is only active after power-up. MAX II devices require an external diode if driven by 5.0 V before power-up.
If VCCIO for MAX II devices is 3.3 V and the I/O clamp diode is enabled, the voltage at point B in
Figure 8–4 is 4.0 V, which meets the MAX II devices reliability limits when the trace voltage
exceeds 4.0 V. 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.
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 8–5 shows an example of typical output drive characteristics of a 5.0-V device.
Figure 8–5. Output Drive Characteristics of a 5.0-v Device
150
IOL
135
120
VCCINT = 5.0 V
VCCIO = 5.0 V
90
IO
Typical
Output
Current (mA)
60
30
IOH
4
1
2
3
5
VO Output Voltage (V)
MAX II Device Handbook
© October 2008 Altera Corporation
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