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
CC
PCI Clamp
(1)
V
CCIO
I
Model as R
1
R
2
I
B
Note to
(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 V
CCIO
for MAX II devices is 3.3 V and the I/O clamp diode is enabled, the voltage at point B in
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, R
2
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.
To compute the required value of R
2
, first calculate the model of the pull-up transistors on the
5.0-V device. This output resistor (R
1
) can be modeled by dividing the 5.0-V device supply
voltage (V
CC
) by the I
OH
: R
1
= V
CC
/I
OH
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
135
120
I
OL
V
CCINT
= 5.0
V
V
CCIO
= 5.0
V
Typical I
O
Output
Current (mA)
90
60
I
OH
30
1
2
3
4
5
V
O
Output
Voltage
(V)
ALTERA [ ALTERA CORPORATION ]
