5–6
Chapter 5: Using MAX V Devices in Multi-Voltage Systems
5.0-V Device Compatibility
Figure 5–4 shows how to drive a MAX V PCI-compliant device with a 5.0-V device.
Figure 5–4. Driving a MAX V PCI-Compliant Device with a 5.0-V Device
MAX V 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 5–4:
(1) This diode is only active after power-up. MAX V devices require an external diode if driven by 5.0 V before
power-up.
If VCCIO for the MAX V devices is 3.3 V and you enabled the I/O clamp diode, the
voltage at point B in Figure 5–4 is 4.0 V, which meets the MAX V devices reliability
limits when the trace voltage exceeds 4.0 V. To limit large current draw from the 5.0-V
device, R2 must 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. You can model this output resistor (R1) by dividing the
5.0-V device supply voltage (VCC) by IOH: R1 = VCC/IOH
.
Figure 5–5 shows an example of typical output drive characteristics of a 5.0 V device.
Figure 5–5. 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 V Device Handbook
December 2010 Altera Corporation
INTEL [ INTEL ]
