Chapter 8: Using MAX II Devices in Multi-Voltage Systems
8–5
5.0-V Device Compatibility
To compute the required value of REXT, first calculate the model of the open-drain
transistors on the MAX II device. This output resistor (REXT) can be modeled by
dividing VOL by IOL (REXT = VOL/IOL). Table 8–2 shows the maximum VOL for the 3.3-V
LVTTL/LVCMOS I/O standard for MAX II devices.
f
For more information about I/O standard specifications, refer to the DC and Switching
Characteristics chapter in the MAX II Device Handbook.
Table 8–2. 3.3-V LVTTL/LVCMOS Maximum VOL
I/O Standard
3.3-V LVTTL
3.3-V LVCMOS
Voltage (V)
0.45
0.20
Select REXT so that the MAX II device’s IOL specification is not violated. You can
compute the required pull-up resistor value of REXT by using the equation: REXT
=
(VCC/IOL) – RINT. For example, if an I/O pin is configured as a 3.3-V LVTTL with a 16
mA drive strength, given that the maximum power supply (VCC) is 5.5 V, the value of
R
EXT can be calculated as follows:
Equation 8–1.
(5.5 V – 0.45 V)
REXT = -------------------------------------- = 315.6 Ω
16 mA
This resistor value computation assumes worst-case conditions. You can adjust the
EXT value according to the device configuration drive strength. Additionally, if your
R
system does not see a wide variation in voltage-supply levels, you can adjust these
calculations accordingly.
Because MAX II devices are 3.3-V, 32-bit, 66-MHz PCI compliant, the input circuitry
accepts a maximum high-level input voltage (VIH) of 4.0 V. To drive a MAX II device
with a 5.0-V device, you must connect a resistor (R2) between the MAX II device and
the 5.0-V device. See Figure 8–4.
© October 2008 Altera Corporation
MAX II Device Handbook
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