Chapter 8: Using MAX II Devices in Multi-Voltage Systems
5.0-V Device Compatibility
8–5
To compute the required value of R
EXT
, first calculate the model of the open-drain
transistors on the MAX II device. This output resistor (R
EXT
) can be modeled by
dividing V
OL
by I
OL
(R
EXT
= V
OL
/I
OL
).
shows the maximum V
OL
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
chapter in the
MAX II Device Handbook.
Table 8–2.
3.3-V LVTTL/LVCMOS Maximum V
OL
I/O Standard
3.3-V LVTTL
3.3-V LVCMOS
Voltage (V)
0.45
0.20
Select R
EXT
so that the MAX II device’s I
OL
specification is not violated. You can
compute the required pull-up resistor value of R
EXT
by using the equation: R
EXT
=
(V
CC
/I
OL
) – R
INT
. 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 (V
CC
) is 5.5 V, the value of
R
EXT
can be calculated as follows:
Equation 8–1.
R
EXT
=
(
5.5
V
–
0.45
V
)
=
315.6
Ω
--------------------------------------
-
16
mA
This resistor value computation assumes worst-case conditions. You can adjust the
R
EXT
value according to the device configuration drive strength. Additionally, if your
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 (V
IH
) of 4.0 V. To drive a MAX II device
with a 5.0-V device, you must connect a resistor (R
2
) between the MAX II device and
the 5.0-V device. See
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