Chapter 5: Using MAX V Devices in Multi-Voltage Systems
5–5
5.0-V Device Compatibility
Table 5–1. 3.3-V LVTTL/LVCMOS Programmable Drive Strength (Part 2 of 2)
I/O Standard
IOH/IOL Current Strength Setting (mA)
8
4
3.3-V LVCMOS
To compute the required value of REXT, first calculate the model of the open-drain
transistors on the MAX V device. You can model this output resistor (REXT) by
dividing VOL by IOL (REXT = VOL/IOL). Table 5–2 lists the maximum VOL for the 3.3-V
LVTTL/LVCMOS I/O standard for MAX V devices.
f For more information about I/O standard specifications, refer to the DC and Switching
Characteristics for MAX V Devices chapter.
Table 5–2. 3.3-V LVTTL/LVCMOS Maximum VOL
I/O Standard
Voltage (V)
0.45
3.3-V LVTTL
3.3-V LVCMOS
0.20
Select REXT so that the MAX V 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,
you can calculate the value of REXT as follows:
Equation 5–1.
5.5 V – 0.45 V
REXT = ---------------------------------------- = 315.6
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 V 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 V device
with a 5.0-V device, you must connect a resistor (R2) between the MAX V device and
the 5.0-V device.
December 2010 Altera Corporation
MAX V Device Handbook
ALTERA [ ALTERA CORPORATION ]
