Chapter 8: Using MAX II Devices in Multi-Voltage Systems
8–7
Recommended Operating Condition for 5.0-V Compatibility
As shown above, R1 = 5.0 V/135 mA.
The values usually shown in data sheets reflect typical operating conditions. Subtract
20% from the data sheet value for guard band. This subtraction applied to the above
example gives R1 a value of 30.
Select R2 so that the MAX II device’s IOH specification is not violated. For example, if
the above device has a maximum IOH of 8 mA, given the I/O clamp diode, VIN = VCCIO
+ 0.7 V = 3.7 V. Given that the maximum supply load of a 5.0-V device (VCC) is 5.5 V,
the value of R2 can be calculated as follows:
Equation 8–2.
( 5.5 V – 3.7 V) – (8 mA × 30 Ω)
R2 = ------------------------------------------------------------------------------- = 194 Ω
8 mA
This analysis assumes worst-case conditions. If your system does not see a wide
variation in voltage-supply levels, you can adjust these calculations accordingly.
Because 5.0-V device tolerance in MAX II devices requires use of the I/O clamp, and
this clamp is activated only after power-up, 5.0-V signals may not be driven into the
device until it is configured. The I/O clamp diode is only supported in the EPM1270
and EPM2210 devices’ I/O Bank 3. An external protection diode is needed for other
I/O banks for EPM1270 and EPM2210 devices and all I/O pins in EPM240 and
EPM570 devices.
Recommended Operating Condition for 5.0-V Compatibility
As mentioned earlier, a 5.0-V tolerance can be supported with the I/O clamp diode
enabled with external series/pull-up resistance. To guarantee long term reliability of
the device’s I/O buffer, there are restrictions on the signal duty cycle that drive the
MAX II I/O, which is based on the maximum clamp current. Table 8–3 shows the
maximum signal duty cycle for 3.3-V VCCIO given a PCI clamp current-handling
capability.
Table 8–3. Maximum Signal Duty Cycle
VIN (V) (1)
ICH (mA) (2)
5.00
Max Duty Cycle (%)
4.0
100
90
50
30
17
10
5
4.1
11.67
18.33
25.00
31.67
38.33
45.00
4.2
4.3
4.4
4.5
4.6
Notes to Table 8–3:
(1) VIN is the voltage at the package pin.
(2) The ICH is calculated with a 3.3-V VCCIO. A higher VCCIO value will have a lower ICH value
with the same VIN.
© October 2008 Altera Corporation
MAX II Device Handbook
ALTERA [ ALTERA CORPORATION ]
