UG445 Datasheet, PDF (21/24 Pages) –

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Power, Sequencing, and Slew Rates
• Oxide Lifetime for 65Å gate oxide is ~2.66 years at 5.0V
• Oxide Lifetime for 65Å gate oxide is ~0.24 years at 5.5V
• Oxide Lifetime for 65Å gate oxide is ~10 minutes at 7.5V
Driving 5V with a CPLD
First, determine if 5V is truly necessary for the downstream device. LVTTL, a 5V standard,
can be driven by a LVCMOS33 device and function properly. This is because the input
thresholds for the 5V device are met by the output thresholds for the LVCMOS33 I/O
standard. The information below applies if you still need 5V outputs.
CoolRunner-II devices are not 5V-tolerant.
For other CPLDs, the basic strategy is to configure the I/O structure so that it drives either
a 0, or is high-Z. Then, use an external pull-up resistor (500 Ω to 1 kΩ) to 5V. To do this, run
the output signal logic to the enable of the OBUFT. The logic input of the OBUFT is
connected to ground. In this way, when the signal is low, the output is grounded.
When the signal is high, the output is placed in a Tristate condition, and the external
pull-up pulls the output up to 5V.
UG445_05_052207
Figure 6: Simplified Method of Driving 5V
The following expands upon the first solution. Using a feedback loop allows the output to
drive up to 3.3V (or 2.5V) for a short period of time before being placed in a Tristate
condition and allowing the external pull-up resistor take care of the rest. The advantage of
this is that it has a faster rise, from 0 to 3.3V (or 2.5V), than the first solution.
CPLD I/O User Guide
UG445 (v1.2) January 14, 2014
Figure 7: Fast Rise Time Method of Driving 5V
UG445_06_052207
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