UG445 Datasheet, PDF (23/24 Pages) –

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Power, Sequencing, and Slew Rates
In Figure 8 are two waveforms of a signal with and without the Schmitt Trigger, it is the
noise from the outputs that make the non-Schmitt Trigger input appear jagged.
Non-Schmitt Waveforms
Schmitt Waveforms
Figure 8: Before and After Schmitt Trigger Waveforms
UG445_07_052207
When the input is higher than a certain chosen threshold, the output is high; when the
input is below another (lower) threshold, the output is low; when the input is between the
two, the output retains its value. Thus, the output retains its value until the input changes
sufficiently to trigger a change. This dual-threshold action is called “hysteresis”.
The hysteresis for the CoolRunner-II Schmitt trigger is reported in the data sheet. One
point to note is that the Vt+ min and Vt- max are equal. However, this combination does
not happen together as the Vt levels track each other. When looking at the Vt max of one
characteristic, you must look at the Vt max of the other.
For more information on the CoolRunner-II Schmitt trigger, see the CoolRunner-II I/O
Characteristics Application Note (XAPP382).
Slew Rate
The slew rate for outputs can be selected as either fast or slow. A slow slew rate is a delayed
version of the fast slew rate. The change in the slew rate is achieved by delaying the fast
slew rate output.
CPLD I/O User Guide
www.xilinx.com
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UG445 (v1.2) January 14, 2014