Purpose: A schmitt trigger is a buffer used to convert a slow or noisy input signal into a clean one before passing it to the FPGA. This is a simple, low-cost solution for a user working with low slew-rate signals. Using schmitt-trigger buffers guarantees a fast, noise-free, input signal to the FPGA.
Actel recommends that you use a schmitt trigger to buffer a signal if input slew rates fall below the values outlined in the specification for SX-A and RTSX-S devices. Depending on the application, different schmitt-trigger buffers can be used to fulfill the requirements.
Schmitt-trigger buffers are categorized in three configurations:
Fixed threshold voltages with non-inverted outputs
Fixed threshold voltages and inverted outputs
Variable threshold voltages with non-inverted outputs
With the aid of schmitt-trigger buffers, low slew-rate applications can also be handled with ease. Implementation of these buffers is simple, not expensive, and flexible in that different configurations are possible depending on the application. The characteristics of schmitt-trigger buffers (e.g. threshold voltage) can be fixed or user-adjustable if required.
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Families |
Supported |
|
Fusion |
Yes |
|
ProASIC3E |
Yes |
|
ProASIC3 |
No |
|
ProASIC PLUS |
Yes* |
|
Axcelerator |
No |
|
ProASIC |
No |
|
SX-A |
No |
|
SX |
No |
|
RTSX-S |
No |
|
eX |
No |
|
MX |
No |
*Although ProASIC PLUS supports the schmitt-trigger attribute,you cannot edit this attribute with the MultiView Navigator tools. Instead, it has to be instantiated in the schematic or the netlist.
Values: A schmitt trigger has two possible states: on or off. The trigger for this circuit to change states is the input voltage level. That is, the output state depends on the input level, and will change only as the input crosses a pre-defined threshold.
Default value: The default value is On.
For more information, please see the "Using Schmitt Triggers for Low Slew-Rate Input" Application Note on the Actel web site.