Intel® Quartus® Prime Standard Edition User Guide: Design Recommendations

ID 683323
Date 9/24/2018
Public
Download
Document Table of Contents

1.2.1.4. Use Synchronous Pulse Generators

Use synchronous techniques to design pulse generators.
Figure 2. Asynchronous Pulse GeneratorsThe figure shows two methods for asynchronous pulse generation. The first method uses a delay chain to generate a single pulse (pulse generator). The second method generates a series of pulses (multivibrators).

In the first method, a trigger signal feeds both inputs of a 2-input AND gate, and the design adds inverters to one of the inputs to create a delay chain. The width of the pulse depends on the time differences between the path that feeds the gate directly and the path that goes through the delay chain. This is the same mechanism responsible for the generation of glitches in combinational logic following a change of input values. This technique artificially increases the width of the glitch.

In the second method, a register’s output drives its asynchronous reset signal through a delay chain. The register resets itself asynchronously after a certain delay. The Compiler can determine the pulse width only after placement and routing, when routing and propagation delays are known. You cannot reliably create a specific pulse width when creating HDL code, and it cannot be set by EDA tools. The pulse may not be wide enough for the application under all PVT conditions. Also, the pulse width changes if you change to a different device. Additionally, verification is difficult because static timing analysis cannot verify the pulse width.

Multivibrators use a glitch generator to create pulses, together with a combinational loop that turns the circuit into an oscillator. This method creates additional problems because of the number of pulses involved. Additionally, when the structures generate multiple pulses, they also create a new artificial clock in the design that must be analyzed by design tools.

Figure 3. Recommended Synchronous Pulse-Generation Technique

The pulse width is always equal to the clock period. This pulse generator is predictable, can be verified with timing analysis, and is easily moved to other architectures, devices, or speed grades.