When designing integrated circuits (ICs), electrical engineers must consider propagation delay in VLSI CMOS design. The propagation delay of a logic gate is defined as the time it takes for the effect of a change in input to be evident at the output. In other words, propagation delay is the amount of time it takes for the input to reach the output.
Propagation delay in VLSI is normally described as the time difference between when the transitional input reaches 50% of its final value and when the output reaches 50% of its final value. This demonstrates the influence of input change. In this case, 50% is defined as the logic threshold at which output (or, more specifically, any signal) is presumed to flip states. It is represented by the symbol ‘tpd’. It is also known as gate delay.
Why Consider Propagation Delay?
Modern integrated circuits can contain billions of gates and operate at extraordinary speeds. Inconsistent propagation delay in an integrated circuit can result in data mistakes or race situations on the chip. As a result, propagation delay is a significant consideration in high-speed circuit design and a limiting factor in the processing speed, or frequency (in hertz), that a processor can operate at.
What influences propagation delay in VLSI?
The propagation delay of a logic gate is not constant and is determined by two factors:
1. Input transition time causes output transition:
The longer the transition time at the input, the longer the cell’s propagation delay. Signals should transition quicker to reduce propagation delays.
2. Output load of the logic gate:
The greater the capacitive load at the cell’s output, the greater the effort (time required) to charge it. As a result, the propagation latency increases.
How to Avoid Propagation Delay in VLSI Digital Circuits?
Reduce your clock frequency:
The most apparent option is to reduce your clock frequency. Your time will improve if you can run your FPGA slower.
Divide your logic into stages (pipeline):
The more robust solution is to divide your logic into steps. The propagation delay will be reduced and your design will satisfy timing constraints if you perform less “stuff” between two Flip-Flops.
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