This is the time
that an incoming signal
takes to propagate
from the input
to the output
of a logic gate
. It´s a main factor
of the speed
, such as the CPU
of your computer
. The reason for this delay
usually lies, like almost always in electrical engineering
, in the time it takes to charge
. Consider the following CMOS inverter
A ---+ +---- B
To propagate a change in the input
A to the output B your have to open one transistor
and to close
the other one. This means that the gate capacitor
s have to be charge
d. There are also other, so-called parasitic capacitances
, like from interconnect
s, that have to be considered. Due to the minuscule
size of today´s logic structures this process won´t take long, but cannot be neglected.
Thís charge/discharge mechanism is also the main reason why a logic circuit needs power to operate.
- Device complexity The more stages a device has, the longer takes the signal to propagate through them. Important: This does not necessarily have an impact on throughput. When you´ve got some sort of pipeline structure, you can have more than one data set processed at the time.
- Gate capacity The smaller, less time is taken. That means that in the future, with structures becoming smaller and smaller, devices will become faster. This also helps keeping power consumption low, as there is less charge needed. On the opposite side, parasitic capacitances become more important.
- Structure size The smaller structures become, the higher their resistance, limiting the charge/discharge current.
- Transistor design Today´s transistors have a much lower on resistance, this allows higher currents and lower propagation delays.
- Heat radiation Charging means current, current means heat. The faster you load, the higher the current. In addition, faster circuits can have more charge/discharge cycles per given time. This means that faster circuits radiate more heat. You remember the times of those 486/25 machines that didn´t even need a fan? Now think of today´s quazillion GHz computers... (Yeah, they´re even back to water cooling like in the good old Cray times)