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 of
devices, 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/dis
charge a
capacitor. Consider the following
CMOS inverter:
VDD
|
|
|-
+--|
| |-
| |
A ---+ +---- B
| |
| |-
+--|
|-
|
|
VSS
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 capacitors have to be
charged/
discharged. There are also other, so-called
parasitic capacitances, like from
interconnects, 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)