layout of large transistors in clock signal
Techniques for drawing the layout of large transistors
• Optimizing signal and power connections in terms of resistance and
capacitance.
• Healthy substrate connections—remember also that the clock in a chip is
generally one of the highest speed signals and may generate a significant
amount of noise and coupling into the substrate. Generally these transistors
are isolated with independent guard rings.
• Techniques to reduce supply resistance include busing wide connections
from power supply pads and a large number of vias.
• Electromigration rules must be strictly followed.
• The timing characteristics of clock signals are critical, so extraction and simulation
of the layout is a must.
• If there are different clocks that need to be synchronized, then the layout
should be symmetrical between them. A common technique is to use one
clock cell that is configurable as it is used in different locations.
Re: layout of large transistors in clock signal
In addition to this I would like to add
Placement: Ideally placed near the external clock pad as well as power supply
pads. The clock generator itself is a large consumer of power; therefore, it is
a source of power-supply noise. This noise should be isolated from the rest
of the chip by connecting the generator to independent or power pads that
are nearby.
Buffer stage design: the clock buffer cells can be
extremely large (thousands of microns in transistor width). Each stage in the
buffer chain should be designed to minimize the area and power consumption.
The transistors are laid out using special methodologies for reduced
power connection resistance, minimum input capacitance on the gates and
most importantly minimum output capacitance.