When gets to choose between one of these two switching strategy (either header or footer) area, IR drop,efficiency & design architectural issues are the key matrix.
Actually, both could be used. However, it would generate a big IR drop, which, in turn, could cause large standard cells delays.
Considering about Header switch which is being implemented by using pMOS to control VDD supply. pMOS transistors are less leaky than nMOS transistor of same size.The disadvantage is that pMOS is having less driving capability than nMOS of a same size (mobility of electrons is higher than holes)
Considering about Footer switch which is being implemented by nMOStransistors to control VSS supply. The advantage of footer is having high drive strength & less IR . Thus, to achieve the same drive strength and IR drop, a design with footers would require less switches than a design with headers. However nMOS transistors are more leakier than pMOS & design become more sensitive to ground noise on the virtual GND coupled through Footer switch.
Despite the obvious advantage of Footer switch ,Header Switch are generally being used in Power gating designs as in multi voltage designs demands for level shifters on signals between blocks with different supply voltages. These elements have a common ground and two power supplies. In this case, footers should not be used.
Actually, both could be used. However, it would generate a big IR drop, which, in turn, could cause large standard cells delays.
Considering about Header switch which is being implemented by using pMOS to control VDD supply. pMOS transistors are less leaky than nMOS transistor of same size.The disadvantage is that pMOS is having less driving capability than nMOS of a same size (mobility of electrons is higher than holes)
Considering about Footer switch which is being implemented by nMOStransistors to control VSS supply. The advantage of footer is having high drive strength & less IR . Thus, to achieve the same drive strength and IR drop, a design with footers would require less switches than a design with headers. However nMOS transistors are more leakier than pMOS & design become more sensitive to ground noise on the virtual GND coupled through Footer switch.
Despite the obvious advantage of Footer switch ,Header Switch are generally being used in Power gating designs as in multi voltage designs demands for level shifters on signals between blocks with different supply voltages. These elements have a common ground and two power supplies. In this case, footers should not be used.
Nice explanation Abhishek. keep posting and keep educating us
ReplyDeleteTopic had been covered in depth and explained in lucid manner
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