Fanout Gate Capacitance C fanout : fanout capacitance due to the inputs of subsequent gates, C G. C fanout =C G1 +C G2 +C G3 …. Assumption: Each fanout is an inverter.
Input Capacitance Calculation C OL : overlap capacitance C GN, C GP : Thin Oxide Capacitance
Worst Case Analysis Assumption The thin-oxide capacitance is voltage dependent. The worst case analysis uses C ox WL to compute its worst case value.
Thin Oxide Capacitance:C g C G =WLC ox =WL(ε ox /t ox )=WC g Unit of C g : fF/μm [Worst Case Analysis]
CgCg toxLCg 110 nm51.61 fF/μm 7.5 nm0.35 μm1.65 fF/μm 2.2 nm0.1 μm1.61 fF/μm Cg is approximately 1.61 fF/μm for the last 25 years. Exception: the 0.18 μm process, which has a Cg of 1.0 fF/ μm. [Worst Case Analysis]
Thin Oxide Capacitance:C ol
Components of C ol C ol =C f +C ov C f :fringing capacitance C ov : overlap capacitance
Redefine C g For 0.13 μm, – C g (due to t ox alone): 1.6 fF/μm [Hodges, p.72] – C ol (due to C ov and C f ): 0.25 fF/ μm [Hodges, p.80] – Redefine Cg [Hodges, p.259] as C g =C ox L+2C ol C g =1.6 fF/μm fF/μm=2 fF/μm C g has been constant for over 20 years – Multipy Cg by W to obtain the total capacitance due to t ox, C ov and C f [Worst Case Analysis]
Gate Capacitance of an Inverter C G =C g (W n +W p ) C G =2fF/μm(W n +W p ) [Worst Case Analysis]
Input Capacitance of a 3-input NAND Gate 2W 3W C G =C g (W n +W p )=C g (3W+2W)= C g (5W)
Fanout Gate capacitance of n Inverters C fanout =2fF/μm[(W n +W p ) 1 +(W n +W p ) 2 …(W n +W p ) n ] [Worst Case Analysis] For NANDs, NORs, apply the above equation with appropiate widths.
Self-Capacitance Calculation 1.Eliminate capacitors not connected to the output 2.Assume the transistors are either on (Saturation) or off (Cutoff). 3.C GD is negligible in either saturation or cutoff.
Calculation of Self-Capacitance of an Inverter C self =C DBn +C DBP +2C OL +2C OL C DBn =C jn W n C DBp =C jp W p C OL =C ol W C self =C jn W n +C jp W p +2C ol (W n +W p ) Assume C jn =C jp C self =C eff (W n +W p ) For 0.13: C eff =1 fF/μm [Hodges, p. 261]
Self-Capacitance of a NOR Condition: A=0 B=0→1 CDB4, CSB3 do not need to be charged.→NOT THE WORST CASE CDB3 is charged, while CDB1 and CDB2 are discharged. To avoid double counting, CDB1 and CDB2 will be called CDB12.
Self-Capacitance of NOR Constant Voltage at X
Self-Capacitance of a NOR CDB4 and CSB3 need to be charged CDB3 is charged, while CDB1 and CDB2 are discharged WORST CASE!!
Self-Capacitance of NOR WORST CASE!!
Wire Capacitance Ignore wire capacitance if the length of a wire is less than a few microns. Include wires longer than a few microns – C wire =C int L wire – C int =0.2 fF/um For very long wires use distributed model
Example 6.4 Capacitance Calculation for Inverter
Conclusion Propagation delay depends on the arrival time of inputs – In a series stack, the delay increases as the late arriving input is further from the output.
Sequence: A: charges X B: charges Y C: discharges X, Y, CL Worst Case