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Propagation delay in digital gates
ESE 314 / Fall 2012 / Lecture 10
Let’s first take a look at simple switched capacitor network
1
Propagation delay in inverter
Consider ideal (zero rise/fall times) input signal
Definition of inverter propagation delay
Clearly, minimum clock period:
And, hence, maximum clock frequency:q y
2
Power dissipation
Total power dissipated:
major part
Power‐delay product:Power delay product:
For CMOS inverter operating at This is energy that is being For CMOS inverter operating at maximum clock frequency:
dissipated when output switches high‐low or low‐high:
Power‐delay product decreases when VDD is reduced.What about maximum operation clock?What about maximum operation clock?
Energy‐delay product:
3
Let’s first compute tPHLStill assume zero rise time of input pulse.
Expected V (t)Expected VOUT(t).
NMOS output IVp
hence
VTDD
DD VV2
V
4
Let’s first compute tPHLApproximate analysis
4A
Let’s estimate tPLH
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MOSFET capacitances revisited
Internal (fundamental)
Linear regime:
Saturation:
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MOSFET capacitances revisited
External (parasitic)
External and internal capacitances are in parallel.
6A
Dynamic operation of CMOS inverter
Propagation delay tP is determined when inverter drives identical inverter.
Equivalent load capacitance
7
Another popular way of estimation of propagation delay
Equivalently
RON – nonlinear resistor, depends on VOUT , i.e. on time
8
Carrier velocity saturation in MOSFET channel
8A
Intrinsic Delay
9
Inverter Propagation Delay
10