Page 264 - A Practical Guide from Design Planning to Manufacturing

236 Chapter Seven

layout match the result. Inevitably there are changes that must be
made in the circuit design as a result of simulations using real layout.
These circuit changes force changes in the layout. The process repeats
until all the circuit checks are passed using real layout data.
Experienced circuit designers will be deliberately conservative in
their prelayout assumptions in order to minimize the number of times
the circuit and layout must be revised, but overly pessimistic initial
assumptions will lead to over design. Layout and circuit design must
work together to make a circuit with the necessary speed and robust-
ness that does not waste die area or power.
Earlier steps in the design flow treat transistors as ideal switches that
are either on or off. Circuit design simulates the electrical properties of
transistors to predict their true behavior. ON transistors draw only a
limited amount of current. OFF transistors do not draw zero current.
Transistor size matters. It is possible for circuits to be both too fast and
too slow. Voltages are sometimes between the maximum and minimum
values. Taking into account all these nonideal behaviors allows circuit
design to bridge the gap between the logical world of 1’s and 0’s and the
real world of transistors.

Key Concepts and Terms
1T memory cell Leakage power
Active power Mindelay, maxdelay
Clock skew, clock jitter MOSFET
Cross talk NMOS, PMOS, CMOS
Cutoff, saturation, linear P-to-N ratio
Fanout Required window
Flip-flop Subthreshold leakage
Gate leakage Supply noise
Gate, source, and drain Threshold voltage
Inverter, NAND, NOR Transfer curve
Latch Valid window

Review Questions
1. What has to be true for a MOSFET to be in the linear region?
2. What is the significance of a MOSFET’s threshold voltage?
3. Draw the transistor diagram for a CMOS gate implementing the
function NOT(WX + YZ).
4. Should a three-input NOR have a higher or lower P-to-N ratio than
a two-input NOR? Why?