Ratioed Circuits

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Transcript Ratioed Circuits

Ratioed Circuits
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Ratioed circuits use weak pull-up and stronger pull-down networks.
The input capacitance is reduced and hence logical effort.
Correct operation depends on correct pull-up and pull-down ratio.
Ratioed circuits dissipate static power and must be used sparingly.
The pull-up network for ratioed CMOS (pseudo-nMOS) uses a single pMOS
whose gate terminal is grounded (device is always on).
To compute the logical effort of the pseudo-nMOS gates we use the full
complementary CMOS inverter as reference.
Assume that the full complementary CMOS inverter delivers current I in
both rising and falling transition and that a properly sized pMOS device in a
pseudo-NMOS circuit has its width ½ that of the nMOS pull-down network.
The logical effort is given by the ratio of the pseudo-nMOS inverter input
capacitance to that of the unit CMOS inverter input capacitance.
Ratioed Circuits
• The objective is to have the nMOS pull-down network sized
appropriately to effectively fight the pMOS device for correct output.
• The output current is therefore the difference between the pull-down
current and the pull-up current.
• The parasitic delay p is determined by comparing the parasitic
output capacitance of the pseudo-nMOS inverter to that of the unit
sized inverter.
• A Pseudo-nMOS NAND can be shown to be generally slower than
static CMOS, but pseudo-nMOS style shows improved delay for
gates such as the NOR gate. Why?
• Ganged CMOS is widely known as Majority gate.
• A single configuration can perform different logic functions
depending on input values.
Cascode Voltage Switch
• Cascode voltage switch logic (CVSL) eliminates the static power
consumption of the pseudo-nMOS design style.
• It has a pair of nMOS pull-down networks to compute an output and
its complementary value from associated input signals and their
complementary values.
• The pull-down networks are complementary with one side having
series transistors while the other has a parallel combination.
• CVSL has the potential to improve switching speed since the gate
input capacitance is reduced.
Dynamic Circuits
• The drawbacks of ratioed logic include:
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Slow rising transitions
Contention on the falling transitions
Static power dissipation and
Non-zero output low voltage
• Dynamic circuits require a precharge and evaluate phase and use a
clock to circumvent the problems of the pseudo-nMOS approach.
• Dynamic circuits are fast because of lower input capacitance and no
contention during switching.
• Dynamic circuits have zero static power dissipation??????
• The clock and the associated logic consume considerable dynamic
power.
• Dynamic circuits are sensitive to noise during evaluation.
Domino Logic
• Floating nodes could result when the input transitions from a HIGH
to a LOW before the precharge signal is asserted. When this
condition occurs both the nMOS pull-down network and the pMOS
(precharge devices are OFF.
• Domino logic allows for the placement of static and dynamic circuits
to eliminate floating nodes??????
• Domino gates are non-inverting.
• Dual-rail domino logic gates encode each signal with a pair of wires.
• The output and input signal pairs are denoted with _h and _l to
indicate their status (high or low).
• Inputs and their complementary values are accepted by dual rail
domino.
• These gates have both a precharge and an evaluate clock.