Presentation on theme: "Transmission Gate Based Circuits. Elmore Delay (HO) – Application of Elmore Delay to Mux Design (Ex. 7.4) – Logical Effort of CMOS Transmission Gate ("— Presentation transcript:
Transmission Gate Based Circuits
Elmore Delay (HO) – Application of Elmore Delay to Mux Design (Ex. 7.4) – Logical Effort of CMOS Transmission Gate ( Dynamic D-Latch Dynamic Logic
Distributed RC line as a lumped RC Ladder
NMOS TG as a D-Latch CLK=1, Q=D CLK=1 0, Q last is stored on C 2 CLK=0, high impedance state.
Problems with NMOS TG
CMOS TG as a latch
Use feedback to statically hold the logic value when the latch is off (1) We can NOT drive a load from internal Q
Use feedback to statically hold the logic value when the latch is off (2)
No Feedback when the latch is ON
Problem & Solution CLK Problem: If D and Q prev are different: Driver + TG1 will drive Q to a different value while INV2 and NMOS of TG2 will drive Q to Q prev Solution: Size the forward path so that it is stronger than the feedback path.
Adjust V S Knob: – χ as defined in EQ – Increase (W N L P )/(L N W P ) Decreased V S. – Decrease (W N L P )/( L N W P ) Increased V S.
Increase W P to adjust V S W N /L N =200nm/200nm W P /L P =200nm/200nm W N /L N =200nm/200nm W P /L P =460nm/200nm
Typical D-Latch Implementation in CMOS CLK Q now =0 Q prev =1 1 0 Optional
Typical D-Latch Implementation in CMOS CLK Q now =1 Q prev =0 01 Optional
Schematic of a TG Based D latch
Simulation of D-Latch
Zoom in to a transition
Positive Edge D Flip-flop D is only transmitted to the output on the rising edge of CLK
Positive Edge D FF (CLK=0)
Positive Edge D FF (CLK=1)
Dynamic NAND CLK=0 (Pre-Charged Phase) NMOS is OFF. OUT is charged to VDD. CLK=1 (Logic Evaluation Phase) NMOS is ON. If either A or B is GND, OUT=VDD. If A=B=1, OUT=GND Precharge Phase is only a small portion of the clock cycle. Disadvantage: All dynamic logic circuits require a clock.
General Structure of a Dynamic Gate Disadvantage: All dynamic logic circuits require a clock
Examples Example 7.6 P7.5 (a) P7.5 (b)
Problem of Domino Logic Gates 1. During the precharge phase, the output voltage is high. 2.There is an active path to ground as soon as the foot transistor is turned on. 3.Once an output node has been discharged, it cannot go high until the next precharge phase.
Solution 1.Define each stage as a dynamic gate plus an inverter. 2.The output of each stage is now 0 during precharge. Therefore all NMOS transistors are off during precharge and can only be turned on during the evaluation phase. Disadvantage: An inverter can not be created!
Domino Cascaded Gates During the pre-charge phase (Φ=0), Y1, Y2 and Y3 are charged to VDD simultaneously. Φ =0 does not have to last very long since all stages are pre-charged simultaneously. Φ has a high duty cycle. Note: There is no direct current from VDD to GND.
Implement the expression
Propagation Delay of Domino Cascaded Gates The propagation delay is determined by: 1.The falling edge of the dynamic block 2.The rising edge of the inverter Y 1,Y 2 and Y 3 fall like dominos.
Improve the fall time of a Dynamic Block Design a domino stage with a stronger pull- down. – Increase the sizes of NMOS devices. The NMOS devices do not have to fight with the pull up network. So the switch voltage is lower. (VTN of the NMOS) Static Inverter Domino Gate
Improve the Rise Time of an Inverter Design a static inverter with strong pull-up – Increase the size of the PMOS device. Decrease W N L P /L N W P Increased V s of the inverter Static Inverter Domino Gate
Logical Effort Comparison 5/3 2/3 assuming that CLK is does not arrive prior to either A or B
Dynamic NOR Gate The Dynamic NOR gate is a faster circuit because only one NMOS device is driven The pull-down transistors do not fight with the pull-up devices.
Limitations of Domino Logic Charge Sharing V x (initially)=0 V*=(C out )/(C x +C out )V DD
Minimizing the effect of Charge Sharing Using Keepers The keepers keep VX at VDD and reduce charge sharing to minimum. The keeper transistor is weak enough (small W/L ratio by using a large L) that when X=VDDGND. NMOS can prevail over weak PMOS. Disadvantage: large driver requirement of INV Keepers X
Enhancement The INV sees a minimum length device. The effective pull-up strength is controlled by the long device.