Domino logic
WHY does domino logic exist?
Background: how a dynamic gate works
The domino fix
WHAT this buys you — monotonicity. Trace the levels:
| Phase | Dynamic node | Output |
|---|---|---|
| Precharge () | HIGH () | LOW (0) |
| Evaluate () | stays HIGH or falls to 0 | stays LOW or rises to 1 |
So during evaluation, starts LOW and can only ever rise (0→1) — never fall. This is called a monotonically rising signal.
HOW to build one — worked construction
Goal: build a domino AND-gate Y = A·B.
- Pick the function for the PDN. A domino output is , and goes LOW (so goes HIGH) when the PDN conducts. We want , so the PDN must conduct when . Why this step? The dynamic node computes the complement at ; the inverter un-complements it, so PDN logic directly matches the non-inverted output.
- PDN = series NMOS for AND. Two NMOS then in series between and the evaluate transistor. Why? Series NMOS conduct only when both are on = logical AND.
- Add clocked precharge PMOS from to , gate = . Why? Sets HIGH ⇒ LOW during precharge (the required monotonic start).
- Add clocked evaluate NMOS at the bottom, gate = . Why? Blocks any discharge during precharge so charges cleanly.
- Add static inverter . Why? Provides drive, restores levels, and produces the monotonic non-inverting output for cascading.
The big limitation
Recall Feynman: explain to a 12-year-old
Imagine a row of dominoes standing up. You (the clock) first stand them all up — that's precharge. Then you tip the first one — evaluate. Each domino can only fall (go from up to down), never jump back up on its own. Because they only fall in one direction, one after another, they never knock each other over by accident. In the circuit, "standing up" = output is LOW, "fallen" = output rose to HIGH, and this one-way behavior lets us chain the gates safely and quickly.
Flashcards
Why can't plain dynamic gates be directly cascaded?
What single element turns a dynamic gate into a domino gate?
What is the state of a domino output right after precharge?
What key property makes domino cascadable?
What is the fundamental limitation of domino logic?
Why does a floating dynamic node hold its value?
What is charge sharing and its fix?
Charge-sharing final voltage of node X (cap C_X at Vdd) with internal node (cap C_a at 0)?
For a domino AND gate Y=A·B, how is the PDN arranged?
Role of the keeper PMOS?
Connections
- Dynamic CMOS Logic — the precharge/evaluate base that domino builds on
- Static CMOS Logic — the inverter used, and the fallback for inverting functions
- Charge Sharing and Keepers — the reliability problem and cure
- Clocking and Precharge Schemes — timing of
- NP / Zipper Domino — variants that overcome the non-inverting limit
- Pull-Down Network Design — mapping Boolean functions to NMOS networks
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, dynamic logic bahut fast aur chhoti hoti hai kyunki usme clock ke do phase hote hain: precharge me node upar (HIGH) charge ho jaata hai, aur evaluate me agar pull-down network ON ho to node discharge ho jaata hai. Problem ye hai ki agar aap ek dynamic gate ka output seedha doosre dynamic gate me daalo, to evaluate ke shuru me sabhi outputs HIGH hote hain — isse downstream gate galat discharge ho jaata hai. Ye ek race condition hai aur dynamic node ka charge wapas nahi aata, to answer galat aa jaata hai.
Domino logic is problem ka jugaad hai: dynamic gate ke output pe ek chhota static inverter laga do. Ab actual output ho gaya. Precharge me HIGH tha, to LOW hoga. Evaluate me ya to LOW rahega ya 0 se 1 ki taraf badhega — kabhi girega nahi. Isko monotonically rising kehte hain. Bilkul dominoes ki tarah — ek girta hai tabhi agla girta hai, koi galti se dusre ko nahi giraata. Isliye ab gates ko safely cascade kar sakte ho.
Ek important limitation yaad rakhna: domino output hamesha non-inverting hota hai, isliye seedhe domino chain se inverting function (NAND/NOR jaisa) nahi ban sakta. Uske liye dual-rail ya doosre tricks use karte hain. Aur ek practical dikkat hai charge sharing — jab ka charge andar wale PDN nodes ke saath baant jaata hai to gir jaata hai, jisse galat ho sakta hai; iska ilaaj hai ek chhota keeper PMOS jo ko thoda-thoda charge deta rehta hai. Bas itna samajh lo aur domino clear ho jaayega.