Reading1: An Introduction to Asynchronous Circuit Design Al Davis Steve Nowick University of Utah Columbia University

Signaling Protocol SenderReceiver req ack data Signaling Protocol: communication protocol req: initiate an action ack: signal completion of that action

Signaling Protocol Control signaling 1. Two-phase handshaking protocol 2. Four-phase handshaking protocol Data signaling 1. Bundled Data with two-phase, four-phase HPs 2. Dual-rail Data with two-phase, four-phase HPs,

Control Signaling Protocol Four -phase Handshaking protocol Level signaling or return to zero SenderReceiver req ack data

Control Signaling Protocol Two-phase Handshaking protocol Transition signaling or Non-return to zero SenderReceiver req ack data

Data Signaling Protocol Bundled Data Signaling 1. Similar to synchronous circuits 2. Measure maximum delay of each circuit piece Com. Logic Delay A B C SenderReceiver req ack data a. Bundled data Computation b. Bundle Data transfer 3. Require n+2 wires

Data Signaling Protocol Dual­Rail Data signaling 1. Two wires per bit, encoded to show validity = no data (spacer), 01 = 0, 10 = 1, 11 = error Com. Logic A B C A1=0 A0=0 A1=0 A0=1 A1=1 A0=0 A1=1 A0=1 a. No data b. valid 0 c. valid 1 d. illegal 3. Require 2n wires Register

Signaling Protocol: EX Bundling Constraint VS Delay-Insensitive adders

Completion Detection Circuits Self-timed component with completion detection circuit. Ack: completion of dual-rail signal DoneReset=1: completion of computation DoneReset=0: completion of reset

Classes of Asynchronous Circuits Classification based on delay model: Gate and wire 1. Delay-insensitive (DI) circuits. 2. Quasi­delay­insensitive (quasi­DI or QDI) circuits 3. Speed­independent (SI) circuits 4. Self­timed (ST)circuits DISTQDI Async SI

Delay-insensitive circuits Arbitrary (unbounded but finite) delays on gates and wires Most robust (reliable) circuits Very small class (Martin)

Quasi­delay­insensitive circuits Delay insensitive with isochronous forks Delay in isochronous forks assumed to be similar Weakest compromise to pure delay-insensitivity needed to build practical circuits fork A BCBC

Speed­independent circuits Arbitrary delays on gates Wires have no delay Introduced by David Muller in the 50’s

Self­timed circuits Communication between elements is delay­insensitive Elements may be DI, QDI, SI or well­bounded Introduced by Seitz

Hazards Hazards: unwanted glitches 0 1 glitch Combinational and sequential Hazards May not be severe: No inverter no hazard

Combinational Hazards Static hazards: Dynamic hazards Static 0 hazard Static 1 hazard

Combinational Hazards Static Hazard in Single Input Change

Combinational Hazards Static Hazard in Multiple Input Change

Combinational Hazards Dynamic Hazard in Multiple Input Change

Petri Net Petri Net :bipartite graph places (circle) transitions (bar) Token (dot) A R1 B If A is true then B is true

Petri Net A R1 C Input places of a transition: Output places of a transition: Transition enable: all conditions of a transition are true Transition fire: removes the tokens from the input places and insert tokens to output places B A R1 C B Transition Firing

Petri Net: (Interface Net) C-element XYXY Z

Petri Net Arbiter

Petri Net: I-Net I-Net describes the allowed interface behavior I-Net ==> ISG (Interface State Graph) (finite state machine) X Y Z X Y

Petri Net: X+ Z+ Y+ ISG ==> Encoded ISG X-Y- X+Y+ Z- X-Y XY Z

Translation Method: Wireprefix*[a?;b!] Toggleprefix*[a?;b!;a?;c!] C-elementprefix*[a?||b?;c!] Mergeprefix*[a?|b?;c!] Basic DI Elements :

Translation Method: Operators: ? an input to the wire ! an output of the wire ; concatenation * repetition | choice || weave pref prefix-closure operator

Modulo-3 Counter MOD3=prefix*[a?;q!;a?;q!;a?;p!]

Tangram Compiler-based approach by van Berkel at Philips Research Laboratories and Eindhoven University of Technology Tangram: based on CSP, is a specification language for concurrent systems. A Tangram program for a 1­place buffer, BUF1: (a?W&b!W ) * | [x : var W | #[a?x; b!x]] |

Tangram A Tangram program for a 1­place buffer, BUF1: (a?W & b!W ) * | [x : var W | #[a?x; b!x]] | Interface: an input port, a, an output port, b, data type, W command [x : var W | #[a?x; b!x]] x is local variable ; : sequencing # : infinite repetition T : transfer go

Tangram: Buf1 Channel (arc): c, d, e, wx, rx, a, b A channel has two ports Active port: black dot initiates a request Passive port: white dot returns an acknowledgment Buf1: data is received on port a stored in internal variable x; data is then sent out on port b.

Tangram: Buf2=(a?W&c!W )*| [b:chan W|(BUF 1 (a; b) || BUF 2 (b; c))] | New command ||:parallel composition. :synchronizer Major goal of Tangram rapid turnaround time low­power implementation. portable electronics: an error corrector for a digital compact cassette player counters, decoders, image generators

Micropipeline 4-stage pipeline

Micropipeline