Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!lll-tis!ames!amdahl!oliveb!sun!wings!bobc
From: bobc%wings@Sun.COM (Bob Clark)
Newsgroups: comp.arch
Subject: Re: Cycle stretching
Message-ID: <42976@sun.uucp>
Date: 24 Feb 88 03:01:22 GMT
References: <844@daisy.UUCP> <28200107@ccvaxa>
Sender: news@sun.uucp
Reply-To: bobc@sun.UUCP (Bob Clark)
Organization: Sun Microsystems, Mountain View
Lines: 43

In article <28200107@ccvaxa> aglew@ccvaxa.UUCP writes:
>
>    Am I correct in saying that a trivial way of obtaining a self timed
>    circuit is to take a "normal" circuit, say an adder, and put a timing
>    circuit beside it that will produce a pulse when the adder is finished?
>    And that there are "transformations" that will more closely intertwine
>    the timing circuit with the function, so that they share gates?
>    Doesn't this require extremely accurate parametrization of the device's
>    performance, more than is required for non-self-timed systems?

You have defined two approaches:

1) Take a functional module, characterize the worst case delay, and
add a delay line in parallel with the function.  Use the output of
the delay line to determine that the function is complete.

	This is the trivial approach, and buys you nothing over standard
	synchronous design.  It is a way of modifying a synchronous
	circuit to work withing an otherwise self-timed system.

2) Design an entirely new ciruit to implement the function, whose
state changes are controlled in such a way that the final state change
indicates completion of the function.

	This is the truly self-timed approach, and requires careful
	definition of the state changes.  One approach is to design
	an asynchronous state machine, whose states are carefully
	designed so that only a single bit of the state code can
	change at a time.  This requires no characterization of the
	circuit speed, and is referred to in the literature as
	the "one-hot" approach.

	An alternative is to assemble your macro self-timed circuit
	out of micro-self-timed modules, such as C-elements.  As
	others have mentioned, Ivan Sutherland and others have been
	working in this area recently, and I would guess that some
	work has gone on sporadically since the early days of
	computing.

It is possible to design circuits whose functional completion does
not require parametrization of the device's perfromance.

Bob Clark
Sun Microsystems