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From: alvitar@xavax.com (Phillip Harbison)
Newsgroups: comp.arch
Subject: Re: Back to Basics, Which Flip Flop Do you use ?
Message-ID: <1990May3.074336.2692@xavax.com>
Date: 3 May 90 07:43:36 GMT
Sender: alvitar@xavax.com
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There are still some good reasons to use JK flip flops.  Here are a few.

[1] Most JK flops are internally symetrical.  The pins J,S, and Q could
just as easily have been K, R, and /Q.  As a result, there is little or
no skew between Q and /Q.  This is useful if you are trying to divide a
clock by 2 and create alternate phase clocks with minimal overlap.

[2] Alot of JK flops are negative edge triggered.  This can be useful
if your design can't spare the extra nanoseconds for an inverter.

[3] JK flops can simplify state machine logic, since the designer only
must generate only the transition terms.  With a D flop, one also has
to generate hold terms.  Take the following state machine example:

	Inputs:	A, B, and C			Notation: * = and
	Output: Q					  + = or
	Rules:	Q -> 1 if A * B * /C,			  / = not
		Q -> 0 if /A * B * C,
		otherwise Q(n) = Q(n-1).

	Solution using a JK flip flop:
		J = A * B * /C
		K = /A * B * C
	Solution using a D flip flop:
		D = (A * B * /C) + (/(/A * B * C) * Q)
		or
		D = (A * B * /C) + ((A + /B + /C) * Q)

This may not seem like much additional logic, but it gets worse with
more complex state machines (this example is truly trivial).

-- 
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