Check page 511.  There it shows a four-bit counter, composed of 
"blob-clocked" JK FF's.  The action there occurs on the falling edge,
which indicates to me that "no-blob" JK FF's should trigger on the
rising edge.

BUT, and this is a very big but, even if there is a "standard", recall
that standards usually aren't.  Consider any discussion of RS-232
ports, and all the caveats and such that arise around those.

For the final word, I would:
A.  Always check the data sheet.
B.  Go back to the logic circuit for a JK FF and see whether that
inversion is inherent, or if it is added later on.  IIRC (it's been a few
years since I last discussed the internals of a JK), that bubble is
inherent in the design, but is moved outside to make it more
obvious to the reader that the signal is negative-triggering.

I hope I did more than further muddy the waters.  This thread should
be VERY educational.

Mike H.

On Wed, 6 Oct 2004 16:33:18 +0200, Wouter van Ooijen <obf> wrote:
> asked on sci.electronics.design, but no real answer yet. let's ask here:
> 
> I am confused about the edge on which a FF triggers. My intuition would
> say that any logic symbol with a plain wedgie (no inverting blob) should
> trigger on the positive edge, but maybe that's too simple.
> 
> H&H 2nd edition P508/509 states that a JK triggers on the negative edge,
> and an edge-triggered FF on the positive edge. But it is not clear
> whether that refers to the implementation as shown, or to the bare logic
> symbol. Most JK symbols do have the inverting blob on the clock input.
> 
> P510 suggests that both dividers behave the same, which would imply that
> the plain JK symbol triggers on the negative edge, yet P511 states that
> the shown ripple divider clocks on the negative edge, the JK symbols do
> have the inverting blob, so the plain JK symbol would clock on the
> positive edge.
> 
> My tentative conclusion is that a plain (no inverting blob) D FF clocks
> on the positive edge. But on which edge clocks a plain JK FF? Positive,
> neative, or worse: there is no standard?
> 
> Wouter van Ooijen
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