Velocity Reviews > VHDL > Will this "asynchronous handshaking" feasible in real circuits?

# Will this "asynchronous handshaking" feasible in real circuits?

HUANG Huan
Guest
Posts: n/a

 02-12-2004
"sig1" and "sig2" are two asynchronous signals. Both are low active.
Assume that at any time the negative pulses of these two signals do
not overlap and there is enough time between the two pulses.

process (ack, sig1)
begin
if ack = '1' then
elsif rising_edge(sig1) then
end if;
end process;

begin
ack <= '0';
elsif rising_edge(sig2) then
ack <= '1';
end if;
end process;

I use Xilinx FPGA to do the post-place-and-route simulation.
The waveform is as follows:

_______ _______________________________________
| |
sig1 |_____|

__________________
| |

________________________ _________________________
| |
sig2 |_____|

___
| |
ack ________________________________| |___________________

The pulse width of "ack" is about 4ns. Because the signal "ready" is
actually a flip-flop and the signal "ack" is the asynchrounous reset
of this flip-flop, will the pulse width of "ack" too short to clear
the flip-flop in real circuits?

Thank you!

VhdlCohen
Guest
Posts: n/a

 02-13-2004
>The pulse width of "ack" is about 4ns. Because the signal "ready" is
>actually a flip-flop and the signal "ack" is the asynchrounous reset
>of this flip-flop, will the pulse width of "ack" too short to clear
>the flip-flop in real circuits?
>

Living dangerously! Speed is also a function of device lot, technology,
temperature, voltage, and I doubt that you can guarantee performance thru all
these variables.

How about using clocks and reclocking critical signals to avoid metastability.

On a scale of 1 to 5, where 5 is great, I would score that design a 1 because
of reliability.
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Dave Higton
Guest
Posts: n/a

 02-24-2004
In message <(E-Mail Removed)>
(E-Mail Removed) (VhdlCohen) wrote:

> >The pulse width of "ack" is about 4ns. Because the signal "ready" is
> >actually a flip-flop and the signal "ack" is the asynchrounous reset
> >of this flip-flop, will the pulse width of "ack" too short to clear
> >the flip-flop in real circuits?
> >

>
> Living dangerously! Speed is also a function of device lot, technology,
> temperature, voltage, and I doubt that you can guarantee performance thru
> all these variables.
>
> How about using clocks and reclocking critical signals to avoid
> metastability.
>
> On a scale of 1 to 5, where 5 is great, I would score that design a 1
> because of reliability.

Sorry to come to this late; but I don't understand your objection.
This is a causal system; ack can't go inactive again until ready has
already gone to 0. I can't see how it could possibly fail. Propagation
delay times make it /more/ secure.

If ack were used to clear another flip-flop too, then I'd agree with
you.

Dave