[SI-LIST] Re: setup & hold Time for source synchronous
- From: "Lai, Ricky (Eng Hou)" <Ricky.Lai@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 7 Aug 2001 11:38:06 -0500
I agree with your analysis and your equation can generally apply to almost
all source-synchronous design. However, on most of the source-synchronous
systems that I have seen, the drivers and receivers are well balanced, i.e.
T_available_setup_time_at_driver = T_available_hold_time_at_driver
and
Tsetup_at_receiver = Thold_at_receiver.
In such cases, the board designers will route the board where the data and
clock (or strobe) with identical trace length. As a result, the
Tflight_skew_max will be identical in both equations.
Ricky
-----Original Message-----
From: Bill Chen [mailto:billchen@xxxxxxxxxxx]
Sent: Monday, August 06, 2001 8:01 PM
To: Issa, Elie; Lai, Ricky (Eng Hou); si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: setup & hold Time for source synchronous
I don't agree with Richy's Equations.
In the equation given, Tflight_skew_max is subtracted
in both setup and hold equations and this is incorrect.
If the Tfligh_skew reduces your setup margin, it may
increase your hold time margin. On the other hand, if it
improves you setup margin, it will reduce your hold time
margin. That's the reason that design engineers sometimes
on purposely skew the data / clock signals to balance
setup / hold time margins.
Here is my version of setup/hold equations for your
reference:
Tsetup_margin = T_available_setup_time_at_driver -
T_interconnect_flight_skew_clk_leading_data_max -
Tsetup_at_receiver
Thold_margin = T_available_hold_time_at_driver -
T_interconnect_flight_skew_clk_trailing_data_max -
Thold_at_receiver
Where: T_available_setup_time_at_driver and
T_available_hold_time_at_driver
are Time data_valid_before_clock_min and
Time data_valid_after_clock_min
and
T_interconnect_flight_skew_clk_leading_data_max,
T_interconnect_flight_skew_clk_trailing_data_max
are skew between data and clock signals with reference
to clock signal. please note that these two terms may
be positive or negative. So the skew may increase or
decrease setup or hold time margin.
For example, if clock is actually skew more than data
signals, the T_interconnect_flight_skew_clk_leading_data_max
becomes a negative number, the maximum time clock leading
data should use the minimum absolute skew between clock and
data {Think about Max(-1ns, -1.2ns, -1.5ns) = -1ns}.
And in this situation, the setup margin is improved
comparing with 0 skew between clk/data. Same principal
holds true for the other term.
Let me know if you have any comments!
Thanks!
======================================================
Bill (Qinghua) Chen, Ph.D.
Technical Leader
Andiamo Systems Inc.
375 E. Tasman Dr. Bldg 6 Phone: (408)527-4982
San Jose, CA 95134 Email: billchen@xxxxxxxxxxx
======================================================
Let me know if you have any questions.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Issa, Elie
Sent: Friday, August 03, 2001 2:49 PM
To: Lai, Ricky (Eng Hou); 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: setup & hold Time for source synchronous
Thanks!
-----Original Message-----
From: Lai, Ricky (Eng Hou)
Sent: Friday, August 03, 2001 1:44 PM
To: Issa, Elie; 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: setup & hold Time for source synchronous
The correct equation should be:
Setup margin = Budget - (T_data_valid_before_stb_min) - (Tflight_skew_max) -
Tsetup
Hold margin = Budget - (T_data_valid_after_stb_min) - (Tflight_skew_max) -
Thold
-----Original Message-----
From: Issa, Elie
Sent: Friday, August 03, 2001 11:26 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] setup & hold Time for source synchronous
Dear SI Gurus:
I need your help in verifying some derivation for setup & hold equations
for GTL bus
for source synchronous clocks.
The equation for setup is:
Tco(strobe)min + Tflight(strobe)min - Tco(data)max -Tflight(data)max
-Tsetup= Tmargin_setup
The equation for hold time was:
Tco(data)min + Tflight(data)min -Tco(strobe)max -Tflight(strobe)max
-Thold= Tmargin_hold
The skew for the strobe & data flight time should be very small. Whenever,
I fit the variable given in the
specs into the equation, the margin comes negative.
I would think that there must be a ValidBefore and ValidAfter variable to be
included in the equation.
Yet the specs claim the ValidBefore is only [Tco(strobe)min-- Tco(data)max].
Any input would be greatly appreciated!
Regards
Elie
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