[SI-LIST] Re: Matched Length Constaint Approximation for a bus running between 20-50MHz

  • From: Adeel Malik <AdeelM@xxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 19 Aug 2002 10:12:51 +0500

Hello Mike,
           Thanks for the reply. In your rough calculation you have
specified  the max. length of the bus to be about 4.8inches running at 50MHz
but what about the length differnce among the various bus signals ?. Do you
want to say that while routing the bus I need to adhere to 2 rules 1) The
maximum length of the bus is constrained to about 4.8 inches and 2) The
lenght of the bus signals may lie between 0 to 4.8 inches ?.

Secondly, let assume that I am using series terminators with a tolerance of
5% to match the output resistance of the driver to that of transmission line
(whose characteristic impedance has a tolerance of about 10% as specified by
the fabricator) giving rise to the relection-coefficient of  about 0.075 at
the driver side. How much conservative should I be in estimating the "no. of
round trips" for the bus to quiet down or to settle the signal within the
range ?. Can I afford to scale the no.of round trips from 10 to let say 5 ?.

Again many thanks for the reply,
Adeel Malik

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Michael Khusid
Sent: Saturday, August 17, 2002 3:19 AM
To: 'Michael Smith '
Cc: 'si-list@xxxxxxxxxxxxx '
Subject: [SI-LIST] Re: Matched Length Constaint Approximation for a bus
running between 20-50MHz



>I have two questions related to reflective switching.  First does
reflective switching provide an easy termination scheme by only
requiring series terminators at the drivers?  This would drive half
height waves down the line and full height would be achieved on the
reflected pass. 

Many reflective switching topologies require no discrete resistors in their
topologies at all. Sometimes, ASIC vendors place resistors on chip to slow
down the drivers, and those series resistors serve to dissipate the energy.
Otherwise, the energy dissipates through natural parasitics, so it takes
several roundtrips for net to quiet down.

>My second question is what do these schemes do for the clock.  I
wouldn't think you would want a half height clock traversing the line
and hovering at the switching threshold.  Do they drive a full height
clock down the line?  If not how do they avoid multiple clock edges?

In multiple load reflective switching buses, clocks are normally kept
point-to-point. That allows to use same half-height drivers, so that full
height wave happens at the far end. So, if you have four devices, you would
have four different clock nets of the similar length going to the device
clock inputs. Sometimes designers would put two clock inputs at the net. It
is acceptable as long as both inputs are at the far end of the line, less
than a risetime apart. In this case, a rising edge looks a bit ugly, yet the
multiple clock edges do not occur. 

Also, clock nets typically need terminations.

Mike Khusid
SI/HF Application Engineer
Ansoft Corporation
www.ansoft.com

>Thanks in advance, 

>Michael Smith
>Hardware Engineer
>iZ Technology Corp.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Michael Khusid
Sent: Friday, August 16, 2002 12:50 PM
To: 'Adeel Malik '
Cc: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: Matched Length Constaint Approximation for a bus
running between 20-50MHz


Adeel,

The maximum flight time can be roughly calculated by formula:

tflight_time < tperiod - tco - treceiver_setup - tcrosstalk - tjitter

where tco is clock to output delay.

The question is what is the flight time.

Consider a bus with a driver and several receivers.

Driver---Receiver1---Recever2--------------Recever3

Most 20-50MHz buses today use reflective switching, so receiver1 has to
wait
for signal to travel from Driver to Receiver3 and back all the way.
That's
one full round-trip time delay. Also, such slow speed buses have no
terminions (eg. PCI33), so it takes a few round trips for reflections to
die
out. In a conservative design, consider 5 to 10 round trips for a bus to
quiet down.

So, let's do a calculation.
tperiod = 20ns
tco = 2ns
treceiver_setup = 2ns (I am guessing)
tcrosstalk = 1ns
tjigger = 0.5ns (guessing on last two)

tflight time < 14.5ns
Let's say you want to be conservative and allow for 10 round trips.
round_trip_delay = 14.5/10 = 1.45ns
line_delay = round_trip_delay / 2 
line_length = line_delay / speed = 0.5 * 1.45ns / 0.15 ns/inch =
4.83inches

Note that this is a very rough calculation. It gets more much more
interesting with Ts/stubs on the line, connectors and terminations.
Besides,
there are no good ways to estimate time delay caused by crosstalk on the
line. If you want to be more accurate, I would recommend using
simulation
software.

Mike Khusid
SI/HF Application Engineer
Ansoft Corporation
www.ansoft.com

-----Original Message-----
From: Adeel Malik
To: si-list@xxxxxxxxxxxxx
Sent: 8/16/02 8:14 AM
Subject: [SI-LIST] Matched Length Constaint Approximation for a bus
running
between  20-50MHz


Hi All,
         In order to accurately calculate the maximum lenght difference
to
meet the setup and hold times among the bus signals (address, control
and
data) , one needs to find the flight time of the traces, clock-to-output
delay of the Flip-Flops and other logic involved ,cycle-time period and
other things...... 
But if someone is designing the bus for a 20-50MHz range, I don't think
that
there is any need to precisely calculate all the afore-mentioned
parameters
because I know that the cycle-time period for a bus running at e.g 50MHz
is
about 20ns while the delay of the outer-layer PCB Track is about 150
ps/inch
and clock to output delay of the flip-flop in the memory is about 1-2ns
leaving at least 10-15ns of time-margin. So can someone give me any
crude
approximation to determine the maximum lenght difference among the
microstrip traces running in 20- 50MHz range.
Regads,
ADEEL MALIK,

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list


For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  




------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: