[SI-LIST] Re: Matched Length Constaint Approximation for abu s ru nning between 20-50MHz

Note:  Tco is spec'd into a given test load.
         Depending on what you are driving into
         this time may be larger or smaller.

-----Original Message-----
From: Michael Khusid [mailto:mkhusid@xxxxxxxxxx]
Sent: Friday, August 16, 2002 2:50 PM
To: 'Adeel Malik '
Cc: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: Matched Length Constaint Approximation for a bus
ru nning 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:
http://www.freelists.org/webpage/si-list


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

List archives are viewable at:     
                http://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:
http://www.freelists.org/webpage/si-list

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

List archives are viewable at:     
                http://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:
http://www.freelists.org/webpage/si-list

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

List archives are viewable at:     
                http://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: