RE: [DC] How to minimiZe PCB trace propagation time K.Balasubramanian The equation for propagation velocity is: v = c /sqrt(Er) Where: v = velocity in meters per second c = speed of light in a vacuum ~ 3 x 10^8 m/s sqrt(Er) = square root of your dielectric constant. This means that the only two things affect the velocity of your signal. The first is the dielectric constant, or permittivity, of the material. The second factor that can impact is whether or not the trace is microstrip or stripline. For example with a permittivity of 4.1, a microstrip will have a propagation velocity of: 135.2 ps/in And a stripline signal will have a velocity of: 171.4 ps/in The reason that the microstrip will propagate faster is that half of its dielectric environment is composed of air, with a permittivity of ~1, thus making it faster. The long and short is that if your signal is already on the surface of the board, then there is little if anything you can do except change the permittivity. If the signal is a stripline, consider moving it to the surface. Another option would be to adjust the microcontroller code to compensate. Finally, you could run a piece of wire over the board so that its dielectric material consists mainly of air. Bear in mind that I didn't say that any or all of these are practical. Those darn laws of physics... Hope this helps, Devon ======================================================================= Devon J. Post - C.I.D. Mayo Foundation <mailto:[log in to unmask]> 200 First Street SW Voice: (507) 538-5479 Sn 2-134C Fax: (507) 284-9171 Rochester, MN 55905 Internet: http://www.mayo.edu/sppdg/sppdg_home_page.html ======================================================================= -----Original Message----- From: Balasubramanian Krishnan [mailto:[log in to unmask]] Sent: 31 August, 2006 8:21 AM To: [log in to unmask] Subject: [DC] How to minimise PCB trace propagation time Dear Experts, In one of our design we see that signal round trip delay (clock 'high to low' transition and data arrival) is beyond the timing requirement of the microcontroller used. The microcontroller expects the data to arrive within 14 nanoseconds after the clock pulse is activated but the data is reaching only after 16 nanoseconds delay and hence the device fails. No over shoots or undershoots are noticed in the clock signal, the clock frequency is 24 MHz with rise time of about 4 to 5 nano seconds. The PCB trace length cannot be reduced. Costly PCB material (for low relative permittivity) is not acceptable being a cost effective product. How to handle this issue? Your expert guidance will be of great use. Sincerely K.Balasubramanian Project Leader - Hardware. ---------------------------------------------------------------------------- ----- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ---------------------------------------------------------------------------- ----- --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ---------------------------------------------------------------------------------