The formula you quote to calculate the output resistance will give the the guaranteed value at 2.4V output voltage in the high state. With only this value, your concern appears to be substantiated, because this does not provide the rise time you need. Most outputs, however, are nonlinear: if you pull them further away from the steady state, the current goes up more than what you could predict the the steady-state calculation. The highest available current is usually at least two to three times higher than the steady state guaranteed current. If you need proof/disproof whether your output can deliver the required transition time with the heaviest load, the surest way would be to get trusted simulation models for the output (IBIS or SPICE) and simulate the waveform. Regards, Istvan Novak SUN Microsystems ----- Original Message ----- From: "pom gud" <pomgud@xxxxxxxxx> To: <si-list@xxxxxxxxxxxxx> Sent: Sunday, January 25, 2004 4:33 AM Subject: [SI-LIST] Problems with system bus design > Hello, > I am using a 32-bitprocessor in current design. > I have one technical doubt about system bus design. > In processor data sheet, in DC Electrical > Specification section, it is mentioned that drive > capability of data bus is 6 mA and output load is of > 30 pF. I am referring a book "High Speed Digital > Design; A hand book of Black Magic" by Howard Johnson > for peripheral bus analysis. As per formulas given in > this book i found that it is not possible for > processor peripheral data bus to drive 30 pF load with > 6 mA drive capability and meet rise time requirement > of 1.5 nS. The design calculations are as follows, > > Output driver resistance = (VCC- VOH)/IOH = > (3.3 - 2.4)/ 6 mA = 150 ohm > So rise time = 2.2*R*C = 2.2*150*30pF > = 9.9 nS > > I have a lot of confusion about these calculations. > > Hoping for a quick reply > > Thanks and Regards > pomgud > > > __________________________________ > Do you Yahoo!? > Yahoo! SiteBuilder - Free web site building tool. Try it! > http://webhosting.yahoo.com/ps/sb/ > ------------------------------------------------------------------ > 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 technical documents are available at: > http://www.si-list.org > > 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 technical documents are available at: http://www.si-list.org 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