From: Dr Bruce Griffiths <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Gate propagation delay jitter Date: Wed, 11 Apr 2007 09:32:26 +1200 Message-ID: <[EMAIL PROTECTED]>
> > One of the wizards on the fpga newsgroup has a war story about clock jitter > > due to outputs near the clock input switching. I forget the details. It > > may > > have been a single ended clock input. > > > Hal > > Yes ground bounce can play havoc with the effective switching thresholds. > One would expect this effect to be much worse with single ended clocks. You would also keep clear of the MGT/GTP pins unless you are driving/receiving very slow signals rarely. Ground-bounce is a real issue with these things and as you are into large pin-counts, it becomes a real mess. You do want to plan your bypass capacitance with care (both in-board planes and on-board chip- caps). They have also started to do things that I and probably others proposed, put bypass caps on the BGA carrier PCB. That alone greatly simplifies the design. Single ended clocks is indeed more sensitive, but many people forget that balanced traces are more sensitive when you break the balance, such as in a non-symmetric bend or when you have a non-symmetric (often unintentional) coupling to a nearby trace. In one design we had a bent balanced ECL clock, but at excessive stress-testing (running 0x0000/0xffff patterns all the time) we had sever clock troubles. I troubleshooted it by doing a TDT, using the reciprocity and sensing the coupling to suspected lines. It was the bending where the fault was. For the next layout time we had clearence around that clockpaths bendings and the problem have not been seen since! A balanced ECL signal is still a small signal compared to the full swing of 3.3V CMOS and with the high current rate thus drivers where set for. Balancing alone isn't a magical bullet, it is a tool among others but still needs to be handled with respect. Also, that's why you have a TDR/TDT capable scope collecting dust in the lab. When you need it, you bloody well need THAT scope. It has payed its price many times over. Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
