Hi Richard, Did you add snap blocks to the counters prior to the xaui links, triggered on a sync, to ensure that the ibobs are indeed synchronized?
- Andrew On 8/28/09 2:02 AM, "Richard Armstrong" <[email protected]> wrote: > Dear CASPERites, > > I've been trying for some time to synchronise two XAUI streams from two > separate IBOB boards in a BEE2 board. I've not met with much success. I'd be > really grateful any ideas or help from anyone who may have experience with > this, or perhaps a better general understanding of the issues I'm > encountering. > > The test system I've got is as follows: I'm clocking my IBOBs synchronously > and aligning them with a 1PPS input. Each IBOB has a 31-bit counter which is > reset off a 'sync' pulse. The sync pulse is generated by a circuit that is > aligned with the 1PPS signal. I'm sending the counter value multiplexed with > the sync in a demux-by-2 XAUI block to the BEE2. At the BEE2, I'm testing the > alignment of the counters. > > In my mind, as long as the ibob data rate is low enough, the XAUI interface > should provide complete clock-domain isolation. That is, the bee2 should be > able to be clocked at any rate, including a lower rate than the ibob > (provided, again that the data transfer rate is low enough). > > Worryingly, and unexpectedly, I find the counter values way out of alignment, > and perhaps worse than this, there seems to be jitter on the difference > between the counter values from cycle to cycle. Andrew and Peter helped me out > for a couple of days on their way to Paris. We corrected some of the design > errors (namely, selecting demux-by-2 for 32-bit data), made sure the ibob > clock was running slower than the bee2 clock, and changed the length of the > sync pulse. However, we were unable to resolve the fundamental issues. > > This week I've been trying to clock the BEE2 off an IBOB-generated clock with > an LVTTL-to-LVPECL clock driver. I changed the BEE2 design and changed a > jumper to select user-clock on the BEE2 board. Unfortunately, this doesn't > seem to help much (although, I could be doing it wrong) and I'm finding with > this scheme that the BEE2 has the tendancy to kill the running process after a > few seconds to minutes. > > I know many of the more experienced CASPERites advise asynchronous designs for > this reason. However, I'm pretty sure someone else must have disregarded this > advice (or perhaps discovered this advice) by pursuing XAUI synchronisation. > Were these same issues seen? What's the best clocking scheme in practice > (fully synchronous or downstream boards clcoked faster)? Does anyone have any > ideas on jitter? > > Thanks, > > Richard
