On Wed, 2006-03-22 at 10:09 -0500, Jack Carroll wrote: > On Wed, Mar 22, 2006 at 01:17:57PM +0100, Hans Kristian Rosbach wrote: > > > > Also, would not these pins be less vulnerable to noise than the > > ones at the other far end, thus maybe routing would not need to be > > so strict? After all, the more bits you get, each one will have less > > and less actual impact on the image. So my logic is that the first > > bit toggles a color value of 0/128, so it is absolutely critical to > > get this one right. But the 9'th bit toggles 0/0.5 so some deviance > > probably would not be noticed unless it's more wrong than right. > > In an analog situation, that would often be true. For instance, in > the design of a simple DAC ladder network, more tolerance error and ringing > is acceptable on the lower bits than on the higher ones. > This is a digital situation, though. When the clock edge strobes > the data word into the DAC registers, each bit has either settled at the > register input or it hasn't, so after the clock the bit will be either > completely correct in the register or completely wrong. So every bit's > track on the board must meet the appropriate design rules for impedance, > length, and stray reactive loading. Also, the very high clock rate makes > the transmission line delay through the data and clock tracks a significant > item in the total timing budget. It's necessary to account for the skew > between the clock edge and each bit, when calculating timing margins. It > would almost certainly be necessary to make every bit line the same length, > or close to it, then adjust the delay in the clock line to fit. The signals > coming the shortest distances would require some extra bends so that the > transmission line delay is the same as the signals coming from more distant > pins.
If I had only thought a little more I would have remembered that line length is crucial, thanks for reminding me. Regarding the last bit being correct or not in the digital domain, that is actually what I meant. Do we really care if it gets set to the wrong value if it's for example less than 10% of the time? Not having it at all would possibly be wrong 50% of the time. Would it not be better? -HK _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
