[tryp]

I totally agree about micros. They are wonderfully nice and tidy for small, well defined problems. I've had extremely satisfying success using them for communications glue, motor controllers and thermoelectric cooler controllers.

I'm in a chatty mood tonight, so I hope you'll excuse my indulgence of another few 'graphs on modern memory bus stuff I think is cool that I had to learn under product ship-date duress: Dynamic Termination.

Remember the high-school physics demonstration of the "reflected wave" where you grab one end of a rope that's tied off at the other end, give it a flick, watch the wave travel from your hand to the knot, then reflect back? Well kind of the same thing happens with voltage level on high speed data signals.

If your high school was fortunate enough in these (or those) fiscally troubled times to afford the equipment, the next demonstration tied off the same rope to some sort of damping spring that absorbed your wave's energy and didn't reflect it back at you. Those springs translate to "termination impedance" in electrical circuits.

So in order to avoid reflections, the address/control/clock lines of the memory bus "fly by" each and every DRAM IC until they reach the end of their path, where they "terminate" via resistors that prevent reflection to a voltage potential (half-way between 1- or 0- value potentials so that it's no more work to drive a 1 that 0 or vice-versa).

The data lines are wired directly from the memory controller to the DRAM chips so the DRAM chips have termination built-in to absorb the energy at the end of the line and prevent reflections.

Except that maybe there's more than one DIMM. DDR3 chips let you command them to connect or disconnect their termination resistors. So when you are accessing the closer-to-the-controller DIMM, the next one that's further out can apply termination. And when addressing the one that's furthest out, you disable on-die termination and rely on the actual resistor on the motherboard at the end of the bus.

My hard-learned lesson: the signals don't so so well when you get confused about which rank of chips are closer and which are further away.

[rbanffy]

I understand getting signaling to work in the frequencies we use is hard (when I was in college a lot of people doubted we could go above 50 MHz), but I can't shake the feeling the complexities we have on modern computers are part of a very elaborate Monty Python-esque prank to make our work look like magic to muggles... :-)

[analog31]

My school couldn't even afford the rope. ;-)

I remember reading about the PCI bus, which I think is the first time I heard of a mainstream digital system requiring serious transmission line design.

[dfox]

Actually, one of the selling points of PCI in contrast to competing standards was that it was significantly easier and cheaper in terms of transmission line design due to reflected wave switching (ie. no termination), this essentially means that only the steady state is relevant for signaling. While not good practice by any measure, this means that PCI bus does not have to be physically an linear bus, but works with arbitrary trees that does not contain excessively long paths (see for example aftermarket right-angle multi-slot PCI brackets for SFF and rackmount systems, that bus most of PCI signals for multiple slots together and only break out REQ#, GNT# and IDSEL through few wires to other original board slots).