Network On Chip and Bus Structures.

Transmitting data consumes energy and causes delay. Basic physical parameters:

• Speed of light on silicon and on a PCB is 200 metres per microsecond.
• A clock frequency of 2 GHz has a wavelength of 2E8/2E9 = 10 cm.
• Within a synchronous digital clock domain requires connections to be less than (say) 1/10th of a wavelength.
• Conductor series resistance further slows signal propagation, so need to register a signal in several D-types if it passes from one corner of an 8mm chip to the other!
• Can have several thousand wires per millimetre per layer: fat busses are attractive.
• DRAM is several centimeters away from the SoC and has significant internal delay.

Need to use protocols that are tolerant to registers (4P H/S degrades with reciprocal of delay).

But first let's revist the simple hwen/rwen system used in the `socparts' section.