Electronics Terminology: Crossbar
In microelectronics, a crossbar is a type of interconnect topology commonly used in microcontroller designs employing an on-chip bus or standardised communication protocol.

First, a bit of background: In a "System-on-chip" design, all functional modules communicate via an on-chip bus. In terms of their bus interaction, modules can be classified as:

In a typical "System-on-chip" design there are several masters and slaves, all connected together by the on-chip bus. A typical bus master might be a CPU, DMA engine or DSP module. These all initiate transactions such as instruction fetches or data transfers. A typical slave module might be a UART or a timer.

Let's get back to interconnect and the crossbar: Consider a simple example, where we have two modules acting as masters, ("M1" & "M2") and two acting as slaves, ("S1"& "S2"). Both bus masters can access either slave. Consider these simple scenarios:

  1. M1 attempts to access S1 - With no other master or slave activity, S1 repsonds to M1's request, (a read or write), and the transaction is completed.
  2. Both M1 and M2 attempt to access the same slave, (S1), at the same time - We now potentially have a bus contention, since S1 can only respond to one request at a time. A bus arbiter must be used to resolve this situation. In this case, perhaps the aribter is programmed such that bus master M1 gets priority over M2: M1's transaction request is dealt with first. Our simple interconnect is no longer just a set of wire: It has functionality!
  3. M1 attempts to access S2 at the same time as M2 attempts to access S1 - If an arbiter is being used to control bus transaction, then M1, say, will have priority over M2 and will dealt with first. If , on the other hand, the simple arbiter is replaced with a crossbar, then the simultaneous requests from M1 and M2 can be satisfied concurrently, since they are addressing seperate slaves.

Bottom line: A crossbar is a multi-pathed network switch. It increases system bandwidth at the expense of added complexity.

Cross"bar` (-b?r`), n.

A transverse bar or piece, as a bar across a door, or as the iron bar or stock which passes through the shank of an anchor to insure its turning fluke down.

Russell.

Crossbar shot, a projectile which folds into a sphere for loading, but on leaving the gun expands to a cross with a quarter ball at the end of each arm; -- used in naval actions for cutting the enemy's rigging.

 

© Webster 1913.

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