3.1.2. Bus matrix

TheARM926PXP development chip bus matrix is implemented in a single module. It is constructed using two basic components:

Input stage

This is used to request access to the required slave port and to buffer the AHB address phase signals (for example, HADDR and HTRANS) when access to the slave port is not granted.

Output stage

This is used to arbitrate between the input stages that are requesting access to the slave port.

The interconnect between the various input and output stages define the structure of the bus matrix. For example, Figure 3.1 shows the implementation of the ARM926PXP development chip bus matrix.

Figure 3.1. Bus matrix configuration

The ARM926EJ-S PXP Slave Expansion AHB interfaces that are used have only one peripheral connected to them and the ARM926EJ-S PXP Master Expansion AHB interface has only one master connected to it. Consequently no additional AMBA infrastructure components are required.

The ARM Instruction, DMAC 1, DMAC 2 and LCD AHB interfaces from the PXP subsystem are unused. The ARM926EJ-S PXP memory map is modified in this implementation so that no additional peripherals can be accessed via these interfaces.

The multi-layer interconnections are optimized to remove any redundant connections. For instance, the CLCDC master is intended to fetch significant quantities of data from memory devices. The CLCDC cannot usefully fetch data from communications peripherals on the DMA APB bus. Therefore the DMA APB bridge is not connected as a slave on the CLCDC AHB.

Arbitration between the buses only occurs when two or more masters try to gain access to the same slave. The arbitration priority is fixed as:

  1. CLCDC AHB (highest priority).

  2. DMA0 AHB.

  3. Expansion Master AHB

  4. DMA1 AHB.

  5. ARM Data AHB.

  6. ARM Instruction AHB (lowest priority).

The slave arbitration is performed at the start of every transfer and on the quadword boundary of any incrementing burst.

The ARM926PXP development chip adds four AHB peripherals into the ARM926EJ-S PXP memory map:

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