14.6. Unifying the code buses

For some systems you might want to combine the processor core’s ICode and DCode buses into a single, unified Code bus. To support this for high-speed operation, the processor has the DNOTITRANS input which suppresses the HTRANSI line when HTRANSD becomes active. With DNOTITRANS asserted, if HTRANSI and HTRANSD are to be active simultaneously in corresponding single-cycle address phases, then only HTRANSD is asserted. The ICode transaction is waited internal to the processor. In other words, the external ICode bus is forced into an idle state. The two HTRANS signals are therefore guaranteed never to be simultaneously active, which permits the bus multiplexer to be a very simple device.

Note

DNOTITRANS is a static input that must be tied high to enforce this behavior.

The external ICode/DCode bus multiplexer can be integrated into a Cortex-M3 system as shown in Figure 14.1.

Figure 14.1. ICode/DCode multiplexer

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