ARM Technical Support Knowledge Articles

Section 2.4. Accesses to the MMCI

Applies to: PL181 Multimedia Card Interface V2

Answer

Some of the registers are 32 bit. Is it possible to access these registers with 16 bit bus cycles? Also if this is possible in terms of the bus interface, are there any registers where this would lead to incorrect behavior of the interface?

It is not possible to access the registers in the MMCI with 16 bit bus cycles as the APB interface is a 32-bit interface. If an attempt is made to do so this may be unpredictable, depending on whether read or write accesses are being performed. If an attempt is made to carry out a 16 bit read access from a 32-bit peripheral on the APB, the APB will give out 32-bit data and it will then be up to the processor to deal with the 32-bits of data it gets back. If an attempt is made to perform a 16-bit write to a 32-bit APB register then the complete 32-bit register will be overwritten as the APB bridge cannot indicate which half is valid.

Article last edited on: 2008-09-09 15:47:25

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