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carmfetchcortex-a8databus
Thomas Matthews
Thomas Matthews

Reputation: 57718

ARM - Fetching 32 bits of data using 16-bit data bus

On our embedded platform, some devices have a 16-bit data bus (such as the FPGA). However, we need to read them as 32-bit values (such as 32-bit floating point).

My understanding is that when the 16-bit memory is accessed by a 32-bit data load instruction (LDR), the ARM processor will perform 2 16-bit fetches to assemble a 32-bit quantity.

Is this correct?

For example, I would like the ARM processor to load a uint32_t value from the device with 16-bit data bus without having to make 2 uint16_t reads then combine the uint16_t values into one uint32_t variable.

Do we need to explicitly fetch as 2 16-bit quantities and then assemble as a 32-bit quantity?

What ARM documents contain this clarification?

Are there any specific ARM configuration settings for the ARM to make 2 fetches from a 16-bit data bus to form a 32-bit quantity (in a register)?

FYI: We are using an ARM Cortex A8 and the IAR EW IDE/Compiler.
The platform is running is "system" mode and not "thumb" mode.

Upvotes: 2

Views: 1954

Answers (1)

Dric512
Dric512

Reputation: 3729

If you are accessing memory, when the CPU required 32-bit of memory, it will request it directly (Cortex-A8 does not support 16-bit busses). Some peripheral (In the system) is responsible for converting the 32-bit request to 16-bit to adapt to the bus (Doing 2 transfers).

It means that for the CPU is should be totally transparent.

Upvotes: 0

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