How to Describe Bitfields That Span Multiple Registers in CMSIS-SVD

[bordeaux15]

I'm working on a project where bitfields can span multiple registers (i.e. 8-bit registers, 23-bit bitfields across 3 registers). Example below:

Bitfield_A is 23 bits across 3 8-bit registers Register_1, Register_2, Register_3
Register_1 contains Bitfield_A[7:0]
Register_2 contains Bitfield_A[15:8]
Register_3 contains Bitfield_A[22:16]

I don't see any where in the CMSIS-SVD 1.3.9 schema that can support this structure. The bitRange identifier is used only to specify a bitfields range within a register. But it cannot specify which bits of the bitfields are included in that bitRange. A couple questions:

• Is there a method to support these bitfields split across multiple registers that I'm not seeing?
• Is it possible to add a schema/tag into the bitfield definition that would highlight which bits of given bitfield occur in a given register?

[thorstendb-ARM]

Hi, does the register have a 32bit representation (means, all three (four) can be read as one 32bit register having the upper 8 bit "reserved")? In this case you can use both descriptions (1x 32bit, 3x 8bit) as alternate specification ("union concept").

[JonatanAntoni]

Please note that CMSIS-SVD spec has moved to https://github.com/Open-CMSIS-Pack/svd-spec.

[bordeaux15]

Hi @thorstendb-ARM,

The register doesn't have a 32 bit representation only the multiple 8 bit representations. I need to know the schema used for naming the LSB register starts with _1 and up to _3.

[JonatanAntoni]

Hi @bordeaux15,

I suggest to move this discussion to https://github.com/Open-CMSIS-Pack/svd-spec.
Perhaps raise another issue over there and describe the layout you want to achieve in C code for instance. Do you want to have access to the bitfield with a single read/write access? Or do you need three separate 8-bit accesses? Please take care that LSB/MSB definition depends on hardware representation.

Cheers,
Jonatan

[thorstendb-ARM]

Can you please give an example how you would describe this in C with struct and union?
Are the Registers 1-3 in ascending address space with or without gaps?
What is the access size and thew width of the registers? Are they 8 bit, or do they just contain 8 bit of relevant data inside 32 bit?

Note: SVDConv does currently not support virtual registers that gather information from (several) other registers. The tool is meant to reproduce the physical structure of the peripheral layout, which is then used e.g. for C headerfile generation or displaying tools.