The CMSIS-SVD format is based on XML and was influenced by IP-XACT. Due to the much wider scope and complexity of IP-XACT, it was decided to specify a separate format focused and tailored towards the description of the programmer's view of a device.
CMSIS-SVD XML Hierarchy
CMSIS-SVD Hierarchy Levels
One CMSIS-SVD file contains the description of a single device. A device consists of a processor and at least one peripheral. Each peripheral contains at least one register. A register may consist of one or more fields. The range of values for a field may be further described with enumerated values.
- Device Level: The top level of a System View Description is the device. On this level, information is captured that is specific to the device as a whole. For example, the device name, description, or version. The minimal addressable unit as well as the bit-width of the data bus are required by the debugger to perform the correct target accesses.
Default values for register attributes like register size, reset value, and access permissions can be set for the whole device on this level and are implicitly inherited by the lower levels of the description. If however specified on a lower level, the default setting from a higher level will get overruled.
- Peripherals Level: A peripheral is a named collection of registers. A peripheral is mapped to a defined base address within the device's address space. A peripheral allocates one or more exclusive address blocks relative to its base address, such that all described registers fit into the allocated address blocks. Allocated addresses without an associated register description are automatically considered reserved. The peripheral can be assigned to a group of peripherals and may be associated with one or more interrupts.
- Registers Level: A register is a named, programmable resource that belongs to a peripheral. Registers are mapped to a defined address in the address space of the device. An address is specified relative to the peripheral base address. The description of a register documents the purpose and function of the resource. A debugger requires information about the permitted access to a resource as well as side effects triggered by read and write accesses respectively.
- Fields Level: Registers may be partitioned into chunks of bits of distinct functionality. A chunk is referred to as field. The field names within a single register must be unique. Only architecturally defined fields shall be described. Any bits not being explicitly described are treated as reserved. They are not displayed in the System Viewer and are padded in the bit fields of the device header file. The case-insensitive field named "reserved" is treated as a keyword and each field with this name is ignored.
- Enumerated Values Level: An enumeration maps an unsigned integer constant to a descriptive identifier and, optionally, to a description string. Enumerations are used in C to enhance the readability of source code. Similarly, it can be used by debuggers to provide more instructive information to the programmer, avoiding a lookup in the device documentation.
- Vendor Extensions: The CMSIS-SVD format includes a section named vendorExtensions positioned after the closing tag peripherals. This allows silicon vendors and tool partners to innovate and expand the description beyond the current specification.
Multiple Instantiation
CMSIS-SVD supports the reuse of whole sections of the description. The attribute derivedFrom for the peripheral-, register-, and field-section specifies the source of the section to be copied from. Individual tags can be used to redefine specific elements within a copied section. In case the name of the description source is not unique, the name needs to be qualified hierarchically until the element composite name becomes unique. Hierarchies are separated by a dot. For example, <peripheral name>.<register name>.<field name>.
Array of Elements
A powerfull construct in data structures of the C programming language is the array. An array is a series of data elements of the same type selected via an index. CMSIS-SVD supports arrays of <peripherals>, <cluster> and <register>.
Peripheral Grouping
Peripherals that provide similar functionality (Simple Timer, Complex Timer) can be grouped with the element groupName. All peripherals associated with the same group name are collectively listed under this group in the order they have been specified in the file. Collecting similar or related peripherals into peripheral groups helps structuring the list of peripherals in the debugger.
Descriptions
On each level, the tag description provides verbose information about the respective element. The description field plays an important part in improving the software development productivity as it gives instant access to information that otherwise would need to be looked up in the device documentation.
