While vhdMMIO's field types and interrupts are already quite powerful on their own, their full power is only truly unlocked when they are used together. For instance, you may want to trigger an interrupt when a bus write occurs to an MMIO-to-stream field while the stream is blocked, to notify the software that it did something wrong. To support this and more without needing some external boilerplate logic (after all, the goal is to reduce such boilerplate logic as much as possible!), vhdMMIO supports specification of custom "internal" signals.

An internal signal is automatically inferred when its name is referenced by an event source or sink. The name acts as a unique identifier, so specifying the same name twice will result in the components being tied together.

Internal signals can be scalars (std_logic) or vectors (std_logic_vector), and can be level-based with a single driver, or strobe/event-based with one or more event sources (sometimes called strobers). The kind of internal signal that's associated with a name is usually implied by context. Of course, all endpoints that refer to an internal must agree to its kind, or vhdMMIO will send an error message your way.

Sometimes, you may want to use or drive an data source or sink outside the register file. For instance, there may be a page register outside the register file that you want to use to select which page of registers is accessible. That's where this configuration structure comes in: it lets you associate a port with an internal signal, essentially making it an external signal.

This structure supports the following configuration keys.

This key specifies what kind of I/O port should be made for the internal signal specified by internal.

The following values are supported:

• input: an input port is generated for the internal. This must be the internal signal's only driver.

• strobe: an strobe input port is generated for the internal. The internal can be driven by other strobe sources as well; the result is wired-or.

• output: an input port is generated for the internal.

This key is required.

This key specifies the name and shape of the internal signal.

The following values are supported:

• a string matching [a-zA-Za-z][a-zA-Z0-9_]*: a port is generated for a scalar internal with the given name.

• a string matching [a-zA-Za-z][a-zA-Z0-9_]*:[0-9]+: a port is generated for a vector internal with the given name and width.

This key is required.

This key specifies the name of the port.

The following values are supported:

• null (default): the port is named after the internal signal.

• a string matching [a-zA-Za-z][a-zA-Z0-9_]*: the specified name is used for the port, regardless of the name of the internal signal.

This key is optional unless required by context. If not specified, the default value (null) is used.

The I/O port for the internal signal can optionally be grouped along with other ports in a record. This key specifies the name of the group record.

The following values are supported:

• null (default): port grouping is determined by the global default.

• no: the port is not grouped in a record.

• a string matching [a-zA-Z][a-zA-Z0-9_]*: the port is grouped in a record with the specified name.

This key is optional unless required by context. If not specified, the default value (null) is used.