Fields with stream-to-mmio behavior interface with an incoming AXI4 stream. When the incoming AXI4 stream is valid and the internal register for the field is not, the stream is handshaked and the data is put in the register. The MMIO bus can then read from the field to fetch the data, automatically invalidating the internal register to let the cycle repeat. The field cannot be written by the bus.

By default, the only way for software to know whether data is waiting in the internal holding register is to read and compare with zero, which is always what's returned for an empty holding register. This is of course not ideal at best. vhdmmio provides several options for doing this better, which require a bit more work:

• Set bus-read to valid-wait. In this case, reads will always return valid data because they are blocked until data is available. This is the simplest method, but reading from a stream that isn't going to send anything will deadlock the whole bus.
• Set bus-read to valid-only. In this case, a read from an empty holding register yields a slave error. This is very simple from vhdmmio's standpoint, but requires the bus master to actually support AXI4L error conditions in a convenient way.
• Drive an internal signal with the status of the holding register (full-internal or empty-internal), and monitor it with a status field (internal-status behavior) and/or an internal interrupt.
• Strobe an internal signal when an invalid bus read occurs using underrun-internal and check whether an underrun occurred after the fact using a status field (internal-flag behavior) and/or an internal interrupt.

Finally, vhdmmio allows you to set the reset value of the internal register to a valid value. This effectively imitates a stream transfer, which may be used to start some loop based on sending stream transfers back and forth between systems.

This structure supports the following configuration keys.

Configures what happens when a bus read occurs.

The following values are supported:

• enabled (default): reads from an empty holding register return 0.

• valid-only: reads from an empty holding register return a slave error.

• valid-wait: reads from an empty holding register are blocked until data is received from the stream.

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

Configures the reset value.

The following values are supported:

• null (default): the internal data register resets to 0, with the valid flag cleared.

• no: the internal data register resets to 0, with the valid flag set.

• yes: the internal data register resets to 1, with the valid flag set.

• an integer: the internal data register resets to the given value, with the valid flag set.

• generic: the reset value is controlled through a VHDL generic.

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

Configures driving an internal signal high when the internal data register is valid. This essentially serves as a holding register full signal for stream interface fields.

The following values are supported:

• null (default): the feature is disabled.

• a string matching [a-zA-Z][a-zA-Z0-9_]*: an internal signal with the given name is created (if necessary) and driven by the internal valid register of this field.

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

Configures driving an internal signal high when the internal data register is invalid. This essentially serves as a holding register empty signal for stream interface fields.

The following values are supported:

• null (default): the feature is disabled.

• a string matching [a-zA-Z][a-zA-Z0-9_]*: an internal signal with the given name is created (if necessary) and driven by the one's complement of the internal valid register of this field.

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

Configures strobing an internal signal when a bus read occurs while the stored value is invalid. This is equivalent to an underflow condition for stream to MMIO fields. It is intended to be used for underflow interrupts.

The following values are supported:

• null (default): the feature is disabled.

• a string matching [a-zA-Z][a-zA-Z0-9_]*: an internal signal with the given name is created (if necessary) and strobed when a bus read occurs while the internal valid signal is cleared.

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