This behavior can be seen as both the inverse of a flag and as an extension of strobe: the bits in the field are set by software writing a one to them, and cleared when acknowledged by hardware. They can be used for requests that cannot be handled immediately. By default, software can use an MMIO read to determine whether a command has been acknowledged yet, but this can be disabled to make the field write-only.

This structure supports the following configuration keys.

Configures what happens when a bus read occurs.

The following values are supported:

• enabled (default): normal read access to field, ignoring valid bit.

• error: reads always return a slave error.

• disabled: read access is disabled.

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:

• no (default): 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 (no) is used.

Controls the existence of the ctrl_clear control input signal. When this signal is asserted, the internal data register is cleared. The valid flag is not affected.

The value must be a boolean (default no).

This key is optional unless required by context. If not specified, the default value (no) is used.

Controls the existence of the ctrl_reset control input signal. When this signal is asserted, the field is reset, as if the register file reset input were asserted.

The value must be a boolean (default no).

This key is optional unless required by context. If not specified, the default value (no) is used.

Controls the existence of the ctrl_bit_clear control input signal. This signal is as wide as the field is. When a bit in this input is high, the respective data bit is cleared.

The value must be a boolean (default yes).

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

Configures strobing an internal signal when a bit-set operation to a bit that was already set occurs. This essentially serves as an overflow signal for flag 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 strobed when a bit-set operation occurs to an already-set bit.

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

Configures strobing an internal signal when a bit-clear operation to a bit that was already cleared occurs. This essentially serves as an underflow signal for flag 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 strobed when a bit-clear operation occurs to an already-cleared bit.

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