Similar to flag fields, counters are used to signal events from hardware to software. However, counters allow multiple events occurring between consecutive software read cycles to be registered by counting instead of bit-setting. Like flag, software should use fields of this type by reading the value and then writing the read value to it in order to avoid missing events; the write operation subtracts the written value from the internal register.

When a counter overflows, it simply wraps back to zero. Similarly, if a counter is decremented below zero, it wraps to its maximum value. Optionally, overflow-internal and underflow-internal can be used to detect this condition, in conjuntion with an internal-flag field and/or an internal interrupt.

This structure supports the following configuration keys.

Configure the existence and behavior of the hardware read port.

The following values are supported:

• disabled (default): no read port is generated.

• simple: only the data output is generated.

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

Configure the existence and behavior of the hardware write port.

The following values are supported:

• disabled (default): no write port is generated.

• enabled: a record consisting of a write enable flag and data is generated.

• accumulate: like enabled, but the data is accumulated instead of written.

• subtract: like enabled, but the data is subtracted instead of written.

This key is optional unless required by context. If not specified, the default value (disabled) 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_increment control input signal. When this signal is asserted, the internal data register is incremented.

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.

Controls the existence of the ctrl_decrement control input signal. When this signal is asserted, the internal data register is decremented.

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.

Configures strobing an internal signal when the most significant bit of the internal register flips from high to low during an increment or accumulate operation. This essentially serves as an overflow signal for counter 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 an increment or accumulate operation causes the MSB of the data register to be cleared.

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

Configures strobing an internal signal when the most significant bit of the internal register flips from low to high during a decrement or subtract operation. This essentially serves as an underflow signal for counter 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 decrement or subtract operation causes the MSB of the data register to be set.

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