Truncating Rounder (rounding_truncate)

Truncation, or Floor rounding for a static width signed input.

Design

The input stream is arithmetically right-shifted by binary_point, truncating the integer and discarding all fractional bits, then truncated from above (losing the most-significant bits of the input data), to the output width.

In addition it is possible to enable saturation, which clips the output if the incoming value has a larger fixed point magnitude, than that which can be expressed within the outputs number of bits.

Implementation

The primitive is designed such that a new sample can be inserted into the primitive on every rising edge of clk. The data_valid_in signal allows each input sample to be marked as valid or not. The pipeline advances every rising edge of clk regardless of if valid data is passed to the primitive or not. The data_valid_out signal is a delayed version of data_valid_in that indicates which outputs were calculated based on valid inputs.

The clk_en signal provides a clock enable for the primitive. When set low no data will enter or leave the primitive and all calculations are stopped. When set high the module will operate normally.

Interface

Generics

  • input_width_g (integer): Sets width of data_in signal.

  • output_width_g (integer): Sets width of data_out signal.

  • saturation_en_g (boolean): Enables saturation on the data when an overflow has occurred as a result of the resolution changing from input to output.

Ports

  • clk (std_logic), in: Clock. Inputs and outputs registered on rising edge.

  • reset (std_logic), in: Reset. Active high, synchronous with rising edge of clock.

  • clk_en (std_logic), in: Clock Enable. If low the module will not operate.

  • data_in (signed, input_width_g bits), in: Input value for primitive.

  • data_valid_in (std_logic), in: data_in is valid when data_valid_in is high.

  • binary_point (integer), in: Number of fractional bits in the least significant bits of the data_in input.

  • data_out (signed, output_width_g bits), out: Output value for primitive.

  • data_valid_out (std_logic), out: data_out is valid when data_valid_out is high.

Dependencies

The dependencies to other elements in OpenCPI are:

  • None.

There is also a dependency on:

  • ieee.std_logic_1164

  • ieee.numeric_std

Limitations

Limitations of rounding_truncate are:

  • A small negative bias will be introduced when using this primitive.

  • If this causes an issue convergent rounding may be used instead using the Half Even Rounder Primitive.