Timestamp recovery (timestamp_recovery)

Recovers the timestamp of each incoming sample by taking a periodic timestamp update message and incrementing it by the sample interval between updates.

Design

Recovers the timestamp of each incoming sample by taking a timestamp update message and incrementing it by the sample interval for each new sample received.

timestamp_[seconds/fraction]_out and sample_interval_[seconds/fraction]_out are valid at the point where data_valid_in is high. This means timestamp and sample rate can be registered at the same time as the input data to a component.

Implementation

The timestamp calculation is split over multiple clock cycles to avoid timing issues cause by the high resolution of the timestamp. The primitive is fully pipelined and asserts no backpressure. Once a timestamp update or sample interval update has occurred, a new timestamp can be generated on every rising edge of the module’s clock.

When a new timestamp is received the next data sample will have this timestamp. When a new sample interval is received the next data sample timestamp will be the last timestamp calculated using the old sample interval. Subsequent timestamps are advanced using the new sample rate. Users are advised that after a sample interval change a timestamp opcode should be sent for best results. This is due to the ambiguity of exactly when between the two samples the sample rate change occurred.

Interface

Generics

  • DATA_IN_WIDTH_G (integer): Sets the width of the data interface input.

  • ADDER_WIDTH_G (integer): Sets the size of the adders used to calculate to overall timestamp. 32 bits is recommended in most applications.

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.

  • enable (std_logic), in: Enable. Primitive enabled when high.

  • timestamp_valid_in (std_logic), in: data_in is valid and contains timestamp data when timestamp_valid_in is high.

  • sample_interval_valid_in (std_logic), in: data_in is valid and contains sample interval data when sample_interval_valid_in is high.

  • data_valid_in (std_logic), in: data_in is valid and contains sample data when data_valid_in is high.

  • data_in (std_logic_vector, DATA_IN_WIDTH_G bits), in: Input data to the timestamp recovery primitive.

  • timestamp_units_out (std_logic_vector, 32 bits), out: Contains the integer part of the timestamp. Valid when data_valid_in is high.

  • timestamp_fraction_out (std_logic_vector, 64 bits), out: Contains the fractional part of the timestamp. Valid when data_valid_in is high.

  • sample_interval_units_out (std_logic_vector, 32 bits), out: Contains the integer part of the sample interval. Valid when data_valid_in is high.

  • sample_interval_fraction_out (std_logic_vector, 64 bits), out: Contains the fractional part of the sample interval. Valid when data_valid_in 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 timestamp_recovery are:

  • ADDER_WIDTH_G must be a factor of the full width of the timestamp (96 bits).

  • DATA_IN_WIDTH_G must be a factor of ADDER_WIDTH_G.