Common interfaces

Overview

In this directory, we provide commonly used interfaces used to construct testbenches for DV. These interfaces are instantiated inside tb module for connecting dut signals. They are described in detail below.

clk_if

This is a passive clock interface that is used to wait for clock events in testbenches. This interface has two clocking blocks, cb and cbn, for synchronizing to positive and negative clock edges, respectively. The interface also has the following tasks:

  • wait_clks: waits for specified number of positive clock edges
  • wait_n_clks: waits for specified number of negative clock edges

clk_rst_if

Unlike clk_if, this interface can generate a clock and a reset signal. These are connected as inout signals and the interface observes them passively unless the set_active function is called.

Just like clk_if, this interface has clocking blocks cb and cbn, together with wait_clks and wait_n_clks utility tasks. It also has

  • wait_for_reset: wait for a reset signalled on rst_n

To generate a clock signal, call set_active at the start of the simulation. This is typically called from an initial block in the testbench. To configure the frequency and duty cycle of the generated clock, use the following functions:

  • set_freq_mhz / set_freq_khz: set the clock frequency in MHz / KHz. This is 50MHz by default.
  • set_period_ns: set the clock period in nanoseconds. This is 20ns by default (giving a clock period of 50MHz).
  • set_duty_cycle: set the duty cycle (as a percentage: 1 - 99). This is 50 by default.

The clock can also have jitter added. This is generated as an offset in picoseconds added to randomly selected clock half-periods. It can be enabled and configured with:

  • set_jitter_chance_pc: set the percentage probability of adding a jitter to a given half-period. By default, this is 0 and the clock has no jitter.
  • set_max_jitter_ps: set the maximum jitter to add to each clock half-period in picoseconds. This is 1000ps (1 ns) by default.

To start and stop the clock or apply a reset, use the following tasks. These will have no effect if set_active has not been called.

  • start_clk: start the clock. The clock is started by default, so this task is only needed after a call to stop_clk.
  • stop_clk: stop / gate the clk
  • apply_reset: signal a reset on rst_n. The length of this reset and whether it is synchronous or not can be configured with arguments to the function.

pins_if

This parameterized interface provides the ability to drive or sample any signal in the DUT.

interface pins_if #(
  parameter int Width = 1
) (
  inout [Width-1:0] pins
);

By default, it behaves as a passive interface. The values of the pins can be read with the following functions:

  • sample: sample and return all the pin values
  • sample_pin: sample just the given pin

The interface can also be configured to drive, pull up, or pull down its outputs. To do this, call

  • drive / drive_pin: Drive the output to the given value.
  • drive_en / drive_en_pin: Configure output enable; when enabled, this drives value previously stored by a call to drive or drive_pin.
  • set_pullup_en / set_pullup_en_pin: Configure pull-up setting. If true and output enable is false, drives the output to 1.
  • set_pulldown_en / set_pulldown_en_pin: Configure pull-down setting. If true and both output_enable and pull-up are false, drives the output to 0.

The diagram below gives a schematic view of pins_if. The driver shown is replicated for each bit.

Block diagram