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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 signaled 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_ps: set the clock period in picoseconds. This is 20_000ps 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

entropy_subsys_fifo_exception_if

The IPs in the entropy subsystem (entropy_src, CSRNG and EDN) raise fatal alerts if they detect that data is lost in one of the intermediate FIFOS, either through:

  • Write errors (Overflow) wvalid_i asserted when full
  • Read errors (Underflow) rready_i asserted when empty (i.e. rvalid_o == 0)
  • State errors: Anomalous behavior rvalid_o deasserted (FIFO is not empty) when full

Furthermore some instances of prim packer fifo's also raise recoverable alerts if firmware tries to write data to it when it is not ready.

The events described above need not be treated as errors in general purpose designs. In many general designs where the fifo is capable of applying backpressure (stalling the read or write inputs) these do not have to be an error. One could in principal only signal an alert if it is observed that data was indeed lost (e.g., if wvalid_i is deasserted or data changes before wready_o is high). However most of the entropy complex fifo stages do not respond to such backpressure, and thus the conditions above do indicate a loss of data.

The entropy_subsys_fifo_exception_if.sv has pins that can map either to synchronous FIFOs or to packer fifos. Though some interface ports will be unused in any case the interface parameter IsPackerFifo indicates which ports to use (True for packer FIFOs or False for synchronous FIFOs.

The interface then generates error pulses in the mon_cb clocking block under the signal mon_cb.error_pulses, which has one line per possible error condition. The enumeration entropy_subsys_fifo_exception_pkg::fifo_exception_e lists the types of exceptions and provides the mapping to the corresponding error pulse line.