entropy_noise_source

Sample an asynchronous ring oscillator through a metastable flop into the system clock domain.

TOTAL_LENGTH and TAPPED_LENGTH size the RO; enable_i starts oscillation; detune_i high selects the full-length feedback path and low the shorter tap. sample_clk_i clocks the metastable sample; the bit is double-synchronized onto clk_i as noise_o.

Parameters

Table 1. Parameters
Name	Type	Default	Description
`TOTAL_LENGTH`	`int unsigned`	`17`	Full ring-oscillator stage count.
`TAPPED_LENGTH`	`int unsigned`	`13`	Stage count of the shorter feedback tap, used while detune_i is low.

Ports

Table 2. Ports
Name	Direction	Type	Description
`clk_i`	input	`logic`	System clock.
`rst_ni`	input	`logic`	Active-low asynchronous reset of the sample and synchronizer flops.
`sample_clk_i`	input	`logic`	Clock of the flop that samples the asynchronous ring output.
`enable_i`	input	`logic`	Enables the ring oscillator; while low its output is static.
`detune_i`	input	`logic`	High selects the ring's full TOTAL_LENGTH feedback path, low the shorter TAPPED_LENGTH tap.
`noise_o`	output	`logic`	Ring-oscillator sample, double-synchronized onto clk_i.