verilog-basics-for-lowlevel

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Verilog basics skill for firmware and kernel engineers. Use when reading RTL to understand hardware behavior, reset/clock domains, bus protocols, or collaborating with hardware teams. Activates on queries about Verilog basics, RTL, hardware description, clock domain, reset synchronizer, or MMU in hardware.

3installs

Sourcemohitmishra786/low-level-dev-skills

Added on2026-10-01

NPX Install

npx skill4agent add mohitmishra786/low-level-dev-skills verilog-basics-for-lowlevel

SKILL.md Content

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Verilog Basics for Low-Level Engineers

Purpose

Give firmware and kernel engineers enough Verilog/SystemVerilog literacy to read RTL: modules, clocks/resets, combinational vs sequential logic, bus interfaces, and why hardware behavior explains driver bugs — not a replacement for HDL design courses.

When to Use

Reference manual unclear — RTL clarifies register behavior
Understanding CDC (clock domain crossing) bugs
Correlating DMA/AXI transactions with driver ordering
Reviewing SoC block diagram with hardware team

Workflow

1. Module structure

verilog

module uart_tx (
    input  wire       clk,
    input  wire       rst_n,   /* active-low async reset */
    input  wire       start,
    input  wire [7:0] data,
    output reg        busy
);
    /* sequential logic */
    always @(posedge clk or negedge rst_n) begin
        if (!rst_n)
            busy <= 1'b0;
        else if (start)
            busy <= 1'b1;
        /* ... */
    end
endmodule

wire

= combinational/network;

reg

in

always

block = flip-flop unless combinational

always @(*)

.

2. Synthesizable subset (what firmware folks need)

Construct	Meaning
`posedge clk`	Registered update
`assign x = a & b`	Combinational
`case` / `if` in `always @(*)`	Mux logic
Parameters `#(.WIDTH(32))`	Configurable width

Avoid

#delay

in synthesizable RTL — simulation only.

3. Reset discipline

Async assert, sync deassert common (
```
rst_sync_n
```
)
Firmware must wait post-reset setup times — see RM reset chapter
Multiple reset domains → peripheral may need explicit soft reset bit

4. Bus protocols (reading SoC diagrams)

Bus	Typical use
APB	Slow peripherals, simple reg interface
AHB	Higher throughput on-chip
AXI	DMA, modern SoCs — bursts, channels

Linux

regmap

MMIO maps to APB/AXI slave decode in RTL address map.

5. Clock domains

Signals crossing

clk_a

→

clk_b

need synchronizers (2+ FFs). Metastability causes intermittent firmware bugs — not fixed in software alone.

6. Simulation vs silicon

bash

# Typical open-source sim (conceptual)
iverilog -o sim.vvp design.v tb.v
vvp sim.vvp

QEMU/peripheral models may not match RTL edge cases.

7. Agent usage

/verilog-basics-for-lowlevel Explain this APB register block and when STATUS bit updates relative to WRITE

Common Problems

Symptom	Cause	Fix
Bit toggles once	Pulse in RTL	Poll latch / clear-on-read in driver
Random corruption	CDC	Hardware synchronizer; don't hack delays
Read stale data	Bus bridge buffer	Follow RM ordering / barrier
IRQ stuck	Level vs pulse in RTL	Match handler ACK sequence
Verilog vs VHDL	Mixed SoC docs	Focus on interface signals table

Related Skills

skills/baremetal/mmio-and-bit-manipulation

— register access from C

skills/baremetal/peripherals-from-datasheet

— RM ↔ RTL

skills/baremetal/datasheet-and-refmanual-reading

— doc navigation

```
skills/kernel-dev/device-tree
```
— hardware integration in Linux
```
skills/qemu/protocol-analysis
```
— validate bus timing