Learn the bitwise tricks embedded C runs on by writing real register code for a simulated CH32V003.
▶ Play it in your browser

You type C, like GPIOC->CFGLR |= (GPIO_Speed_10MHz | GPIO_CNF_OUT_PP) << (4*1);. bit-dojo runs it against the chip model and shows every bit before and after. An LED on PC1 lights up when your code is actually right.

Step-by-step visualizer: every operator animated in binary	Bit-level grading: = instead of |= wipes 29 bits
Sandbox: the whole chip, real TSSOP-20 pinout	Works on phones

• 24 levels in 5 chapters: bits & hex → set/clear/toggle/test → fields → real CH32V003 registers → bug hunt.
• Random test cases. Each level runs your code against several starting states. Code that only works for one lucky value fails, e.g. using = where you meant |=.
• A real C subset. It follows C precedence rules, uses 32-bit int/unsigned semantics like RV32, and handles casts, if/else, while/for/do, break/continue and block-scoped locals. int main(void) { … } and #include lines are accepted, so lesson code pastes in. It flags undefined behaviour such as 1 << 31 or a shift count of 32 or more.
• Hardware behaviour, modelled:
  • peripherals with their clock off ignore writes and read back 0
  • BSHR/BCR are write-only set/reset registers
  • INDR is read-only
  • reserved bits get dropped
  • IN_PUPD inputs use the OUTDR bit to pick pull-up or pull-down
  • a button on PD2 pulls the pin to GND
• "What you need" panel on every level. It's generated from the level's own checks. The checks are run over about 40 starting states, and each bit is marked as: must be 1, must be 0, flip, keep, don't care, or copied from another bit. There's also a plain-English sentence, with CFGLR pin slots decoded (e.g. PC1 → out · push-pull · 10MHz). A new level gets this panel without writing anything extra.
• Step-by-step visualizer. Every operator becomes one animated step. For <</>> the bit strip slides over while zeros (or copies of the sign bit) come in and dropped bits fall off. For & | ^ ~ the result is built one column at a time, with the truth-table row lit up. -x is shown as ~x + 1, and reg |= x is shown as reg = reg | x followed by the store. It's available in the Sandbox evaluator (press Enter), after Sandbox runs, and on every level's test case.
• Sandbox with live loops: the whole chip. Put setup code first and while (1) { … } after it, and the loop keeps running until you press Stop. Delay_Ms() really waits, so blink loops blink, and clicking the board's button changes PD2 while the code runs, so "button toggles the LED" works. It comes with example programs, clickable bits and an expression evaluator.
• Cheat sheet: the idioms, the macro values and C operator precedence.

Macro values and register layouts are copied from ch32fun's ch32v003hw.h.

Needs Node 20+. pnpm runs through npx so the machine's global pnpm stays untouched:

npx -y pnpm@11 install
npx -y pnpm@11 dev        # http://localhost:5173
npx -y pnpm@11 test       # C semantics, the chip model, and every level's solution/starter
npx -y pnpm@11 build      # static site in dist/

dist/ is plain static files with relative paths, so any static host works. Pushing to main deploys to GitHub Pages through .github/workflows/pages.yml. The absolute URLs used for the canonical link, Open Graph and sitemap.xml come from SITE_URL, which defaults to https://bit-dojo.elabins.com/. The custom domain is set by public/CNAME. Override it with SITE_URL=https://example.com/ npx -y pnpm@11 build.

src/core/lexer.ts    tokenizer (hex/bin/octal literals, u suffix, comments)
src/core/parser.ts   recursive-descent parser, C precedence table
src/core/interp.ts   evaluator: int vs unsigned, UB detection
src/core/chip.ts     CH32V003 model: RCC + GPIOA/C/D, pins, board
src/core/levels.ts   the curriculum
src/core/runner.ts   grades code against each level's cases
src/core/live.ts     runs sandbox programs live (Delay_Ms waits, while (1) keeps going)
src/core/goal.ts     derives each level's per-bit target from its checks
src/ui/explain.ts    animated step-by-step visualizer (reads the interpreter's trace)
src/ui/*             editor, register view, board SVG, cheat sheet

Add an object to LEVELS in src/core/levels.ts. cases returns starting states, and expect returns the registers or variables to check (with an optional mask) or custom checks. The test suite fails if the solution doesn't pass every case or if the starter passes by accident.

MIT