bit-dojo: learn bitwise register tricks on a CH32V003

A free, interactive game for learning the bitwise operations embedded C is built on. Write real register code, such as GPIOC->CFGLR |= (GPIO_Speed_10MHz | GPIO_CNF_OUT_PP) << (4*1);, against a simulated CH32V003 RISC-V microcontroller, and see every bit change.

What you practise

• Setting, clearing, toggling and testing bits with |=, &= ~, ^= and &
• Writing and reading multi-bit register fields with masks and shifts
• Real CH32V003 registers: RCC clock enables, GPIO CFGLR, OUTDR, INDR and BSHR
• Classic embedded C bugs: clobbering with =, operator precedence, signed shifts and undefined behaviour
• An animated step-by-step visualizer that shows how every operator works in binary

24 levels in 5 chapters

Bits & hex

1. 1. One bit — Write an expression with only bit 5 set.
2. 2. A nibble mask — Write a mask with bits 4, 5, 6 and 7 set — nothing else.
3. 3. Everything but — Write a value with every bit set except bits 4-7.
4. 4. Place a field — Put the 4-bit value 0b1001 into bits 8-11 (everything else 0).

Set · clear · toggle · test

1. 5. Set a bit — Set bit 3 of reg. Leave every other bit alone.
2. 6. Clear a bit — Clear bit 7 of reg. Leave every other bit alone.
3. 7. Toggle a bit — Flip bit 0 of reg: 0 → 1, 1 → 0. Others unchanged.
4. 8. Test a bit — Write an expression that is true (non-zero) exactly when bit 2 of reg is 1.
5. 9. Two at once — Set bits 1 and 6 of reg, and clear bit 0. Others unchanged.

Fields

1. 10. Clear a field — Clear bits 4-7 of reg (make them 0000). Others unchanged.
2. 11. Write a field — Make bits 4-7 of reg hold the value 9 (0b1001). Others unchanged.
3. 12. Read a field — Store the value of bits 8-11 of reg into val (as a number 0-15).

Real CH32V003 registers

1. 13. Wake up port C — Turn on the clock for GPIOC. Don't disturb the other peripherals.
2. 14. PC1 as output — Configure PC1 as a 10 MHz push-pull output. Other pins keep their setup.
3. 15. LED on with BSHR — Drive PC1 high using GPIOC->BSHR. Other outputs unchanged.
4. 16. LED off with BSHR — Drive PC1 low using GPIOC->BSHR. Other outputs unchanged.
5. 17. From reset to light — Starting from reset: light the LED on PC1. Don't break anything else.
6. 18. Button input — Make PD2 an input with the internal pull-up enabled.
7. 19. Is it pressed? — Write an expression that is true when the button on PD2 is pressed.

Bug hunt

1. 20. Where did the UART go? — This enables GPIOC but kills the serial port. Fix it.
2. 21. The LED won't light — PC1 should be a 50 MHz push-pull output. The LED stays dark. Fix it.
3. 22. Never pressed — This should be true when PD2 is pressed, but it never is. Fix it.
4. 23. Silent port — Perfect-looking code, dark LED. Fix it.
5. 24. Bit 31 — Set bit 31 of reg — without undefined behaviour.

Open the sandbox to experiment with a whole simulated chip.