A minimal, no-HAL, bare-metal game for STM32F401CCU6 that draws to a 128×64 SSD1306 OLED over I²C (bit-banged) and uses a single push button on PA0 to “flap” a 3×3 dot through moving pipes.

• Display: SSD1306 128×64, I²C address 0x3C (common)
• I²C pins (bit-banged): PA9 = SCL, PA10 = SDA
• Button: PA0 with internal pull-up (button shorts to GND when pressed)
• Clock: HSI 16 MHz (default)
• Timing: SysTick 1 ms tick for frame pacing (~30 fps target)

• STM32F401CCU6 board (a typical “Black Pill” style board)
• SSD1306 OLED (128×64) that supports I²C

I²C is a 2-wire, open-drain bus:

• SCL (clock) and SDA (data) are both open-drain outputs:
  • Devices can pull the line low, but never drive it high.
  • Pull-up resistors bring the lines high when nobody pulls low.
• Bus levels:
  • Low (0): actively pulled down by a device
  • High (1): released; the pull-up resistor lifts the line

Transfers:

1. START: SDA falls while SCL is high
2. Address + R/W (7-bit addr + 1 R/W bit)
3. ACK/NACK: receiver pulls SDA low to ACK the byte
4. Data bytes (each byte followed by ACK from receiver)
5. STOP: SDA rises while SCL is high

In this project:

• We bit-bang SCL/SDA using PA9/PA10 in open-drain output mode.
• i2c_start(): makes a START condition
• i2c_write_byte(): shifts out 8 bits MSB-first and reads the ACK bit
• i2c_stop(): makes a STOP condition
• SSD1306 requires a control byte before commands/data:
  • 0x00 = commands
  • 0x40 = data stream (display RAM)

SSD1306 addressing & pages:

• The 128×64 buffer is organized as 8 pages (each page = 8 vertical pixels × 128 columns).
• We keep a 1024-byte framebuffer in RAM and push it page-by-page.
• This is simple but means each frame writes 1024 bytes.

SysTick is a 24-bit core timer inside the ARM Cortex-M. You load a reload value; it counts down and interrupts periodically:

• We set reload to F_CPU/1000 - 1 so SysTick fires every 1 ms.
• Interrupt handler increments ms_ticks.
• delay_ms(n) spins until ms_ticks advances by n.
• The main loop runs at a target period (≈33 ms → ~30 FPS), paced by ms_ticks.

Why use it?

• Provides a stable timebase independent of code execution speed.
• Keeps physics & rendering smooth and consistent.

• RCC_AHB1ENR: we set bit 0 to enable the GPIOA clock.
  • Without clocks, peripherals don’t work (their registers won’t respond).

• MODER: 2 bits per pin
  • 00 input, 01 output, 10 alternate function, 11 analog
• OTYPER: output type
  • 0 push-pull, 1 open-drain
• OSPEEDR: output slew rate (we set high so edges are sharp)
• PUPDR: internal pull-ups/pull-downs
  • Button: PA0 uses pull-up (01) so it reads 1 unpressed and 0 when pressed to GND
• IDR: input data register (we read PA0 here)
• BSRR: bit set/reset register
  • Lower 16 bits “set high”, upper 16 bits “set low”
  • With open-drain pins: “set” means release the line; “reset” means drive low

• SYST_RVR (reload value): F_CPU/1000 - 1 for 1 ms
• SYST_CVR (current value): any write clears it
• SYST_CSR (control/status):
  • CLKSOURCE=1 (CPU clock), TICKINT=1 (IRQ), ENABLE=1 (start)

MIT License