Memory-Mapped I/O

Overview

• Memory-mapped I/O allows communication between the processor core and peripheral devices.
• Each on-chip peripheral has registers (control, status, input/output).
• Two approaches for data exchange:
  • Port-mapped I/O: Uses special CPU instructions for I/O operations.
  • Memory-mapped I/O: Uses native load/store instructions and does not require special instructions.

Advantages of Memory-Mapped I/O

• More convenient for interfacing I/O devices.
• Example: Setting a GPIO pin to high using the store instruction (STR).

ARM Cortex-M Microprocessors

• Use memory-mapped I/O with a 32-bit memory address supporting 4 GB.
• Memory space divided into six regions:
  1. Code Region: Stores program code and data (on-chip flash, max 512 MB).
  2. SRAM: Stores data (heaps, stacks); can also store code (max 512 MB).
  3. Peripheral Region: Contains memory addresses for peripherals (e.g., GPIO, ADC).
  4. External Device Region: Reserved for external devices (e.g., SD cards).
  5. External RAM: Off-chip memory for large data storage (max 1 GB).
  6. System Region: Includes NVIC, system timer, etc.

GPIO on STM32L4

• Example of memory-mapped I/O using GPIO on STM32L4:
  • GPIO Port A starts at memory address 48000000 (hex).
  • Port A has 12 registers, each 4 bytes (total 1 KB reserved, only 48 bytes used).
  • Registers: MODER (mode register) at lowest address, ASCR (analog switch control register) at highest.

Setting GPIO Pin Output

• To set pin 14 of Port A high:
  • Set bit 14 of the Output Data Register (ODR) to 1.
  • ODR memory addresses: 48000014 to 48000017 (hex).
  • Uses little-endian format for byte storage.
  • Example C statement uses bitwise OR and dereferencing to access and modify the ODR.

Dereferencing in C

• Dereferencing a pointer retrieves the value at the memory location it points to.
• Asterisk (*) is the dereferencing operator.
• Sequence of load, modify, store operations illustrated in C.*

Struct for GPIO Registers

• Memory-mapped I/O registers represented as a contiguous block of memory using a struct in C.
• volatile qualifier prevents compiler optimization on variables that may change unexpectedly.
• Macro definition for struct pointer (e.g., GPIOA) to improve code readability.
• Access ODR using membership operator (dot or arrow operator).

Assembly Language Example

• Using EQN directive to define constants for base memory address and ODR offset.
• Sequence in assembly:
  1. Load base address into register.
  2. Load ODR value into register.
  3. Modify value.
  4. Store modified value back to ODR.

Further Resources

• Visit book website for additional examples and tutorials.