Peripherals from Datasheet

Purpose

Guide agents through a repeatable methodology for writing peripheral drivers from MCU reference manuals: locating register maps, interpreting bit definitions, following init sequences, respecting timing constraints, and producing maintainable register-level code. Pair with skills/baremetal/datasheet-and-refmanual-reading for doc-navigation methodology (kept as separate skills).

When to Use

• Starting a driver without vendor HAL
• Porting a peripheral between MCU families
• Verifying HAL behavior against the reference manual
• Debugging a peripheral that "should work" per examples

Workflow

1. Reference manual navigation

Typical RM structure
├── Memory map (peripheral base addresses)
├── Peripheral chapter (UART, SPI, GPIO, ...)
│   ├── Functional description
│   ├── Register map (table of offsets)
│   ├── Register bit definitions
│   └── Timing / electrical notes
└── Electrical characteristics (clock limits, setup/hold)

Start with the programming model section before copying register writes.

2. Extract register map

/* From RM — USART base 0x40004400 */
typedef struct {
    volatile uint32_t SR;   /* 0x00 status */
    volatile uint32_t DR;   /* 0x04 data */
    volatile uint32_t BRR;  /* 0x08 baud */
    volatile uint32_t CR1;  /* 0x0C control */
    /* ... */
} USART_TypeDef;

#define USART2 ((USART_TypeDef *)0x40004400UL)

Verify offset column matches struct layout (padding for reserved words).

3. Init sequence checklist

Peripheral bring-up order
├── 1. Enable bus clock (RCC/APB/AHB register)
├── 2. Reset peripheral (if RM requires)
├── 3. Configure pins (GPIO alternate function)
├── 4. Configure peripheral registers (mode, baud, etc.)
├── 5. Enable peripheral (UE, TE, RE bits)
├── 6. Enable NVIC IRQ (if interrupt-driven)
└── 7. Verify status flags before first transaction

Bad — enable UART before clock:

USART2->CR1 |= USART_CR1_UE;  /* USART clock still off — no effect */

4. Bit definition discipline

/* From RM: CR1 M[1:0], PCE, PS, TE, RE, UE */
#define USART_CR1_UE   (1U << 13)
#define USART_CR1_TE   (1U << 3)
#define USART_CR1_RE   (1U << 2)

Document RM section number in comment for audit trail.

5. Timing and busy-wait

/* RM: poll BUSY flag until reset complete */
while (RCC->CR & RCC_CR_PLLRDY == 0)
    ;

Respect startup times (oscillator settle, PLL lock) from electrical characteristics chapter.

6. Good vs bad driver structure

Good — layered, RM-referenced:

void usart2_init(uint32_t baud) {
    rcc_enable_usart2();
    gpio_config_usart2_pins();
    usart2_set_baud(baud);
    USART2->CR1 = USART_CR1_TE | USART_CR1_RE | USART_CR1_UE;
}

Bad — magic numbers, no clock enable:

*(uint32_t*)0x4000440C = 0x2000;  /* what peripheral? which bit? */

7. Agent usage examples

/peripherals-from-datasheet Walk me through USART init from STM32 RM
/peripherals-from-datasheet What sections of the ref manual matter for SPI timing?

Common Problems

Related Skills

• skills/baremetal/datasheet-and-refmanual-reading — fast RM navigation
• skills/baremetal/mmio-and-bit-manipulation — register access patterns
• skills/baremetal/gpio-baremetal — pin mux before peripheral enable
• skills/embedded/linker-scripts — memory map alignment