spi-i2c-baremetal

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Original

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Translation

Chinese

SPI and I2C (Bare-Metal)

SPI和I2C（裸机实现）

Purpose

用途

Implement SPI and I2C master drivers for sensor and memory chips: clock configuration, phase/polarity (SPI), START/ACK sequences (I2C), and common register-oriented transaction patterns.

为传感器和存储芯片实现SPI和I2C主模式驱动：时钟配置、相位/极性（SPI）、START/ACK序列（I2C）以及常见的面向寄存器的事务模式。

When to Use

使用场景

Reading an I2C sensor (WHO_AM_I register)
SPI flash or display bring-up
Debugging NACK or stuck SCL
Replacing HAL_I2C/SPI with minimal code

读取I2C传感器（WHO_AM_I寄存器）
SPI闪存或显示屏启动
调试NACK或SCL引脚卡死问题
用极简代码替代HAL_I2C/SPI

Workflow

工作流程

1. SPI master (STM32)

1. SPI主模式（STM32）

/* Mode 0: CPOL=0, CPHA=0 — check slave datasheet */
SPI1->CR1 = SPI_CR1_MSTR | SPI_CR1_SSM | SPI_CR1_SSI
          | (3 << SPI_CR1_BR_Pos);  /* baud divider */
SPI1->CR1 |= SPI_CR1_SPE;

uint8_t spi_xfer(SPI_TypeDef *spi, uint8_t tx) {
    while (!(spi->SR & SPI_SR_TXE))
        ;
    *(volatile uint8_t *)&spi->DR = tx;
    while (!(spi->SR & SPI_SR_RXNE))
        ;
    return *(volatile uint8_t *)&spi->DR;
}

CS (GPIO bit-bang):

GPIO_CS_LOW();
spi_xfer(SPI1, reg | 0x80);  /* read */
uint8_t val = spi_xfer(SPI1, 0xFF);
GPIO_CS_HIGH();

/* 模式0：CPOL=0，CPHA=0 — 请查阅从设备数据手册 */
SPI1->CR1 = SPI_CR1_MSTR | SPI_CR1_SSM | SPI_CR1_SSI
          | (3 << SPI_CR1_BR_Pos);  /* 波特率分频器 */
SPI1->CR1 |= SPI_CR1_SPE;

uint8_t spi_xfer(SPI_TypeDef *spi, uint8_t tx) {
    while (!(spi->SR & SPI_SR_TXE))
        ;
    *(volatile uint8_t *)&spi->DR = tx;
    while (!(spi->SR & SPI_SR_RXNE))
        ;
    return *(volatile uint8_t *)&spi->DR;
}

CS引脚（GPIO位操作）：

GPIO_CS_LOW();
spi_xfer(SPI1, reg | 0x80);  /* 读操作 */
uint8_t val = spi_xfer(SPI1, 0xFF);
GPIO_CS_HIGH();

2. I2C master — register read

2. I2C主模式 — 寄存器读取

/* START → addr+W → reg → repeated START → addr+R → data → STOP */
bool i2c_read_reg(I2C_TypeDef *i2c, uint8_t dev7, uint8_t reg, uint8_t *out) {
    if (!i2c_start(i2c)) return false;
    if (!i2c_tx(i2c, (dev7 << 1) | 0)) return false;
    if (!i2c_tx(i2c, reg)) return false;
    if (!i2c_restart(i2c)) return false;
    if (!i2c_tx(i2c, (dev7 << 1) | 1)) return false;
    *out = i2c_rx(i2c, false);  /* NACK last byte */
    i2c_stop(i2c);
    return true;
}

Poll

SB

ADDR

TXE

RXNE

BTF

per reference manual.

/* START → 地址+写 → 寄存器 → 重复START → 地址+读 → 数据 → STOP */
bool i2c_read_reg(I2C_TypeDef *i2c, uint8_t dev7, uint8_t reg, uint8_t *out) {
    if (!i2c_start(i2c)) return false;
    if (!i2c_tx(i2c, (dev7 << 1) | 0)) return false;
    if (!i2c_tx(i2c, reg)) return false;
    if (!i2c_restart(i2c)) return false;
    if (!i2c_tx(i2c, (dev7 << 1) | 1)) return false;
    *out = i2c_rx(i2c, false);  /* 最后一个字节发送NACK */
    i2c_stop(i2c);
    return true;
}

请参考手册轮询

SB

、

ADDR

、

TXE

、

RXNE

、

BTF

标志位。

3. Common protocols

3. 常见协议

Pattern	Bus
`reg + write data`	I2C/SPI
`0x80	reg` read (MSB set)
16-bit big-endian length prefix	SPI flash

模式	总线
`寄存器 + 写入数据`	I2C/SPI
`0x80	寄存器` 读取（最高位设为1）
16位大端长度前缀	SPI闪存

4. Agent usage

4. Agent使用方式

/spi-i2c-baremetal I2C read of register 0x0F from device 0x68

/spi-i2c-baremetal I2C read of register 0x0F from device 0x68

Common Problems

常见问题

Symptom	Cause	Fix
I2C NACK	Wrong 7-bit addr (8-bit in datasheet)	Shift addr; check R/W bit
SPI garbage	CPOL/CPHA mismatch	Match slave mode table
Bus stuck SCL low	Slave clock stretch / fault	Bus recovery (clock pulses)
CS glitch	CS timing vs clock	Assert CS before first SCK

症状	原因	解决方法
I2C返回NACK	7位地址错误（数据手册中为8位）	对地址进行移位；检查读/写位
SPI数据异常	CPOL/CPHA不匹配	匹配从设备模式表
SCL引脚持续低电平	从设备时钟拉伸/故障	总线恢复（发送时钟脉冲）
CS引脚出现毛刺	CS时序与时钟不匹配	在第一个SCK脉冲前断言CS