SPI and I2C (Bare-Metal)

Purpose

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.

When to Use

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

Workflow

1. SPI master (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();

2. I2C master — register read

/* 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.

3. Common protocols

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

4. Agent usage

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

Common Problems

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

Related Skills

skills/baremetal/gpio-baremetal — CS, SDA, SCL pins
skills/baremetal/peripherals-from-datasheet — timing requirements
skills/kernel-dev/bus-drivers-i2c-spi — Linux kernel side