mohitmishra786 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 pinsskills/baremetal/peripherals-from-datasheet— timing requirementsskills/kernel-dev/bus-drivers-i2c-spi— Linux kernel side