How to use
- Copy the installation command and review the source package before installing it in your own agent environment.
- Check the source document for required tools, credentials and supported hosts.
- Describe your input and desired deliverable; review the output against the source material.
npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill spi-i2c-baremetalBefore you start
- Installation does not include a model subscription, API allowance or permission to access your files.
- Repository stars and installation counts describe activity, not an independent quality or security test.
Original skill document
Rendered from the source repository. Headings, examples, code, tables, links, and referenced images are preserved.
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 (WHOAMI register)
- SPI flash or display bring-up
- Debugging NACK or stuck SCL
- Replacing HAL_I2C/SPI with minimal code
Workflow
1. SPI master (STM32)
c
/* 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):
c
GPIO_CS_LOW();
spi_xfer(SPI1, reg | 0x80); /* read */
uint8_t val = spi_xfer(SPI1, 0xFF);
GPIO_CS_HIGH();2. I2C master — register read
c
/* 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) | SPI sensors |
| 16-bit big-endian length prefix | SPI flash |
4. Agent usage
/spi-i2c-baremetal I2C read of register 0x0F from device 0x68Common 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
