I2C and SPI Station Wiring Guide

This guide covers electrical safety, pull-up resistors, voltage levels, and the device-tree / sysfs assumptions BenchCI makes when using the i2c-sensor-smoke and spi-flash-verify presets (and any custom bench that uses the i2c or spi transport backends).

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I2C Wiring

Signal Lines

Pin	Function
SDA	Bidirectional data
SCL	Clock (master drives)
GND	Common ground
VCC	Sensor supply (see voltage level section)

Pull-up Resistors

I2C requires pull-up resistors on both SDA and SCL. BenchCI does not configure pull-ups in software — they must be present on the bench hardware.

Recommended values:

Bus speed	Pull-up value	Max cable length
100 kHz (standard)	4.7 kΩ	~1 m
400 kHz (fast)	2.2 kΩ	~30 cm
1 MHz (fast-plus)	1 kΩ	~10 cm

Use resistors to 3.3 V for 3.3 V-tolerant sensors. Use level shifters when mixing 3.3 V and 5 V devices.

Voltage Levels

Most modern SBCs (Raspberry Pi, BeagleBone, NVIDIA Jetson) expose I2C at 3.3 V logic. Connecting a 5 V I2C sensor without a level shifter will damage the SBC’s GPIO bank.

Isolation: If the DUT is connected to mains power or an untested power rail, use an I2C isolator (e.g. ISO1541, ADUM1250) between the bench agent and the DUT.

Short-Circuit Protection

Add a 100 Ω series resistor on SDA and SCL between the bench agent and the DUT. This limits current if the DUT firmware drives a line low while BenchCI is doing a scan.

/dev/i2c-* Availability

BenchCI auto-detects available I2C buses by scanning /dev/i2c-0, /dev/i2c-1, etc. at bench registration time. The detected bus numbers are reported in the capabilities.i2c_buses field of the bench sync payload.

Ensure the i2c-dev kernel module is loaded on the agent host:

sudo modprobe i2c-dev
# To persist across reboots:
echo "i2c-dev" | sudo tee /etc/modules-load.d/i2c-dev.conf

The smbus2 Python package must be installed in the BenchCI agent environment:

pip install smbus2

Grant the agent user access to I2C devices without sudo:

sudo usermod -aG i2c $USER
# Log out and back in, or:
sudo chmod a+rw /dev/i2c-*

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SPI Wiring

Signal Lines

Pin	Function
MOSI	Master Out Slave In
MISO	Master In Slave Out
SCLK	Clock
CS/CE	Chip Select (active low)
GND	Common ground

Voltage Levels

SPI signals are not open-drain — they are push-pull. Connecting 5 V SPI to a 3.3 V SBC without a level shifter will latch-up or destroy the SBC’s SPI controller.

Use a bidirectional level shifter (e.g. TXS0108E, 74LVC245) for mixed-voltage setups.

Short-Circuit Protection

Unlike I2C, SPI lines are driven both ways. Do not add series resistors > 47 Ω — they cause signal integrity issues at speeds above 1 MHz.

/dev/spidev*.* Availability

BenchCI auto-detects SPI devices by scanning /dev/spidev0.0, /dev/spidev0.1, etc. at bench registration time. Detected devices are reported in capabilities.spi_devices.

Enable the SPI interface on Raspberry Pi:

# Via raspi-config → Interface Options → SPI → Enable
# Or add to /boot/config.txt:
dtparam=spi=on

The spidev Python package must be installed:

pip install spidev

Grant the agent user access to SPI devices:

sudo usermod -aG spi $USER
sudo usermod -aG gpio $USER

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Bench Config Reference

I2C Transport (bench.yaml)

nodes:
  dut:
    kind: mcu
    role: target
    transports:
      board_i2c:
        backend: i2c
        bus: 1           # corresponds to /dev/i2c-1
        timeout_ms: 3000

SPI Transport (bench.yaml)

nodes:
  dut:
    kind: mcu
    role: target
    transports:
      flash_spi:
        backend: spi
        bus: 0           # corresponds to /dev/spidev0.X
        device: 0        # corresponds to /dev/spidevX.0
        max_speed_hz: 1000000
        mode: 0
        bits_per_word: 8

Generate a validated starter config with:

benchci init --preset i2c-sensor-smoke
benchci init --preset spi-flash-verify

Station Smoke Suites

The repository includes Station-oriented smoke suites under examples/17-station-i2c-spi-smoke/:

benchci run --bench examples/17-station-i2c-spi-smoke/bench.yaml \
  --suite examples/17-station-i2c-spi-smoke/i2c-smoke.suite.yaml \
  --skip-flash

benchci run --bench examples/17-station-i2c-spi-smoke/bench.yaml \
  --suite examples/17-station-i2c-spi-smoke/spi-smoke.suite.yaml \
  --skip-flash

Update the expected I2C address/register values and SPI response bytes to match the real Station fixture before recording release evidence.