Electronics - STM32 - I2C communication between STM32F3Discovery and LSM303DLHC accelerometer

Both lines must be open-drain.

That means neither master nor slave can actively drive the line high.

They can only pull it low (to GND).

A pull-up resistor (internal or external) brings the line back to high when nobody is pulling it down.

This is what allows multiple devices to share the bus safely (a bit like a mutex on a program).

A typical read transaction has two phases.

In the write phase, the master sends the slave address plus the register it wants to read.

Every transaction has to begin with a write phase and an address (of the device to talk to).

In the read phase, the master sends the slave address again with the read bit set, and the slave clocks

out the data.

So, to read one or more registers from the LSM303DLHC we always use the same pattern:

• Write phase: send the register address (with bit 7 set for auto-increment).
  • End with TC (Transfer Complete) with no STOP yet.
• Read phase: repeated START, read direction, AUTOEND.
  • The hardware generates the STOP after all bytes are received.

I2C Transaction Helpers

We split the I2C logic into small focused functions that map directly to what the hardware does.

i2c_start_write() loads CR2 to begin a write.

No AUTOEND here, because after a write we may want to issue a repeated START for a read rather than a STOP (the mutex analogy).

i2c_start_read() loads CR2 to begin a read, with AUTOEND set so the hardware generates the STOP automatically after the last byte.

i2c_write_byte() waits for TXIS then writes to TXDR. i2c_read_byte() waits for RXNE then reads from RXDR. 

i2c_wait_tc() polls TC before issuing a repeated START. 

i2c_stop() sets the STOP bit in CR2 and waits for BUSY to clear.

The 0x80 bit in the register address tells the LSM303DLHC to auto-increment its internal pointer after each byte.

This lets us read all 6 axis bytes in a single burst starting at address 0x28 from OUT_X_L_A register:

• X_L
• X_H
• Y_L
• Y_H
• Z_L
• Z_H

Otherwise it'd need 6 transactions (instead of 1 for every value).

Build and Flash

The project uses CMake with a separate toolchain file and a run.sh script to wrap the common operations.

In our case we'll use the main command to read I2C and UART data directly from our logic analyser:

$ cd firmware/stm32f3discovery/

$ chmod +x run.sh

$ ./run.sh all