2024-10-23(6): Project: Linux I2C/PWM/SPI

I2C

Our project has two I2C devices that need to be programmed from userspace (we want to create prototypes that can be used in other contexts, and therefore cannot use Linux kernel drivers).

These devices are:

• Proximity Sensor: VL53L1X

• Gyroscope (BNO055)

Go over Linux userspace I2C:

• Linux and I2C (using LM73 Temperature Sensor as Slave)

Explain difference between userspace I2C, and a kernel driver - coincidentally using a driver for a PWM device:

• PWM Userspace Interface (using PCA9685)

PWM

• Motor Control: BTN9960LV

  That device is controlled via two PWM channels. The Raspberry has only one (pin 18 has an alternative setting), so we use a PCA9685 which is connected over I2C.

  ⟶ PWM Userspace Interface (using PCA9685)

• Motor Control: Servo Motor

  Could work over PWM. I suspect not. Read on for SPI.

SPI?

In case PWM does not work for the servos, then chances are that it is PPM (Wikipedia)

• ws2812-spi on Github. Look into ws2812/spi.cpp:

  struct spi_ioc_transfer tr = {
      .tx_buf = (unsigned long)xfer_bytes,
      .len = (uint32_t)xfer_size,
      .speed_hz = s.profile().frequency_hz,
  };
  
  int ret = ioctl(_fd, SPI_IOC_MESSAGE(1), &tr);
  

• https://datasheets.raspberrypi.com/bcm2711/bcm2711-peripherals.pdf

  Things to take into accout when you use SPI to clock out multi-byte waveforms. Take care of “2.3.1. SPI implementation details”, where they say that there is a garanteed gap between two bytes. During that time the MOSI signal is also low (alas it’s SPI).