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USB-I2C.md

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USB to I2C connection

Description

To connect a laptop or desktop to our hardware, you will need a special cable that converts a USB-2 port to the I2C communication protocol our boards and sensors / actuators use.

This is the cable we use, it works well with OSX and Linux/Ubuntu: FTDI C232HM-DDHSL-0 USB I2C cable from DigiKey

Recent versions of OSX and Ubuntu have the drivers you need built in.

Here is the support that is built into OSX 10.14.3:

mac ~ > kextstat | grep -i ftdi
  196    0 0xffffff7f8604b000 0x3000     0x3000     com.apple.driver.AppleUSBFTDI (6.0.0) E7845CEE-2059-399E-95A1-E3D9C07E684B <195 85 23 6 5 3 1>

System Info app, Hardware > USB:
  C232HM-DDHSL-0:
    Product ID:                     0x6014
    Vendor ID:                      0x0403  
    Version:                        9.00
    Serial Number:                  FT2HDNH4
    Speed:                          Up to 480 Mb/sec
    Manufacturer:                   FTDI
    Location ID:                    0x14100000 / 12
    Current Available (mA):         500
    Current Required (mA):          500
    Extra Operating Current (mA):   0

Installation

Since we use Python 3.6 for this appliation, the easiest way to communicate with the cable is this package: pyftdi

Our installation scripts handle setting up the pyftdi system dependencies for OSX and Linux/Ubuntu for you: libusb

Wiring

See the FTDI cable data sheet for the I2C pin out at the bottom of page 8.

This is the I2C pin out we use:

Testing

Refer to the pyftdi docs for python help.

See the USB-I2C cable testing scripts in: ~/openag-device-software/scripts/i2c/*.py

In this picture I am testing a ToF distance sensor that is at I2C address 0x4C. When I connect the sensor to the cable I am able to scan the I2C bus and see the device at that address:

Development

When we are developing our embedded applications and drivers, doing it on a tiny, slow microprocessor can be tedious. This cable lets you directly connect any I2C device directly to your machine for ease of debugging. You can also connect a more complex device that has (multiple) I2C busses, such as our PFC_EDU board v3.0.

In the following picture, you can see that I removed the Beaglebone from my PFC_EDU and I connected the green + yellow SDA lines from the USB-I2C cable to the SDA header pin for the Beaglebone. The orange SCL cable wire goes to the SCL pin and the black cable wire goes to a data ground pin, I used the upper right one in this case.

PFC_EDU v3.0 board connected via the USB-I2C cable to a macbook air:

Here is the I2C bus scan showing:

  • The boards I2C multiplexer at address 0x77.
  • The T6713 C02 sensor at multiplexer channel 2, address 0x15.
  • The SHT25 temperature and humidity sensor at multiplexer channel 2, address 0x40.

On the PFC_EDU board, we use multiplexer channel 3, address 0x47 and 0x4C to controll a DAC for the 6 color LEDs. See the leds.py script to test them.