Objective

Communicate digital data between an Arduino and a PWM module using an I²C bus.

The Arduino I/O pins allow it to bridge the gap directly between digital information and physical signals. However, it can also transmit data to and from other computational devices. The previous examples have implicitly used USB and an asynchronous serial UART for downloading programs and looking at text output. But even simpler bus standards, such as I²C and SPI are useful for connecting an Arduino to I/O modules or even other Arduinos.

This capability is useful for rapid prototyping, since many interface functions are available in modular form. For example, there exist modules:

1. to extend the number of available digital inputs or outputs
2. with specialized electrical behavior such as automatic current regulation or PWM generation
3. which can provide RGB color LED illumination controlled purely by digital data, including PWM generation and power drive

Using digital busses essentially makes the Arduino a node in a network comprising multiple computers, possibly including a host desktop or laptop. Each type of link has different characteristics:

• SPI is normally a point-to-point connection, or sometimes one-to-many. It can be relatively fast (e.g. up to 25 mbps). It requires four wires for bidirectional communication but supports full-duplex transfer with simultaneous input and output. Output-only connections can use just three wires. It is designed for short-range connection to simple devices such as a chain of LED modules or a high-resolution ADC.
• I²C can connect multiple devices on a single bus. It is relatively low speed, typically 400 kbps to one mbps. It uses three wires with separate clock and data but is half-duplex. The protocols are more elaborate and involve specific phases of transmitting device addresses and receiving replies. It is designed for short-range connection within a single system.

In general, using a device-level serial bus requires using two libraries: one to handle the I²C or SPI data transfer, and one to handle the specific protocol for the device. If no device library is available, then this involves understanding the datasheet for the particular device well enough to implement the required protocol. This can be as simple as emitting a few bytes to write a value to a device register, but sophisticated devices can require elaborate programs to handle a variety of messages. For example, a real-time clock-calendar chip can have dozens of registers available over an I²C bus to keep track of time and set alarms.

This exercise will use the Adafruit 815 16 channel 12-bit PWM driver with I2C interface. This module is based on the PCA9685 I2C-bus controlled 16-channel LED controller. Adafruit provides their own tutorial, and library source code on github.

Steps and observations

1. Download or find the complete Adafruit_PWM sketch included with the exercise. This is the pwmtest example from Adafruit-PWM-Servo-Driver-Library, slightly modified into a self-contained sketch. If you have cloned the course materials from github, it will be in the same folder as this page, or it can be downloaded direct from github. Be sure to fetch all top-level files.
2. Load the sketch into the Arduino IDE and use Verify to make sure it compiles on your system.
3. Plug the PWM module into a breadboard and use solid wire to make the following four connections from the Arduino to the module:
   1. 5V -> VCC
   2. GND -> GND
   3. A4 -> SDA
   4. A5 -> SCL
4. Connect the Arduino to your computer and download the sketch.
5. Using an oscilloscope, observe the SCL clock line and the SDA data line. You should see complex digital waveforms. At the right time scale, you should be able to see regular bursts of pulses on the clock line and varying bursts on the data line. Each burst is about 200 microseconds long and represents a command message being sequentially shifted between the two devices.
6. Using an oscilloscope, observe several of the PWM pins on the 3x4 headers. You should see a smoothly varying PWM waveform.
7. Touch an LED between a PWM and GND pin (observing polarity) and you should see a smoothly varying illumination. This module includes 220 ohm resistors in series with the output so no dropping resistor is necessary. If you attach several LEDs, you can see the smooth pattern produced by the example program.

Comments

This example is more about becoming comfortable using vendor libraries than the electronics, given how simple it is to make the electrical connections.

For a challenge, clone the original Adafruit library from github and install it in your Arduino IDE libraries path, then try compiling the original unmodified example.