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I2C

I2C is the most common interface when selecting a microcontroller-level Hardware.

Microcomputers​

Many microcomputers today include JST connectors that use I2C bus. Some companies have standards around the same JST connector, such as Sparkfun QWIIC and Adafruit STEMMA products.

A good example is Arduino UNO R4 WiFi. Arduino is one the Supported Hardware options, providing all the necessary libraries.

Arduino Uno R4 Wifi

Another example is Sparkfun Redboard.

Sparkfun Redboard with DUELink

Note that typical use of non-DUELink I2C on JST connector is done by connecting all modules on the same I2C signals. This is definitely not the case with DUELink. DUELink only uses I2C on the first module. Then that module connects to other modules using the Downlink interface. This typology does not consume I2C addresses and allows for Daisylinking hundreds of modules!

The Protocol​

A DUELink module is always an I2C slave listening to an I2C bus master on address is 0x52 (7bit), which is fixed and can't be changed.

The host (I2C master) can send data as individual bytes or multiple bytes. The module will look at individual bytes and disregard how they're packed in I2C frames. In other words, start, restart, and stop conditions are ignored by the data stream.

The module (I2C slave) will return 0xFF when the host requests data with no available data to return. This is works well in most cases as 0xFF is not used in commands and responses as they are all readable ASCII. However, in cases when the slave needs to send 0xFF as an actual data, it will instead send 0xFE followed by 1. And similarly, to send 0xFE it will instead send 0xFE followed by 2.

This code explains how to process received I2C data.

escape = false;
error = false;
ProcRxI2C(b)
{
    if(escape)
    {
        if(b == 1)
            InsetFifo(0xff);
        else if(b == 2)
            InsertFifo(0xfe);
        else
            error = true;
        escape = false;
    }else
    {
        if(b == 0xfe)
            escape = true;
        else if(b == 0xff)
            return;
        else
            InsertFifo(b);
    }
}

When the device is too busy to accept data, it will use clock stretching feature. Clock stretching support is a standard I2C feature and is required from the host.

Many modules on the market include a JST connector with I2C bus signals. They simply use I2C and most will not have any intelligence. DUELink gracefully allows for these modules to be connected on the same Daisylink. The only rule is you have to connect the non-DUELink modules first in the chain. Access those module like you normally would, as DUELink will simply ignore them.

QWIIC, STEMMA QT, and Modulino with DUELink modules

tip

You can't use any non-DUELink modules that have the same address as DUELink, which is very unlikely!

Pull Up Resistors​

I2C requires pull up resistors to operate, typically 2.2K. Some boards on the market do not include these resistors. Instead, they rely on the internal weak pull up resistors inside the micro. This may cause problems in some cases. DUELink has a PullUp module to solve this issue.

QTPY with Pull-up

Interface Detection​

DUELink modules check their interface pins on power-up for high-level to configure themselves to use the I2C interface. This will work just fine on any system with proper I2C pull-up resistors, as explained before.

This pull-up-rule applies when using Power Inject as well. The problem is that Power Inject may power up a module before the I2C host (master) was powered up causing the I2C pins to not be detected properly. To solve this, use Power Inject in the middle of any chain of module. The Daisylink bus does not use I2C, even if the first module is connected to an I2C host.

Here is an example where the first module is a non-DUELink host (I2C master) that is using I2C bus and has built-in pull-up resistors. Power Inject will cause no issues in this setup. The I2C communication is only between the host and the first module. The rest of the wire will be non-I2C Daisylink bus.

daisylink with power inject