TCA9548A

A simple, but effective, solution for I²C bus management.

This package allows for direct and simple interaction with the channel (segment) selection API. It also provides a I2CBus implementation for creating an abstraction layer for compositing multi-bus multi-sensor deployments.

Adafruit as always.

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I2CBus Abstraction

The I2CTca9548aBus implements this I2CBus interface, creating virtual bus instances for generic sensor consumption. This allows for dynamic deployment environments with high code reuse.

API

The main interface and factory class is Tca9548a. It provides the factory method from which is passed a valid I2CBus.

Channel number for this 8-Channel device is 0 - 7

getChannels

Queries the device and returns and array of enabled channels. If not channels are enabled then the empty array is returned.

setChannels

Sets the devices active channels. Takes in an array of channels

Example:

const i2c = require('i2c-bus');
const { Tca9548a } = require('@johntalton/tca9548a');

// setup device
const i2c1 = await provider.openPromisified(1);
const device = await Tca9548a.from(new I2CAddressedBus(i2c1, 0x70));
const channels = await device.getChannels();

// deconstruct and append channel three
await device.setChannels([...channels, 3]);

Additional API

This package also provides an abstraction layer around Channel Management and the devices in order to expose a I2CTcaBus implementation. This serves as a compliant I2CBus allowing for other Chip / Sensor implementations to abstract Bus Management and Sensor specific code.

// ...
const i2c = // any comliant implementation like require('i2c-bus')
const virtualI2cX = await TcsBus.from(...);

const sensor = await SomeSensor.init(new I2CAddressedBus(virtualI2cX, sensorAddress));

Concept

The TCA provides a management layer that exposes channels. Each channel is an I²C bus segment that can extends the host I²C bus. The management layer allow for forking and joining these channels.

As such, zero or more channel can extend the host bus. Allowing for the host bus to address directly all connected devices to the unified channels.

Consider the following

Assume four sensor are connected to the TCA chip via channel 0 and 1, and each sensor label by it's address.

Using the above bus configuration, the following commands will describe and allow you to control the bus segments.

> node tune off
> i2cdetect
  0x10 0x70
> node tune 0
> i2cdetect
  0x1 0x2 0x10 0x70
> node tune 1
> i2cdetect
  0x3 0x4 0x10 0x70
> node tune 0 1
> i2cdetect
  0x1 0x2 0x3 0x4 0x10 0x70

Make note of the last multi channel tune call. Also the fact that 0x10 and 0x70 is always present from the host perspective as expected.

Overlapping static address multiplexing

A simple / common use case is to use the TCA to select from sensor that would normally share an address (and create errors on the I²C bus) by placing each on individual channels:

In such a configuration the tune command would exclusively select each channel (never selecting more then one at a time), and the i2cdetect call would always return 0xA onto the host bus.

Some care is needed by calling application layer when managing and caching recourses as called from code addressing 0xA (in this example) will result in read/write on different sensors depending on the channel configuration.

Interoperability / Dependencies

This project aims to provide a common / abstract I²C bus implementation such that transparent sensor logic can be written without knowledge of the bus layer.

Wrapper code for i2c-bus and onoff are well supported.