Easyi3C is a leading supplier of embedded system tools that simplify the development and debugging of various communication protocols. The company offers a range of products designed to help engineers and developers use I3C/I2C , USB and MIPI, JEDEC, MCTP and other protocols more efficiently.
I3C Hot-Join is a feature of the MIPI I3C interface that allows a "Target" (slave) device to join an active bus after initialization has already occurred.
In a traditional bus like I2C, devices are typically assumed to be present and powered from the start. I3C Hot-Join enables "plug-and-play" functionality where devices can be physically inserted or powered on later without disrupting ongoing communication.
Power Savings: You can keep specific sensors or peripherals completely powered off until they are needed.
Modular Systems: It supports hardware modularity, such as adding an extension board or a user-replaceable module while the system is running.
Dynamic Discovery: It allows the Controller (master) to discover and assign addresses to new devices without a system reset.
The Hot-Join mechanism relies on In-Band Interrupt (IBI) signaling. Instead of using a dedicated physical pin, the Target uses the existing data lines to alert the Controller.
Bus Idle: The Target waits for the I3C bus to be in an "Idle" state (both SDA and SCL lines are high for a specific duration).
The Request: The Target pulls the SDA line low while SCL is high. This is the same start condition used for an interrupt.
Arbitration: The Target sends a specific "Hot-Join Reserved Address" (0x02).
Note: If multiple devices try to join at once, the one with the lowest physical ID wins arbitration.
Controller ACK: The Controller sees the 0x02 address and responds with an ACK (Acknowledge).
Dynamic Address Assignment (ENTDAA): After acknowledging, the Controller typically issues the ENTDAA (Enter Dynamic Address Assignment) Common Command Code. The new Target then provides its 48-bit Provisional ID, and the Controller assigns it a unique Dynamic Address.
The I3C specification defines two main ways a Target can approach joining:
| Type | Description |
| Active Hot-Join | The Target proactively sends a Hot-Join request as soon as it powers up and sees the bus is idle. |
| Passive Hot-Join | The Target stays quiet until it sees I3C traffic (specifically a broadcast to the 0x7E address). Once it confirms it is on an I3C bus, it emits the request or waits for the next assignment cycle. |
Failsafe Pins: Physical pins on Hot-Join devices must be "failsafe," meaning they don't leak current or interfere with the bus signals when the device is unpowered.
Reserved Address: The address 0x02 is strictly reserved for Hot-Join requests and cannot be used as a standard dynamic address.
Controller Support: The I3C Controller must have Hot-Join enabled (via the ENEC command) to respond to these requests.
Refer to the Intel I3C Hub Spec as follows:
To test I3C Hot Join, we built a test platform using two I3C Hub chips, as shown in the diagram below:
Tower I3C Host Adapter 1 is connected to Controller Port 0 of the first-level I3C Hub, and the relevant configurations for SMBus Agent Target Port 7 are set up. Tower I3C Host Adapter 2 is connected to Controller 1 of the second-level I3C Hub. Following the I3C Hub's specifications, the relevant register operations are performed. At this point, Tower I3C Host Adapter 1 can use IBI to determine that a Hot Join has occurred on controller 0 of the second-level I3C Hub at Target Port 7. The entire testing process can be automated using scripts to easily verify the correctness of the Hot Join function.
Based on our Tower I3C Host Adapter, you can easily test the I3C Hot Join function, meet complex testing needs, build an automated testing environment, and shorten the time to market for chips.
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