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.
In the context of MIPI I3C, RDSON refers to the On-Resistance of the MOSFETs (transistors) used in the bus's I/O drivers or in passive level translators/switches.See the circuit diagram below.
Because I3C operates at much higher speeds (up to 12.5 MHz) than traditional I2C, RDSON becomes a critical factor for signal integrity, power consumption, and backward compatibility.
I3C uses two different driving modes on the same wires. The value of RDSON affects each differently:
Push-Pull Mode (High Speed): In this mode, the controller or target actively drives the signal both High and Low. A low RDSON is essential to ensure a strong, fast transition. If the resistance is too high, the "rise time" and "fall time" slow down, causing data errors at 12.5 Mbps.
Open-Drain Mode (Arbitration/I2C Compatibility):
During the start of a transaction or during "In-Band Interrupts," the bus acts like I2C. The NMOS transistor pulls the line Low. The RDSON of this transistor must be low enough to achieve a valid VOL (Voltage Output Low) level, especially when fighting against the bus pull-up.
RDSON is a major component of the total resistance (R) in the bus's RC time constant (T = R * C).
Capacitive Load (C): The bus has a maximum allowed capacitance (typically 50 pF for I3C).
Effect: High RDSON increases the time it takes for a signal to reach its logic threshold. In I3C, where a clock cycle is only 80 ns, even a small delay caused by high resistance can lead to a "collapsed" eye diagram and communication failure.
If you are using a passive FET-based level shifter (like the TCA39306) to connect I3C devices:
The shifter's FET has an internal RDSON (often around 3.5Ω to 30Ω depending on voltage).
This resistance is added to the trace resistance and the driver's own resistance.
Caution: For 12.5 MHz I3C, "Passive" translators can struggle because their RDSON doesn't provide enough drive strength. This is why "Active" translators with Rise Time Accelerators (RTAs) are preferred for I3C.
| Feature | Importance of Low RDSON |
| Edge Rates | Faster transitions (Slew Rate) for 12.5 MHz+ speeds. |
| Logic Levels | Ensures a clear '0' (Low) voltage near 0V. |
| Power | Lower resistance reduces heat dissipation during high-speed switching. |
| Compatibility | Crucial for "Hot-Join" and arbitration where multiple devices share the line. |
We can use the API provided by the Tower I3C Host Adapter Pro+ to test the RDSON_UP and RDSON_DOWN of the I3C push-pull circuit of the target chip. See the diagram below:
Using the parameters shown in the red box in the image, the RDSON test can be performed. These parameters allow the Tower I3C Host Adapter Pro+ to stop at a specified position when reading data from the I3C trace chip driving bus. At this point, the circuit can be switched, making it easier to calculate the RDSON.
In the next section, we will provide detailed steps for testing the RDSON_UP and RDSON_DOWN of the I3C push-pull circuit of the target chip using the Tower I3C Host Adapter Pro+.
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