// application note

AN0014 Single-port I2C/SMBus/I3C operation for NVMe-MI on Binho Supernova

Many SMBus and MCTP devices hold SCL low while they prepare a response. The Supernova's I3C engine cannot wait for that, so on its I3C port such a target used to lose bits. Adapter firmware 4.6.0 and pyCosmicSDK 1.6.0 add a mode in which legacy I2C transfers on the I3C port wait for the target, up to the SMBus 35 ms timeout. This note explains the mode, supplies a script that reads and writes a stretching target with it, and supplies the firmware of the reference target used to measure it: 3600 of 3600 reads correct at 100 kHz and 400 kHz over three runs, with the target stretching from 0 to 30 ms, and every stretched read correct up to the mode's ceiling of about 750 kHz. The script also runs the SMBus to I3C Basic detection flow of the PCI-SIG out-of-band management specification as far as the adapter can.

Products: Binho Supernova
Protocols: I2C I3C
Topics: NVMe-MI / MCTP
Revision: Rev 1.0 · November 6, 2026

// outcomes

What you'll walk away with

This document covers:

what the clock-stretching mode changes, and what it refuses while it is on
the SCL rates it runs at and how close they come to the request
a script that reads and writes a stretching target through the mode
a reference target that stretches for a configurable time, and its firmware
measured results against that target
the SMBus to I3C Basic detection flow used for NVMe-MI, and which of its steps the adapter performs

It does not cover SMBus framing beyond the PEC check of an ARP Get UDID (block read and write, quick command), MCTP or NVMe-MI messages, or the Supernova's separate I2C port.

// background

Background

An I2C target may hold SCL low after it acknowledges its address, to gain time before it answers. The controller must wait until SCL is released. The I2C specification calls this clock stretching, and SMBus allows it for up to 35 ms (tTIMEOUT). SMBus and MCTP devices use it routinely: a management controller that is fetching a value holds the clock until the value is ready.

The Supernova's I3C port can address legacy I2C targets on the same bus as I3C devices. Its I3C engine, however, keeps clocking while a target holds SCL low, so a stretching target misses bits and the transfer returns wrong data or no acknowledge. Adapter firmware 4.6.0 and pyCosmicSDK 1.6.0 add a mode that fixes this for legacy I2C transfers on the I3C port.

// applicable devices

Applicable devices

Role	Part	Identifier	Basis
Controller	Binho Supernova	hardware revision C, firmware 4.6.1, I3C HV port	measured, this document
Controller	Binho Supernova	firmware 4.5.1 or earlier	refuses the mode with `FW_UNSUPPORTED_COMMAND`
Controller	Binho Pulsar	any	not applicable: the Pulsar has no I3C port
Reference target	Microchip PIC18F16Q20 Curiosity Nano	firmware `stretch_target_pic18q20.c`	measured, this document

// what's covered

What's covered in the note

01 Introduction
02 System overview
03 Hardware setup
04 Software setup
05 The clock-stretching mode
06 Procedure
07 The SMBus to I3C Basic detection flow
08 Measured results
09 Troubleshooting

The full procedure, measured results, and reference code are in the PDF and the assets archive.

// materials

What you'll need

Binho Supernova USB host adapter for I3C, I2C, SPI & UART View product

Software Mission Control 3 or the Cosmic SDK (Python, C / C++ / C#). Get the software

Files The reference scripts and assets used in this note. Download the assets

// our edge

We don't just follow the I3C spec.

We don't watch from the sidelines. Plugfest after plugfest, our team is in the room where vendor I3C implementations come together — finding edge cases, debugging quirks, and validating real silicon against the spec.