// application note

AN0015 Setting the I2C SCL duty cycle and an exact bus frequency with the Binho Supernova and Pulsar

Some I2C targets, and some qualification plans, care how each SCL period is split between high and low, not only how many periods there are per second. Adapter firmware 4.6.0 and pyCosmicSDK 1.6.0 let a Binho Supernova or Pulsar set that split directly, from 20 to 80 percent high in steps of 10, and setting it also makes the bus frequency exact rather than rounded. This note explains the call, supplies a script that applies a ratio and prints what the adapter reports, and gives the ratios measured on the wire at 100 kHz and 400 kHz with a logic analyzer.

Products: Binho Supernova Binho Pulsar
Protocols: I2C
Topics: Bus Timing
Revision: Rev 1.0 · November 6, 2026

// outcomes

What you'll walk away with

This document covers:

what I2cController.set_duty_cycle does and what it leaves alone
the ratios firmware accepts, and what it answers for the ones it refuses
why setting a ratio also makes the frequency exact
a script that applies a ratio, generates traffic to measure, and prints what the adapter reports
the split measured on the wire at 100 kHz and 400 kHz

It does not cover ratios outside 20 to 80 percent, duty-cycle control on the Supernova's I3C port, or SPI. SPI clock timing is a separate firmware change.

// background

Background

An I2C controller chooses a high time and a low time for every SCL period. The I2C specification sets minimums for both (tHIGH and tLOW), and most controllers pick a split on their own. That is fine until a target is sensitive to it, or until a test plan asks for a specific split: a 30:70 clock to check that a target samples correctly with a short high phase, or a 40:60 clock to match the timing of the system the target will ship in.

Until now the Binho adapters chose the split themselves. At 100 kHz the default is about 46 percent high; at 400 kHz, about 40 percent. There was no way to ask for another. Adapter firmware 4.6.0 and pyCosmicSDK 1.6.0 add one call that sets it.

// applicable devices

Applicable devices

Adapter	Identifier	Basis
Binho Supernova	hardware revision C, firmware 4.6.0 pre-release build `80cbb484`	measured, this document
Binho Pulsar	hardware revision C, firmware 4.6.0 pre-release build `ea77618b`	measured during firmware validation, within 1 percent of the Supernova; not re-measured for this document
Either adapter	firmware 4.5.1 or earlier	refuses the call with `FW_UNSUPPORTED_COMMAND`

Note: this revision was written against pre-release builds. The firmware reports its version as 4.5.1 with a build suffix because the version string had not yet been moved to 4.6.0. The call and its behavior are those of the 4.6.0 release.

// what's covered