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I2C pull-up and rise-time budget calculator

RC rise-time and sink-current resistor interval; bus capacitance supplied.

embedded-i2c-pullup.napkin / editable
✓ checks pass

SCENARIO COMPARISON

Compare the numbers.

This source describes an analytical model. Its quantities and checks are the view.

Scenariominimumohmmaximumohm
966.6673540.66
746.6673540.66

All calculated quantities are listed with the inputs. Switch to Source to inspect the equations.

SCENARIOS / CALCULATED VALUESSAME SOURCE

Change an input to see the consequences.

resistor interval feasible3540.66 ohm✓ passes

BASELINE INPUTS

CALCULATED

minimum966.67 ohm
maximum3540.7 ohm

Edits change the baseline source.

Calculated from your source · not saved
Baseline · Baseline
resistor interval feasible3540.66 ohm ✓ passes
alternative · alternative
resistor interval feasible3540.66 ohm ✓ passes

Illustrative first-order example. Checks are editable requirements, not design standards. The broader topic may require additional equations, data, and validation.

Formula and units

This example uses the following equations. Input names and units match the editable source above.

minimum
minimum = (voltage-lowVoltage)/sinkCurrent [ohm]
maximum
maximum = riseTime/(0.8473*capacitance) [ohm]

Original worked example

These are the baseline values in the downloadable source. Edits above let you test your own values.

Given

voltage
3.3 V
low voltage
0.4 V
sink current
3 mA
capacitance
100 pF
rise time
300 ns

Calculated

minimum
966.667 ohm
maximum
3540.66 ohm

Assumptions and limits

RC rise-time and sink-current resistor interval; bus capacitance supplied.

The requirement checks use the limits written in this example. Passing a check does not establish that the model covers every load, material behavior, or operating condition in a real design.

Download this .napkin example ↓

Take it further with your agent

Napkin keeps the inputs, equations, checks and assumptions together in one study. Your agent sets it up; you change the numbers.

Try with Claude ↗

codex 'Use Napkin (https://napkin.engineering/mcp, or the local Napkin plugin) to build a study: I2C pull-up and rise-time budget. Inputs: bus capacitance; supply; sink limit; rise-time target. Outputs: resistor interval; low-state current. Model scope: Declared clock, peripheral and execution assumptions. Read napkin_capabilities and the current snapshot. Ask me for the values that matter, label assumptions with units, author .napkin source, validate it and apply it with napkin_source using the current revision, then open the study. Add checks for the limits I supply. Keep unsupported analyses explicit.'

Claude needs the Napkin connector; Codex needs the Napkin plugin. Set up your agent ↗

The broader study

The topic brief below describes the broader scope. The worked example above states the particular calculation it implements.

  • Inputs: bus capacitance, supply, sink limit, rise-time target
  • Outputs: resistor interval, low-state current
  • Modes: calculate, compare, sweep
  • Drawing: Timing diagram or event simulation

EXECUTABLE WORKED EXAMPLE · FIRST-ORDER MODEL · CHECK THE SCOPE & ASSUMPTIONS

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