Calculator Inputs
Example Data Table
This example matches a common 3.3 V open-drain data line with a moderate bus load.
| VCC | VIH(min) | VOL(max) | Sink Current | Leakage | Capacitance | Rise Time | Safe Range | Suggested Value |
|---|---|---|---|---|---|---|---|---|
| 3.3 V | 2.31 V | 0.4 V | 3.0 mA | 10 µA | 100 pF | 300 ns | 1.063 kΩ to 3.186 kΩ | 1.8 kΩ |
Formula Used
1) Lower resistor limit from sink current
Rmin = (VCC − VOL(max)) / Isink(max)
2) Upper resistor limit from rise time
Rmax,rise = trise / (0.8473 × Cbus)
3) Upper resistor limit from leakage
Rmax,leak = (VCC − VIH(min)) / Ileak(total)
4) Recommended choice
The calculator narrows the safe window with the chosen margin, then aims near the geometric mean and snaps to the selected resistor series.
The factor 0.8473 comes from the 30% to 70% rise-time span of a first-order RC curve.
How to Use This Calculator
Step 1
Enter the line name, supply voltage, and the receiving logic-high threshold.
Step 2
Add the maximum low-state voltage and the current the driving device can safely sink.
Step 3
Enter total leakage current, total bus capacitance, and the required 30% to 70% rise time.
Step 4
Choose a resistor series, apply a safety margin, calculate, then review the safe range, selected value, current, power, and graph.
Use the export buttons after calculation to save the result set as CSV or PDF.
FAQs
1) Why does a pull up resistor need a range?
Too small a resistor forces excessive sink current when the line goes low. Too large a resistor slows the rising edge and may let leakage drag the high level down. A safe design satisfies both sides together.
2) Why is capacitance important?
The resistor and bus capacitance create an RC network. Larger capacitance stores more charge, so the line rises more slowly for the same resistor. That is why long traces, cables, and multiple inputs often require smaller pull ups.
3) What does leakage current change?
Leakage current steals some pull-up current even when the line should be high. If the resistor is too large, the leakage drop can lower the final high voltage below the receiver threshold, causing unreliable logic detection.
4) Why use the 30% to 70% rise time?
Many digital bus specifications measure rise time between 30% and 70% of supply voltage. For a simple RC waveform, that span equals 0.8473RC. The calculator uses that relationship to estimate the maximum allowed resistor.
5) When should I choose E96 instead of E12?
Use E96 when you want tighter matching to the computed target or when the safe window is narrow. E12 is fine for broader windows, prototypes, and parts bins where coarse steps are acceptable.
6) Does this work for I2C and open-drain GPIO?
Yes. It fits I2C, SMBus, wired-AND lines, interrupt lines, and open-drain or open-collector GPIO nodes. Enter the correct thresholds, capacitance, leakage, and sink capability for the specific interface you are designing.
7) Why add a safety margin?
Real boards vary with temperature, tolerance, voltage, and measurement uncertainty. A margin shrinks the usable window, giving extra room before the design hits sink-current, timing, or leakage limits in real operation.
8) Can the selected value still be outside the safe window?
Yes, if your preferred series has no standard value inside the safe range or your electrical constraints conflict. In that case, adjust capacitance, relax timing, increase sink capability, or choose a denser resistor series.