What is the formula for I²C Pull-up & Bus Rise Time Checker?

Core formula: t_r ≈ 0.8473·R_p·C_bus (30%→70% of Vdd). The tool returns results instantly using this formula and the selected parameters.

I²C Pull-up & Bus Rise Time Checker

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Check whether the rise time from pull-up and bus capacitance meets the I²C spec, and back out the maximum capacitance and pull-up current — for when the waveform will not climb.

Key points

I²C Pull-up & Bus Rise Time Checker: Check whether the rise time from pull-up and bus capacitance meets the I²C spec, and back out the maximum capacitance and pull-up current — for when the waveform will not climb. Formula: t_r ≈ 0.8473·R_p·C_bus (30%→70% of Vdd). Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. Smaller Rp gives sharper edges but more dissipation; 400 kbit/s usually uses 2.2–4.7kΩ, 1 Mbit/s needs ≤1kΩ and low bus capacitance.

Inputs

t_r ≈ 0.8473·R_p·C_bus (30%→70% of Vdd)

Calculation model (updates with parameters)

Bus rise time t_r	398.2 ns
Spec limit	300 ns	Fast mode 400 kbit/s
Margin	-98 ns	fail — reduce Rp
Max capacitance for this Rp	75 pF
Pull-up current @VOL	617 µA	I²C spec limits this to 3mA
Static pull-up dissipation	2.317 mW	Flows while SCL is low
Suggested Rp ceiling	3.541 kΩ	Estimated from bus capacitance and mode limit; stay at or below

⚠ 上literTime 398ns 超过 Fast mode 400 kbit/s upper limit 300ns，需减小 Rp 或缩短总线。

Engineering notes

Smaller Rp gives sharper edges but more dissipation; 400 kbit/s usually uses 2.2–4.7kΩ, 1 Mbit/s needs ≤1kΩ and low bus capacitance.

FAQ

What is I²C Pull-up & Bus Rise Time Checker for?: Check whether the rise time from pull-up and bus capacitance meets the I²C spec, and back out the maximum capacitance and pull-up current — for when the waveform will not climb.
What is the formula for I²C Pull-up & Bus Rise Time Checker?: Core formula: t_r ≈ 0.8473·R_p·C_bus (30%→70% of Vdd). The tool returns results instantly using this formula and the selected parameters.
What engineering notes should I keep in mind when using I²C Pull-up & Bus Rise Time Checker?: Smaller Rp gives sharper edges but more dissipation; 400 kbit/s usually uses 2.2–4.7kΩ, 1 Mbit/s needs ≤1kΩ and low bus capacitance.