I2C Pull-Up Resistor
I2C bus pull-up: compute Rp_max / Rp_min from bus capacitance, speed, and device sink current; recommend an E24 value and check rise time and low level.
Key points
I2C Pull-Up Resistor: I2C bus pull-up: compute Rp_max / Rp_min from bus capacitance, speed, and device sink current; recommend an E24 value and check rise time and low level. Formula: Rp(max) = tr/(0.8473·Cb) ｜ Rp(min) = (Vdd − Vol)/Iol ｜ tr = 0.8473·Rp·Cb. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. I2C pull-ups trade speed against power / sink current: smaller resistors give faster edges but higher low-level current; larger ones save power but slow the edges. Rp_max comes from rise time and bus capacitance; Rp_min from the device Iol at Vol. The recommended value must lie between them.
Inputs
Rp(max) = tr/(0.8473·Cb) ｜ Rp(min) = (Vdd − Vol)/Iol ｜ tr = 0.8473·Rp·CbCalculation model (updates with parameters)
| Rp_max (rise-time limit) | 1.77 kΩ | tr ≤ 300 ns，Cb 200 pF |
| Rp_min (sink-current limit) | 966.7 Ω | (Vdd − Vol)/Iol = (3.30−0.40)/3 mA |
| Recommended pull-up Rp (E24) | 1.3 kΩ | 在 [Rp_min, Rp_max] 内取几何中点附近 |
| Actual rise time tr | 220 ns | 0.8473·Rp·Cb，需 ≤ 300 ns |
| Low-level sink current | 2.231 mA | 需 ≤ Iol 3 mA（Vol 0.40 V） |
| Low-level pull-up dissipation | 6.469 mW | 单条线被拉低时的dissipation，空闲时几乎为零 |
| Maximum Cb for this Rp | 272 pF | tr_max/(0.8473·Rp) |
| Bus capacitance check | 通过 ✓ | I2C 规范建议 Cb ≤400pF |
Engineering notes
- I2C pull-ups trade speed against power / sink current: smaller resistors give faster edges but higher low-level current; larger ones save power but slow the edges.
- Rp_max comes from rise time and bus capacitance; Rp_min from the device Iol at Vol. The recommended value must lie between them.
- Bus capacitance includes all device pins, PCB traces, and connectors; the specification recommends ≤ 400 pF. For long buses or many devices, reduce speed or use active pull-ups.
FAQ
- What is I2C Pull-Up Resistor for?
- I2C bus pull-up: compute Rp_max / Rp_min from bus capacitance, speed, and device sink current; recommend an E24 value and check rise time and low level.
- What is the formula for I2C Pull-Up Resistor?
- Core formula: Rp(max) = tr/(0.8473·Cb) ｜ Rp(min) = (Vdd − Vol)/Iol ｜ tr = 0.8473·Rp·Cb. 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 Resistor?
- I2C pull-ups trade speed against power / sink current: smaller resistors give faster edges but higher low-level current; larger ones save power but slow the edges. Rp_max comes from rise time and bus capacitance; Rp_min from the device Iol at Vol. The recommended value must lie between them. Bus capacitance includes all device pins, PCB traces, and connectors; the specification recommends ≤ 400 pF. For long buses or many devices, reduce speed or use active pull-ups.