I2C Pull up resistor calculator

I²C Pull up resistor calculator

I²C Mode

Standard

Fast

Fast Plus

HS

VDD (V)

Cb (pF)

About this tool

You can use this tool to calculate the minimum and maximum allowed values of the pull up resistors required for your I²C bus, based on the desired operation mode (source). It is based on the official I²C bus specifications and is taking under consideration the constraints set for each mode. For calculating the min and max values of the resistors, we are using the following formulas:

Min Rp

Rp_min = (VDD – V_OLmax) / I_OL

where:

• VDD: Power supply voltage
• V_OLmax: Maximum LOW-level output voltage
• I_OL: LOW-level output current

Max Rp

Rp_max = t_r / 0.8473 x C_b

where:

• t_r: Maximum rise time of signal ( SDA / SCL / SDAH / SCLH)
• C_b: Maximum capacitive load for bus line

I²C operation modes

The following operation modes are defined by the specification:

• Standard mode - max bit rate 100 kbit/s
• Fast mode - max bit rate 400 kbit/s
• Fast mode Plus - max bit rate 1 Mbit/s
• High speed mode - max bit rate 3.4 Mbit/s (depending on Cb)
• Ultra Fast mode - max bit rate 5 Mbit/s (unidirectional)

Ultra Fast mode devices offer push-pull drivers, eliminating the pull-up resistors.

Electrical Specifications

The calculator uses the following electrical specifications for each I²C mode:

Parameter	Standard Mode	Fast Mode	Fast Mode Plus	HS Mode Cb<=100pF	HS Mode Cb>100pF
Min VDD	2V	Cb<400: None Cb=400: 3V	None	None	None
Max Fsclk	100 KHz	400Khz	1MHz	3.4MHz	1.7MHz
Max VOL	0.4 V	VDD>2V: 0.4V VDD<=2V: 0.2xVDD	VDD>2V: 0.4V VDD<=2V: 0.2xVDD	VDD>2V: 0.4V VDD<=2V: 0.2xVDD	VDD>2V: 0.4V VDD<=2V: 0.2xVDD
IOL	3mA	Cb<400: 3mA Cb=400: 6mA	20mA	3mA	3mA
Max tr	1000ns	300ns	120ns	80ns	160ns
Max Cb	400pF	400pF	550pF	100pF	400pF

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Resources

• I²C bus specification and user manual – NPX
• I²C bus pull up resistor calculation – Texas Instruments
• I²C – Wikipedia

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