Revisions to Is there a correct resistance value for I2C pull-up resistors?

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edited Jul 27, 2015 at 14:54

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The correct pullup resistance for the I²C bus depends on the total capacitance on the bus and the frequency you want to operate the bus at.

The formula from the ATmega168 datasheet (which I believe comes from the official I²C spec) is --

$$Freq<100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{1000ns}{C_{bus}}$$$$\text{Freq}<100\text{kHz} \implies R_{\text{min}}=\frac{V_{cc}-0.4\text{V}}{3\text{mA}}, R_{\text{max}}=\frac{1000\text{ns}}{C_{\text{bus}}}$$

$$Freq>100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{300ns}{C_{bus}}$$$$\text{Freq}>100\text{kHz} \implies R_{\text{min}}=\frac{V_{cc}-0.4\text{V}}{3\text{mA}}, R_{\text{max}}=\frac{300\text{ns}}{C_{\text{bus}}}$$

The Microchip 24LC256 specifies a maximum pin capacitance of 10pF (which is fairly typical). Count up the number of devices you have in parallel on the bus and use the formula above to calculate a range of values that will work.

If you are powering off of batteries I would use values that are at the high end of the range. If there are no power limits on the power source or power dissipation issues in the ICs I would use values on the lower end of the range.

I sell some kits with an I²C RTC (DS1337). I include 4K7 resistors in the kit which seems like a reasonable compromise for most users.

The correct pullup resistance for the I²C bus depends on the total capacitance on the bus and the frequency you want to operate the bus at.

The formula from the ATmega168 datasheet (which I believe comes from the official I²C spec) is --

$$Freq<100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{1000ns}{C_{bus}}$$

$$Freq>100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{300ns}{C_{bus}}$$

The Microchip 24LC256 specifies a maximum pin capacitance of 10pF (which is fairly typical). Count up the number of devices you have in parallel on the bus and use the formula above to calculate a range of values that will work.

If you are powering off of batteries I would use values that are at the high end of the range. If there are no power limits on the power source or power dissipation issues in the ICs I would use values on the lower end of the range.

I sell some kits with an I²C RTC (DS1337). I include 4K7 resistors in the kit which seems like a reasonable compromise for most users.

The correct pullup resistance for the I²C bus depends on the total capacitance on the bus and the frequency you want to operate the bus at.

The formula from the ATmega168 datasheet (which I believe comes from the official I²C spec) is --

$$\text{Freq}<100\text{kHz} \implies R_{\text{min}}=\frac{V_{cc}-0.4\text{V}}{3\text{mA}}, R_{\text{max}}=\frac{1000\text{ns}}{C_{\text{bus}}}$$

$$\text{Freq}>100\text{kHz} \implies R_{\text{min}}=\frac{V_{cc}-0.4\text{V}}{3\text{mA}}, R_{\text{max}}=\frac{300\text{ns}}{C_{\text{bus}}}$$

The Microchip 24LC256 specifies a maximum pin capacitance of 10pF (which is fairly typical). Count up the number of devices you have in parallel on the bus and use the formula above to calculate a range of values that will work.

If you are powering off of batteries I would use values that are at the high end of the range. If there are no power limits on the power source or power dissipation issues in the ICs I would use values on the lower end of the range.

I sell some kits with an I²C RTC (DS1337). I include 4K7 resistors in the kit which seems like a reasonable compromise for most users.

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edited Jan 6, 2014 at 11:34

Joe Hass

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The correct pullup resistance for the I²C bus depends on the total capacitance on the bus and the frequency you want to operate the bus at.

The formula from the ATmega168 datasheet (which I believe comes from the official I²C spec) is --

$$Freq<100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{1000nS}{C_{bus}}$$$$Freq<100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{1000ns}{C_{bus}}$$

$$Freq>100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{300nS}{C_{bus}}$$$$Freq>100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{300ns}{C_{bus}}$$

The Microchip 24LC256 specifies a maximum pin capacitance of 10pF (which is fairly typical). Count up the number of devices you have in parallel on the bus and use the formula above to calculate a range of values that will work.

If you are powering off of batteries I would use values that are at the high end of the range. If there are no power limits on the power source or power dissipation issues in the ICs I would use values on the lower end of the range.

I sell some kits with an I²C RTC (DS1337). I include 4K7 resistors in the kit which seems like a reasonable compromise for most users.

The correct pullup resistance for the I²C bus depends on the total capacitance on the bus and the frequency you want to operate the bus at.

The formula from the ATmega168 datasheet (which I believe comes from the official I²C spec) is --

$$Freq<100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{1000nS}{C_{bus}}$$

$$Freq>100kHz \implies R_{min}=\frac{V_{cc}-0.4V}{3mA}, R_{max}=\frac{300nS}{C_{bus}}$$