Minimum value I2C-pull up resistor for arduino

Asked May 28 '18 at 13:58

Active May 28 '18 at 14:12

Viewed 313 times

I'm trying to determine the minimum value of pull resistor for Arduino Uno

The datasheets says; 0,4 Vol for 20 mA Iol. Let's assume VDD = 5V

For formula is: Rmin=Vcc−0.4V/20mA = 4,6V / 20mA = 230 ohms.

But I saw that people use 0,4V for 3mA. Than the minimal value is 1,6K ohms.

So whats the minimum value of pull-up for arduino?

edited May 28 '18 at 14:12

asked May 28 '18 at 13:58

Kono

2

Your I2C slave also has to pull down the pins for ack/nack. So you can't use only the Arduino figures to find the minimum value. – Oldfart May 28 '18 at 14:49
My slaves also have 0,4 Vol for 20 mA – Kono May 28 '18 at 14:55
related: Is there a correct resistance value for I2C pull-up resistors? – Nick Alexeev May 28 '18 at 16:14
Yea I know, but they use 3mA there. I'm wondering why they use the 3mA. Many devices like arduino can go much higher – Kono May 28 '18 at 16:22
Umm.. where, exactly, are you getting the 0.4V @ 20mA number from? The MCU datasheet says 1.0V max at 20mA, 5V Vdd and max temperature. – Spehro Pefhany May 28 '18 at 17:13
Most I2C slave devices are designed with a 3mA drive capability - eg http://www.ti.com/lit/ds/symlink/pcf8574.pdf. You have to select the pull-up for the weakest device on the bus. – Kevin White May 28 '18 at 17:20
Yea but all of my devices can sink 20mA – Kono May 28 '18 at 17:34
@spehroPefhany Correct; 0,4V is typical value, 1V is max value. But that does not change anything on my question – Kono May 28 '18 at 17:42
@Berkay - I gave you the reason a for why the typical assumption is 3mA drive capability. In addition to being able to provide the current for the pull-up you also need to provide current to discharge the capacitance of the line (the spec says up to 400pF) in a reasonable time. Why do you want to go so low? - it won't improve the performance and will just take more power. – Kevin White May 28 '18 at 23:13
@KevinWhite I'm driving 800pf of load. So I will use somthing like 900 ohms – Kono May 29 '18 at 09:50

Minimum value I2C-pull up resistor for arduino

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