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According to the I2C bus specification The I2C pull up resistor value mainly depends on the I2C mode (Standard / Fast / Fast Plus / HS), VDD and the bus capacitive load. It's minimum value is calculated using the following equation: Rpmin = (VDD – VOLmax) / IOL. Where: VDD: Power supply voltage; VOLmax: Maximum LOW-level output voltage
A pullup resistor value needs to be small enough to overcome whatever load is on the net enough to keep the voltage out of the grey zone. (example: above 2V for 3.3V logic). Sometimes a pullup is used to hold a signal that is otherwise undriven. In this case, the load current will be the leakage current of any connected devices.
A larger resistor means less current is needed, but the line will pull up more slowly (especially on a long line) and this may limit your rate. A smaller resistor pulls up faster, but at some point you'll burn out the driver pin (check the data sheet). 3.3K to 10K is probably fine; if it's 3.3V maybe 2.2K-4.7K
However, working out the calculations for the pull-up resistor bounds gives some rather odd values. Calculating the I2C minimum pullup resistor value: Rmin = Vcc − 0.4 3mA = 966.7Ω R m i n = V c c − 0.4 3 m A = 966.7 Ω. Looking at the uC datasheets, on page 92 lists the max pin input capacitance of 10pF. However for the LCD, it has on ...
I had a look at the I2C specification here. Interestingly, 300ns only appears as the rise time requirement when you are in fast mode, which is the mode for 400kbps. The time it takes for the signal to rise to VIH is: 0.2 = e−300ns/(200pFR) 0.2 = e − 300 n s / (200 p F R) Solving for R. = × 300 200 ≈ 932 R = − 1. .
answered Oct 5, 2020 at 10:34. 166. Add a comment. Yes, you need external pull up resistors. The for the STM32 microcontrollers is 40 k Ω Ω (assuming they're even active). You can calculate a correct value, but if you just want to get working, a 4.7 k Ω Ω resistor will get you started. answered Oct 5, 2020 at 10:37.
Yes. 2k2-3k3 is fine. If you're using one of the off-the-shelf I2C peripherals like the LCD display, IO expanders, etc then warning: most of them require 5V, and ESP32 runs on 3V3. You must not connect the pullups on the ESP side to +5V, but to 3V3! I've been using this simple voltage translation schematic with success:
2. I'm trying to determine the minimum value of pull resistor for Arduino Uno. The datasheets says for arduino; VOL = 0,4 for IOL = 20 mA. (VDD = 5V) For formula for calculation of minimal pull-up is: Rmin=Vcc−0.4V/20mA = 4,6V / 20mA = 230 ohms. But I saw that people use 0,4V for 3mA.
5. I'm using 100 kHz I2C for communication with 12 slaves. I found that MAX capacitance is about 400 PF. when I calculate the pull up resistor value, it looks like i can go mucher higher in capacitance. Is this correct? Using 5V, and VOL = 0.4V for IOL = 3mA. Rmin=5V−0.4V /3mA = 1533 ohm This means I can go as low as 1533 ohm. If C bus is 650pf.
So, I understand like, suppose my I2C is running at 400kHz in both scenarios. And if the devices are placed far apart, it would take a little more time to charge the bus capacitance and to avoid this extra time, we need to have a strong pull-ups (Low pull up resistor values) on the SDA and SCL lines.