Pull Up Resistance

[Dot Liljenquist]

I was wondering if anyone knows the correct way to measure the existing value of a sensor's pullup resistance. I have an existing I2C ToF distance sensor which is sealed in a casing which has four wires (Vcc, Gnd, SDA & SCL) . Is it as simple as measuring the SDA and SCL lines against the ground or is there more to it?

well, a pullup would connect from signal to VCC, so that's worth a try, not ground.
But, it's very likely to not yield useful info. Why? Because I wouldn't expect the instrument to have a built in pullup. Doing that means that every sensor adds a pullup, and that quickly begins to load the signals unacceptably. So if there IS a measurable resistance, say 4.7k ohm, then your device is limiting the number of other devices you can add to your I2C bus, which shouldn't be the case. I'm hoping you'll read a very high resistance, which will mean that you, as the system designer, have the freedom to add your pullup where YOU want to.

I would disagree. These sensor boards are not designed for a professional user. These things a purely hobby stuff. Why else would you sell a board with the actual chip on it and then encapsulate it. If the sensor only has one I2C address then you are not going to use more than one module on the same bus anyway are you?

If you are not sure if pull-up is there or not: you could enable internal MCU GPIO pin pull-up. This should result in a proper 3V3 level when not in use. But this pull-up might be two weak for operation (often 10K internal MCU pull-up but way too large, e.g. for 100 KHz often 2K2 used).
But when you see now, instead of a floating signal - there is now a stable 3V3 on SDA and SCL - it tells you: there are NO pull-ups outside.

I2C specifies an acceptable (max.) current of 3 mA, not a resistance. Connect and power all devices connected to the bus and measure the short circuit current from SCL and SDA to GND. If it exceeds 3 mA then some pullup resistors have to be removed.

@Grumpy_Mike But you are right, in a sense. Just like most of the el cheapie MAX485 adapter boards have the 120 ohm resistor installed A-B, resulting in bus overload if more than a few are attached. User beware, I guess. Typical newbie trap.

I have an existing I2C ToF distance sensor which is sealed in a casing which has four wires (Vcc, Gnd, SDA & SCL) . Is it as simple as measuring the SDA and SCL lines against the ground or is there more to it?

You can get an approximation by using a pull down resistor, maybe in the 2K range to start. Power it up then measure the voltage from ground to SCL and SDA, when you get a useful reading you can use Ohm's law to calculate the resistance. Both are open drain or open collector so if not asserted you should come out reasonable close. What you are doing is forming a voltage divider and your resistor is the lower one, the pull up will be the sum of all of the pull up resistors on each lead.

I would connect only power/ground to the sensor.
First measure voltage on the I2C pins, to see if it's internals work with 3.3volt or 5volt-logic.
Then measure current between SCL and ground, repeat for SDA and ground.
Now you know logic voltage and pull up current.
Working out pull up resistance is a matter of using ohms law.
Leo..

Thanks for all the feedback. I finally decided to bust open the custom box the ToF sensor was enclosed in to find a terabee evo-mini I2C ToF sensor. I was able to confirm that the sensor does indeed have a 10K pullup on SDA/SCL as per the manual (page 25) evo_mini_manual . I'm using this with a Mega which also has permanent 10K pullups, so I'm thinking of de-soldering the pullup on the mega board since there is no way to remove them using code (there soldered in...). I'm using this sensor as it's much more accurate for reading unlevel surfaces (such as grain containers). It uses 3.3v logic, but I'm using a level shifter to make it work with the mega's 5 volts... Thanks again for all the help...

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When I put together a test circuit using the recommended 510k ohm pull up on the enable input, the regulator wouldn't turn on. I measured the resistance to ground at the enable pin and it measured about 14k ohms, rather than the 1 Meg typical specified in the data sheet . Based on that measurement I used a 24k ohm pullup to VIN, and the regulator turned on and worked properly. Is the data sheet value for the pull down wrong, or is my device defective?

I only built one circuit, so I can't check against a different board. I just wanted to know if 1Meg was the correct value for the internal pull down. I have attached my circuit, which didn't work with R1 = 510k, but did work with R1 = 24k. Measured resistance from pin 5 to ground was 14k without R1.

Yes, as it says in 7.3.6, "To enable the device, connect a pull-up resistor R4 (typical 510 KΩ to limit the quiescent current) to the VIN pin. So there is a voltage divider consisting of the external 510k resistor and the internal Rpd "(typical 1Megohm)". But my question is whether Rpd is really 1Meg, because it wasn't on the one device I tested. Note that Figure 13 shows a zener diode on the EN input, so a supply voltage larger than 7 Volts can be used, so long as the current into the EN pin is limited (7.3.5 paragraph 2).

In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal.[1] It is typically used in combination with components such as switches and transistors, which physically interrupt the connection of subsequent components to ground or to VCC. Closing the switch creates a direct connection to ground or VCC, but when the switch is open, the rest of the circuit would be left floating (i.e. it would have an indeterminate voltage).

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