buscapacitancei2c
We are trying to calculate how many I2C slaves we can hook up the I2C bus.
We tried to run a square wave through a serial resistor and measure the rise time but we get a strange result. More nodes decrease the capacitance, not increasing it as expected.
Is there another way to get the capacitance?
Looking at the datasheet datasheet, it seems like those diodes can have a significant reverse current -- up to several milliamps at 5V if they're warm. They also have a nontrival junction capacitance, on the order of 100 pF.
The main question is whether this is an AC problem or a DC problem. If the DC level is correctly pulled up to 5V but the rise times are too slow during transmission, try using stiffer pull-ups, no more than 1000 ohms. (You do have pull-up resistors, right? You need pull-ups for I2C.) If the DC level is stuck at 3.6V, try measuring the diode current with a multimeter to make sure they're not broken. You could also try removing the 100 ohm resistor.
I can think of three possible mechanisms that may result in the system starting to work if you touch the SCL line with an oscilloscope probe. All three indicate a fault in the unit.
First, it may be a mechanical issue such as a bad solder. When you poke it with the probe, it may repair the connection and signal starts to flow.
Second, it may be a noise issue; crosstalk, coupled noise from any source, and the application of the probe's capacitance dampens that noise. But that would mean that your I2C bus's pull-up resistors are far too small. They should be something like a few kilo-ohms.
Third, your SCL and SDA signals might be switching at the same time, which is a no-no in I2C bus. You shouldn't change both signal states in the same operation; there should be a time delay between any edge in SDA signal to any edge in SCL signal. If the both switch at the same time, the application of the probe capacitance to SCL may delay the SCL edge enough that the I2C device's input data stabilizes before it sees the edge in the clock signal. And this failure mechanism would indicate an error in your microcontroller code.
Edit: For a reference, here is a picture of a proper i2c waveform from the datasheet of the microcontroller which the asker is using.
Image source: LPC1769 datasheet page 59
Best Answer
The answer is "it depends." It depends on how strongly each device can drive the I2C lines; on how small of pull-up resistors all can tolerate; on what speed the I2C is clocking at; and how much parasitic capacitance is induced by the bus wiring length and topology. Smaller pull-ups, slower clock, and shorter (external-PCB, good spacing) bus distances all reduce capacitance and thus allow more devices. Even prototyping on a solderless breadboard is not ideal, as the pin connections will likely introduce significantly more capacitance than say, a 2-sided PCB will. All of these aspects could be calculated and/or simulated, but Renesas has a good point in that "testing it" may be the fastest and most accurate way to arrive at a real-world answer.