Hi,
I'm not sure if this is the correct forum but I have a few questions regarding the selection of resistor pull-ups values. Currently I'm designing a I2C isolation circuit and when dealing with pull-ups I'm unsure what pull-up values to apply to signal buses. Is there a general rule to follow. I've attached a picture of the schematic as well. If you can point me in the right direction it would be greatly appreciated.
Thank you in advance.
General rule use 4K7 for 5V I2C bus lines.
Or 1K8 for 3V3 systems.
Not too much of an expert here..
Pull-up resistor value basically depends on wire capacitance.
There seem to be several I2C standards. One is 400pf/3mA.
The P82B96DRG4 seems to have that standard in the input side.
That means the two 1K input resistors are too low in value (5mA).
4k7 pull-up is widely used if the bus capacitance is low (short wires/tracks).
The output of the IC is another standard. 4000pf/30mA.
Basically, use 4k7 per bus wire if the wires are short and you only have two devices on the bus.
Lower values to 3k3 if the wire is a bit longer ( I use 8meters of Cat6 with 3k3).
Don't go lower than ~1k7 if one of the device on the bus has the 400pf standard.
Leo..
ThreeDMD:
Hi,I'm not sure if this is the correct forum but I have a few questions regarding the selection of resistor pull-ups values. Currently I'm designing a I2C isolation circuit and when dealing with pull-ups I'm unsure what pull-up values to apply to signal buses. Is there a general rule to follow. I've attached a picture of the schematic as well. If you can point me in the right direction it would be greatly appreciated.
Thank you in advance.
Question? Why are you using 100pf capacitors? I would think 100nf or 1uf would be a better choice. The whole point of a bypass capacitor is to provide highspeed logic switching current. 0.1nf is not very much charge.
Chuck.
Good spotting. Missed that.
Another one is a missing LED current limiting resistor (1k) on pin14 of the 6310.
Leo..
