External resistor pull-ups on I2C bus

[CMOS]

I2C Pull-ups

Hi
I want to know whether it is absolutely necessary to use external resistor pull-ups on I2C bus as it is shown in philips documents? I want to interface only a single I2C chip on Port-3 of 8051 which already has internal pull-ups. I don't think for only one chip it will be required but just want to confirm.

 

[farhada]

I2C Pull-ups

It will not work properly wihout pulups, the I2C is based on high impedance bus, even if you only have one master.

But if your device has already an internal pullup on signals, you maybe able to handle it without major ptoblems,

BR,
/Farhad

 

[Microemission]

Re: I2C Pull-ups

But if your device has already an internal pullup on signals, you maybe able to handle it without major ptoblems,
/Farhad

I've done this on a app with at90s8515 and it works great!

 

[M!k]

Re: I2C Pull-ups

try it, but I'd recommend checking the signals with an oscilloscope. These 2k - 3k pullups are responsible for proper rectangle signals. This will change as the internal pullups of the 8051 are much higher. It will then result in a lower possible clock rate.


Mik

 

[manu]

Re: I2C Pull-ups

Internals pull ups of your µC aren't really 'trues' resistors, but transistors arrays.
Use 4.7k/10k resistors.

 

[CMOS]

Re: I2C Pull-ups

Internals pull ups of your µC aren't really 'trues' resistors, but transistors arrays.
Use 4.7k/10k resistors.

What difference does a passive resistor makes as a pull-up as compared to internal active pull-ups?

 

[IanP]

Re: I2C Pull-ups

The internal pullup "resistors" are field-effect transistors.
They are therefore non-linear.
Because of multifunction of pins the pullup circuit is not just a single MOS transistor but has at least two parts (NMOS) and 3P-MOS in CMOS.

Passive resistor can be practically treated as a linear device...