I2C and internal pull-ups

Protech · Joined: 25 Jun 2008 Posts: 1

Hello,

I want to use the internal pull-ups of an 16F886 with the I2C functions.
I'm using pin RB1 for SDA and pin RB4 for SCL. So I'm not using the hardware pins for the I2C. It is possible to switch the pull-ups individually. So far there is no problem. But at the point I'm using the i2c_start() function, the pull-up is switched off. I think it should be possible to let the pull-up high in case of using the pin as an input with a software i2c. But this will not work.

Is it not possible to use the I2C with the internal pull-up ?

Maybe someone can help me

Hermann

Ttelmah · Guest

I'm afraid you have to add external pull-ups.
The internal pull-ups just aren't strong enough to drive an I2C bus.
They are typically equivalent to between about a 100KR to 15KR resistor.
I2C, at 100K, even with a low bus capacitance of perhaps 70pF, requires a minimum of about 17KR for it's pull-ups, and with a more typical 100pF, the minimum is about 9KR. You might _just_ get it to work at a very low I2C rate, if your device has particularly 'strong' pull-ups, but it is never going to be reliable.

Best Wishes

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

Internal pull-ups are only used when the pin is in input mode. As soon as the TRIS is changed to make the pin an output the pull-up is disabled. With this being said, when the pin tries to drive the bus the pull-up will not be available. External resistors are required.

Ronald

Ttelmah · Guest

I think the 'idea' ought to work, but is so far outside the specs, that it is unlikely to be even remotely practical!.
The pull up is disabled when TRIS is set to 0, making the pin an output, but if the chip's logic diagram is right, should come on again when it is set back to '1'. In software I2C, the technique used, is to put a '0' into the output latch, and then set TRIS to '1', when you want a '1' out (driver turns off, and the line is pulled up by the resistor on it), and then set the TRIS to '0', when you want to pull the line down. As such, if the internal pull up is enabled, it should pull the line high, when needed. However the available pull-up current, is off the end of the graph for the I2C bus spec. Two limits apply, the one on bus capacitance (which limits the speed the link could be run), and the spec in I2C, that the inputs can require up to 10uA pull-up current. The graphs f 'maximum pull-up resistor', basically end just under 20K, and the PIC internal pull-up, is unlikely to be lower than this.
'Nice idea', but it isn't going to work...

Best Wishes