topic I2C with internal GPIO pull-up in Kinetis Microcontrollers

I2C with internal GPIO pull-up

gschelotto — Tue, 06 Aug 2019 15:24:01 GMT

Hi,

Is there any way to use I2C KL03Z peripheral with the internal GPIOs pull-up for the SDA & SCL lines? If so, how can I do this?

I've forgot to put the external ones in my custom board... :~(

Thanks in advance,
gaston

Re: I2C with internal GPIO pull-up

BlackNight — Tue, 06 Aug 2019 17:33:58 GMT

You have to check the data sheet of the KL03Z how strong the pull-ups are. Typically they will be in the 100kOhm range, so while it 'can work', I never would use that in a product as they are too weak. It is simply not reliable. Depending on the devices on your bus and the lines, typically you need something in the 4.7-10k range. See as well http://www.ti.com/lit/an/slva689/slva689.pdf

I hope this helps,

Erich

Re: I2C with internal GPIO pull-up

gschelotto — Wed, 07 Aug 2019 10:07:25 GMT

As one colleague would say: It's not a mistake, it's just a Hardware optimization.

You're right. I'd need external pull-up resistors in the 4k7-10k range, as usual...

However, I have planned to investigate a little more to see how far I can go. The fact is that I'm finally able to use the I2C bus successfully and reliably using the internal pull-up. Of course, under certain considerations I will explain below.

Internal pull-up strength of KL03Z. Luckily, for this device family, they fall in the range of 20k-50k
Bus capacitance and rise time: In my application, it is a single master-slave I2C bus without branches and with a length of 2mm (the microprocessor and the slave are placed together). Therefore I can have a very small capacity that I can estimate in about 4pF+ 8pF x 2 of devices pin capacitance (Cb total 20pF). For the maximum rise time of SDA/SCL lines, the I2C spec for the standard mode (100KHz) is tr_max = 1000ns. Then the max pull-up resistance is Rpu_max = tr_max / (Cb x 0.8473) = 59k
Finally, I don't need to go at 100KHz. For my application 10kHz is enough. Here's some results of the bus lines.

Thanks for the given hints!

regards,
gaston

Re: I2C with internal GPIO pull-up

kerryzhou — Wed, 07 Aug 2019 10:27:24 GMT

Hi Gaston Schelotto,

Please also note, PTB3 and PTB4 is the true open drain pins, if you are using these two pins, you must add the external pull up resistor.

Even with the internal pull up, in the practical project, it's better to add the external pull up resistor.

Have a great day,
Kerry

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