For educational purposes, I would like to write my own i2c protocol. Is it possible to write a custom i2c library to communicate using regular ports, or should I use the SCL and SDA ports? Can I mimic SDA port by using an external transistor with pull-up resistor connected to digital pin?
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\$\begingroup\$ The NXP I2C specification was originally written before dedicated hardware was available. Bit-banging a GPIO is the original way it was done, \$\endgroup\$– user319836Commented Sep 30, 2023 at 16:17
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\$\begingroup\$ I was looking at a design where the I2C bus interface available to a FPGA was actually 2 output and 1 input pins, with a CPLD on the board connecting the 3 pins on the FPGA to the actual SCL and SDA signals connected to other I2C devices on the board. As per I2C SCL from CPLD is push-pull rather than open-collector this lead to some restrictions on use of I2C. \$\endgroup\$– Chester GillonCommented Sep 30, 2023 at 17:51
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\$\begingroup\$ Continued. If it helps i2c_bit_banged.c is the source code for the bit-banged driver which was created for the above board, which runs under Linux on a x86-64 processor rather than an Arduino. Given the relatively fast processor, the driver inserts the delays from the I2C spec. \$\endgroup\$– Chester GillonCommented Sep 30, 2023 at 17:54
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Is it possible, yes, and has been done many times already.
It can be done with GPIO ports.
You can mimic SDA port with an external transistor and a pull-up, but usually nobody does it because it can be done without the external transistor by simply using the GPIO ports. No external hardware at all, only the pull-up resistors that are needed anyway.
What you seem to be trying to do is just called software bit-banged I2C interface.
Since you tagged "Arduino", I would be very surprised if the Arduino framework does not already have a software based I2C implementation built-in you can simply use so you don't even have to write it yourself.
Edit: Just confirmed, Arduino comes with "SoftWire" library .
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\$\begingroup\$ Thank you for the answer! Yes, I know that Arduino has native I2C library. I'd like to write a custom one for better understanding. Also I'm planing to program chips and to do that I'd like to try it first on Arduino board. \$\endgroup\$ Commented Sep 30, 2023 at 15:47
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\$\begingroup\$ @AntonTsvayer Arduino has both a native I2C library that uses the I2C hardware and it also has a software bit-bang library which you want to replicate. Feel free to reinvent the wheel, you can use the SoftWire as a base to learn or write your own. \$\endgroup\$– JustmeCommented Sep 30, 2023 at 15:57