It is important to make the VT transistor open and close as quickly as possible. It would seem that there are no obstacles to its instantaneous opening. However, because of so called Miller Cmil capacitance, local negative feedback occurs, and it prevents the transistor from fast entering into the saturation state. You may object after drawing attention to the value of R resistor, and offer to reduce its capacity. But then we will face with a significant increase in the current consumption of chips, which is, of course, also not good.
Limitations imposed on the minimum value of Rpu resistor are connected with the fact that current not exceeding 3 mA must flow through Rpu resistor in the line state of "0". It is easy to see that for the voltage Vdd = 5 V, R value is limited to the par value of 1.7 kOhm. Accordingly, the capacity of the line should not exceed 200 pF.
Another important technical characteristic of the bus is its compatibility. Previously developed elements that have only low-speed exchange capabilities should connect with high-speed ones and vice versa. Let us now consider the question of compatibility of devices connected to the bus in greater detail.
Thus, the 12C bus can be connected to three types of interfaces: low-speed, fast-speed, Hs-mode. It is understood that the data exchange can be performed at a rate available to the slowest interface.
If you connect devices of different types on the classical scheme, different exchange rates may not be realized. Therefore, it is necessary to group the devices, and introduce the bridges between the groups. The assignment of a bridge is to disconnect the slower elements at the time of exchange between the faster ones.
Additional advantage ensured by the bridge is the ability to use the power bus of two voltages (VDD1 and VDD2). The bridge is switched on as a master / slave device of Hs-mode and controlled by SDAH, SCLH, SDA, SCL signals. It can also be a part of the individual chips (e.g., microcontrollers) as an independent device.
We should also note that the 12C bus allows combining the devices with different supply voltages. It’s desirable to connect pull-up resistors to the supply voltage of 5 V.