I'm developing desktop clock with mini-meteo station. My display is Kingbright CC25-12EWA if this matters. I want to power up the whole system from one 18650 Liitokala battery so I bound to about 3.3 volts. The core MCU is atmega8a running at 8 MHz and BOD set to 2.7 volts.
For my meteo measurements I use Bosch's BMP280 or BME280 (in case when humidity matters). These parts are working pretty good with 3.3 volts. I use not the bare chips but small PCBs from AliExpress (so-called GY-modules for Arduino). They are small, have necessary capacitors and some set of pullups soldered, e. g. for using I2C by default. There are also two 10k pull-ups for SDA and SCL lines. I've tested BMP280 with my current setup running from 18650 and things looks fine.
But for RTC module I've chosen another ready-to-use module from AliExpress. It has DS1307 IC for RTC handling, another EEPROM chip (not needed for my purpose), CR2032 holder for backup battery and all necessary parts such as oscillator, capacitors and pull-ups.
Main problem with this setup is the DS1307 IC - it wants 5 volts for VCC and I2C operation. I decided that with such small power consumption of DS1307 it's more wisely to use DC-DC boost converter and step my 3.3 volts to 5 for higher VCC. I've found appropriate module on AliExpress and already ordered it.
The last thing that I want to use both BMP and DS1307 on the same TWI bus but because of higher VCC for RTC module I won't be able to use direct connection of DS1307's SDA and SCL lines to my TWI bus - RTC module already have 4.7k pullups for these two lines to the higher VCC and I want to avoid resoldering of ready-to-use RTC PCB.
Obviously I need TWI level converter and I've found that there are a lot of different solutions for this problem. As for me using two BSS138 mosfets is the simplest way. But all of examples of level converters utilize symmetric pull-ups at both sides - 3.3 and 5. In my case I have two 10k pull-ups at the 3.3 side (BMP modules have them) and two 4.7k pull-ups at 5 side (RTC PCB module use them).
Finally, there is my question. Is there any pitfalls of using famous two-BSS138 level converter (or any other solution) with different rated pullups at the both sides? May be your own solutions...