STM32F103: Can a timer can interfere with uart?
I think the problem is the time spent by timer interrupt routine and you have not configured the priorities. Either make the timer routine efficient enough, or make a DMA buffer for the UART, or raise the priority (lower priority number) of the uart interrupt (which may mean visual glitches on the display, in this case make critical parts of timer code non-interruptable and see if it is sufficient to help the UART)

Also you can unify all of them in one timer interrupt routine. You either make the timer frequency fast enough for the UART without DMA buffer and poll it &amp; run the normal timer code with a lower frequency, or, make a DMA buffer and run the timer as slow as the normal code needs and poll the UART buffer. This makes things more deterministic in contrast of polling in the process mode.