STM32 USART 1-wire communication
Short answer:  

you don't need external circuitry if you enable open drain mode for the UART Tx pin when configured as single wire mode. Check in detail with your actual MCU.  



Referring to the application note from maxim:  



From the same Note:  


  Since most UART transmit data (TXD) pins are not open drain, an
  external open-drain buffer circuit is usually needed. This circuit can
  be constructed out of discrete components as shown in Figure 2a, or
  integrated solutions such as the Fairchild NC7WZ07 shown in Figure 2b
  are available. In both circuits, the 4.7kΩ pullup resistor provides
  the logic high on the 1-Wire bus.  


From STM32 document: