I am trying to use STM32 which uses USART protocol to communicate with a UART protocol device. For the USART, I only use the tx and rx pin which mean it is working in asynchronous mode. I am new to STM32 and a bit confuse about the USART and UART communication.
Yes, they are compatible in asynchronous mode. The USART can do synchronous communication, which the UART cannot, but both can do asynchronous. They are compatible at the register level, the UART registers just don't have the synchronous setting bit implemented. You can set up the USART exactly the same way as a UART to get asynchronous communication.