Complete synchronous USART implementation
Unlike RS-232, which is a standard, an "USART" is not a standard in any way. An MCU containing an USART peripheral, as its name says, contains a "Universal Synchronous-Asynchronous Receiver/Transmitter". This is what the peripheral does: it can send and receive data either with a clock line or without. That's it. The specification of the actual implementation is up to the chip manufacturer. Configuration registers, possible configurations, the fact that the port has half or full duplex, etc., depend on the specific implementation. Generally those peripherals are flexible enough to be used to communicate with RS-232 (through a voltage converter of course), IrDA infrared ports, smartcards, serial buses like LIN, etc.

Compare for example the USART descriptions in the datasheets of different MCUs (I chose those MCUs because I have some familiarity with their datasheets, but it can be any other ones containing an USART...):


ATMega328P (see "USART0" on page 143)
PIC18F66K80 (see "EUSART0" on page 339, also note that it is named an "EUSART" here, where the E stays for enhanced, as it has more functions than other USARTs from the same MCU range)
STM32F4xx (see "USART" on page 965)


You will see that these peripherals have a lot of common functionality, but are implemented in different (although relatively similar) ways.

To study USARTs, therefore have a look at different MCUs so you will have a more general idea.