Although assembly language is much easier to use since the mnemonics make it immediately clear what is meant by a certain instruction, it must be pointed out that assembly language is coupled to the specific microprocessor. A program called an assembler is used to convert the application program written in assembly language to machine language. Even though programming in assembly language is time consuming, assembly language programs can be very efficient and should be used especially in applications where speed, access to all functions on board, and size of executable code are important. For example, to add two numbers, the instruction in assembly language is ADD. Assembly languages were developed that express elementary computer operations as mnemonics instead of numeric instructions. Programming in machine language is incredibly slow and easily leads to errors. A computer chip understands machine language only, that is, the language of 0’s and 1’s. Programming languages provide the link between human thought processes and the binary words of machine language that control computer actions, in other words, instructions written by a programmer that the computer can execute. Martin Plonus, in Electronics and Communications for Scientists and Engineers (Second Edition), 2020 8.2.3 Communicating with a computer: Programming languages However, it is not known when a packet will begin or end as it is not tied to any timing convention. In most data communications systems, data is transmitted in packets, usually synchronous. The term “asynchronous” also refers to the random start of any transmission. These codes take the form of multibit words that form the packet overhead. Data is transmitted in blocks or frames of words and special fields define the beginning and end of each frame. However, synchronous transmission does have its overhead. Synchronous transmission is faster than asynchronous because there are fewer overhead bits. Digital counters keep track of word boundaries by counting bits and words. Each data word is transmitted directly one after the other in a block where the beginning or end is defined by selected bits or words. The overhead of the stop and start bits can be eliminated or at least minimized by using synchronous transmission. While UARTs are available as an IC today, they are more likely to be integrated into an embedded controller or other IC. Control logic provides input and output control signals used in some protocols. The received byte is then transferred in parallel to the buffer and then to an external microcontroller or a PC. The start, stop, and parity bits are monitored and actions are taken accordingly.
Received data comes in serially to the lower shift register. The UART is at the heart of most asynchronous interfaces.
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