Software-Based UART Trade-Offs

　　For μCs that lack an internal UART， the simple and seemingly obvious way to implement serial communications is through software. Extra hardware is not required， and the μC then handles its own communications. The designer can indeed eliminate a hardware UART by creating one in software， but that arrangement has its own problems and costs. Except in the simplest cases， the true cost of a software UART must include the percentage of computational time demanded from the CPU. Realistically， a software UART is more costly than a hardware UART.

　　Software UARTs require substantial resources. In most cases a counter/timer （crucial in μCs） is needed to generate time slices for the serial bit cells. At least two I/O ports are required for the serial input and output （RX and TX）， and RX should have an interrupt capability that allows incoming start bits to synchronize the incoming data （Figure 1）。 If handshaking is required （via the CTS and RTS terminals， for example）， the system may require other port pins as well. Because reliable reception requires that the maximum interrupt latency be kept well below one-half of a bit interval， the interrupt requirement complicates system designs （Figure 2）。 Small microprocessors （μPs） can be overwhelmed， especially at high baud rates （Figure 3）.