In a typical system where data frequencies have not been prearranged and are unknown, the transmitter starts sending data at a relatively low data rate, and the receiver measures the edge-to-edge bit times as accurately possible. Both the transmitter and receiver must agree on the data rate so that the receiver can decide when to “sample” the signal to record a given bit. UARTs are somewhat unique among serial busses in that they use just a data signal, without a timing signal or clock. The UART may be a separate chip, but it is more likely to be an IP block that is captured as a peripheral circuit in another, larger IC (like a processor). Duplex port modelsĪ UART communication system requires two UART devices, one on the transmit side, and one on the receive side. Bidirectional communications can be “full-duplex", with information flowing in both directions simultaneously, or “half-duplex", where information only flows in one direction at a time. A given UART may use just one signal for unidirectional “simplex" communication, or two signals for bidirectional communications. UARTs use one signal to transmit data, and a second independent signal to receive data. For example, the serial port that was commonly found on PCs until quite recently used the “RS-232" standard to send serial/UART data at a higher voltage over cables to external devices like modems or printers. Some UART communication systems use “logic levels", or the native I/O voltage used by the UART’s host ICs, to move data around a given circuit board others may use higher voltage “line buffers" to move data over external serial cables. The UART circuit blocks are independent from (and are probably not aware of) the physical signaling method used to transport data. The term UART actually refers only to the digital send and receive circuit blocks, and not the transmission system itself. UARTs have been used for more than 60 years, and they were the primary means of moving serial data between digital systems for most of those years, (in fact the terms UART and serial port assumed a common meaning for many years). The receiver side must know the bit-clock frequency used by the transmitter, or it must process the received data signal and extract the bit-clock frequency on the fly. UARTs are called “asynchronous" because no timing signal is used as a part of the communication system‚ only the data signal is passed between devices. A “UART" (Universal Asynchronous Receiver Transmitter) is a device/IP block that is used for asynchronous serial data communication.
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