Universal Asynchronous Receiver Transmitter (UART) is also a common peripheral found on most MCUs. UART is a full-duplex asynchronous bus protocol with a transmit and receive function that are independent. The transmit function is a push-pull pin. Figure 2-6 shows that by using a push pull to open-drain converter and an external pullup resistor, the transmit function can be connected to the single-wire interface. The receive pin can then be tied to the same interface for reading bus reset response and data back from the single-wire devices.

The UART bus has a well-defined frame format. The frame itself consists of a mandatory start bit, variable number of data bits, an optional parity, and one or two stop bits. For interfacing the UART peripheral from a host MCU to the single wire bus, use the 8-N-1 format, where there is a start bit, followed by 8 data bits, no parity and 1 stop bit as shown in Figure 2-7.

Figure 2-8 shows that the UART baud rate is set to 187.5kbps and the host transmits 0h00 for bus reset and receives 0hFC or 0hFE for a bus reset response.

When writing to the devices, the UART baud rate is set to 500kbps and transmits 0hC0 for a logic '0' and 0hFF for a logic '1'.

When reading from the devices, the UART baud rate is set to 500kbps and transmits 0hFF. If the device sends a logic '0', the host on the receive pin can read the data as 0hFC or 0hF8. However if the device sends a logic '1', the host reads the data as 0hFF or 0hFE.

Table 2-1 provides the reference baud rates required by bus reset and data bits for standard and overdrive bus speed.