The block diagram shown on this slide illustrates how two devices, an MCU and a serial transceiver can transmit or receive data that is at different voltage levels. The MCU is a 1.8V device and transceiver can be a 3.3V or 5V device. Using the XR16M890 or XR20M1280 eliminates the need for external level shifters. The built-in level shifters allow the XR16M890 or XR20M1280 to interface with different logic voltages on both the CPU bus interface and the UART modem interface. The serial transceiver can be operating at 5V because the UART has 5V tolerant inputs. The GPIOs can be connected to a different power supply if it is communicating with another device that is operating at a voltage in the 1.8V to 3.3V range. The voltage to the core can be at any voltage ranging from 1.8V to 3.3V.