Rx Sampling Clock[(OSR+1) × Baud Rate]Baud Rate =Modulo Divide By(1 through 8191)LPUART ASYNCHModule ClockLPUART ASYNCH Module ClockDivide By(OSR+1)OSRSBR[12:0]Baud Rate GeneratorOff If [SBR12:SBR0] =0SBR[12:0] × (OSR+1)Tx Baud RateFigure 48-3. LPUART baud rate generationBaud rate generation is subject to two sources of error:• Integer division of the asynchronous LPUART baud clock may not give the exacttarget frequency.• Synchronization with the asynchronous LPUART baud clock can cause phase shift.48.3.2 Transmitter functional descriptionThis section describes the overall block diagram for the LPUART transmitter, as well asspecialized functions for sending break and idle characters.The transmitter output (LPUART_TX) idle state defaults to logic high, CTRL[TXINV] iscleared following reset. The transmitter output is inverted by setting CTRL[TXINV]. Thetransmitter is enabled by setting the CTRL[TE] bit. This queues a preamble character thatis one full character frame of the idle state. The transmitter then remains idle until data isavailable in the transmit data buffer. Programs store data into the transmit data buffer bywriting to the LPUART data register.The central element of the LPUART transmitter is the transmit shift register that is 10-bitto 13 bits long depending on the setting in the CTRL[M], BAUD[M10] andBAUD[SBNS] control bits. For the remainder of this section, assume CTRL[M],BAUD[M10] and BAUD[SBNS] are cleared, selecting the normal 8-bit data mode. In 8-bit data mode, the shift register holds a start bit, eight data bits, and a stop bit. When thetransmit shift register is available for a new character, the value waiting in the transmitdata register is transferred to the shift register, synchronized with the baud rate clock, andthe transmit data register empty (STAT[TDRE]) status flag is set to indicate anothercharacter may be written to the transmit data buffer at LPUART_DATA.If no new character is waiting in the transmit data buffer after a stop bit is shifted out theLPUART_TX pin, the transmitter sets the transmit complete flag and enters an idlemode, with LPUART_TX high, waiting for more characters to transmit.Chapter 48 Low Power Universal Asynchronous Receiver/Transmitter (LPUART)K22F Sub-Family Reference Manual, Rev. 4, 08/2016NXP Semiconductors 1323
