The SIR transmit encoder modulates the non return-to-zero (NRZ) transmit bit stream output from the UART. The IrDA SIR physical layer specifies the use of a return to zero, inverted (RZI) modulation scheme that represents logic 0 as an infrared light pulse. The modulated output pulse stream is transmitted to an external output driver and infrared light emitting diode (LED).
The frequency of IrLPBaud16 is set up by writing the appropriate divisor value to the IrDA Low Power Counter register, UART.IR_LOWPR.
The active-Low encoder output is normally Low for the marking state (no light pulse). The encoder outputs a high pulse to generate an infrared light pulse representing a logic 0 or spacing state.