SWRU626 December 2025 CC3501E , CC3551E
When the UART.CTL[1] SIREN bit is set, the IrDA (SIR) encoder and decoder are enabled and provide hardware bit shaping for IrDA communication. In this protocol, from the transmitter perspective, a zero is transmitted as a high pulse and a one is transmitted as a zero.
The width of the pulse is specified as 3/16th of the selected bit period. The SIR decoder converts the IrDA compliant receive signal into a bit stream for the UART core. The SIR receive logic interprets a high state as a logic one and low pulses as logic zeros. For more details please refer to the SIR Physical Layer Link Specification Version 1.1
Setting the UART.CTL SIRLP[2] bit enables low power mode. In the low power mode, the width of the pulse is set to 3 times the time period of the IrLPBaud16 signal. The IrLPBaud16 signal is generated by dividing down the SOC CLK (80MHz) according to the low-power divisor value written to the UARTILPR register. The low-power divisor value is calculated as follows:
where FIrLPBaud16 is nominally 1.8432 MHz.
The divisor must be selected such that 1.42 MHz < FIrLPBaud16 < 2.12 MHz, results in a low-power pulse duration of 1.41-2.11μs (three times the period of IrLPBaud16).
The time period of the nSIROUT bit stream is still equal to the programmed bit period and is governed by the period of the Baud16 signal.
The SIR receiver section contains a 4-bit binary counter, which operates on SOC CLK in the normal mode with the Baud16 signal as an enable signal. When the input SIRIN signal is high, the decoded output signal, RXD, is driven high to indicate a 1. A low on the SIRIN line is sampled multiple times on IrLPBaud16 for glitch rejection and converted into a low on the RXD line. The time period for which the RXD line is pulled low corresponds to approximately one bit period at the programmed bit rate. After the counter rolls over, input sampling restarts. The SIRLPSync signal is the SOC CLK-synchronised version of the SIRLP mode selection bit in the UART.CTL register. This signal determines the IrDA encoding strategy i.e. whether the IrDA transmitter block is to operate in the normal mode or in the low power mode. The SIRENSync signal is the SIREN bit in the UART.CTL register, which has been synchronised to SOC CLK. This signal enables / disables the SIR section.