26.3.4 Serial IR (SIR)

The UART peripheral includes an IrDA SIR encoder and decoder block. The IrDA SIR block provides functionality that converts between an asynchronous UART data stream and a half-duplex serial SIR interface. No analog processing is performed on-chip. The role of the SIR block is to provide a digital encoded output and decoded input to the UART. When enabled, the SIR block uses the UnTx and UnRx pins for the SIR protocol. These signals should be connected to an infrared transceiver to implement an IrDA SIR physical layer link. The SIR block can receive and transmit, but it is only half-duplex so it cannot do both at the same time. Transmission must be stopped before data can be received. The IrDA SIR physical layer specifies a minimum 10-ms delay between transmission and reception. The SIR block has two modes of operation:

• In normal IrDA mode, a zero logic level is transmitted as a high pulse of 3/16th duration of the selected baud rate bit period on the output pin, while logic one levels are transmitted as a static LOW signal. These levels control the driver of an infrared transmitter, sending a pulse of light for each zero. On the reception side, the incoming light pulses energize the photo transistor base of the receiver, pulling its output LOW and driving the UART input pin LOW.
• In low-power IrDA mode, the width of the transmitted infrared pulse is set to three times the period of the internally generated IrLPBaud16 signal (1.63 µs, assuming a nominal 1.8432-MHz frequency) by changing the appropriate bit in the UARTCTL register (see Section 26.5.8).

Whether the device is in normal or low-power IrDA mode, a start bit is deemed valid if the decoder is still low one period of IrLPBaud16 after the low was first detected. This enables a normal-mode UART to receive data from a low-power mode UART that can transmit pulses as small as 1.41 µs. Thus, for both low-power and normal mode operation, the ILPDVSR field in the UARTILPR register must be programmed such that 1.42 MHz < f_IrLPBaud16< 2.12 MHz, resulting in a low-power pulse duration of 1.41 to 2.11 μs (three times the period of IrLPBaud16). The minimum frequency of IrLPBaud16 ensures that pulses less than one period of IrLPBaud16 are rejected, but pulses greater than 1.4 μs are accepted as valid pulses.

Figure 26-3 shows the UART transmit and receive signals, with and without IrDA modulation.

Figure 26-3 IrDA Data Modulation

In both normal and low-power IrDA modes:

• During transmission, the UART data bit is used as the base for encoding
• During reception, the decoded bits are transferred to the UART receive logic

The IrDA SIR physical layer specifies a half-duplex communication link, with a minimum 10-ms delay between transmission and reception. This delay must be generated by software because it is not automatically supported by the UART. The delay is required because the infrared receiver electronics might become biased or even saturated from the optical power coupled from the adjacent transmitter LED. This delay is known as latency or receiver setup time.