RM0319 datasheet(250/368 Pages) STMICROELECTRONICS

250 / 368 page

Fast infrared port (IRDA)

RM0319

250/368

Doc ID 022640 Rev 3

Transmission state

The wrapper unit builds the TX frame out of IrLAP frame which should be transmitted. The

TX frame is then sent (LSB first) to the modulation unit. The encoding mode depends on the

used infrared mode, which is determined by the 2-bit field MODE of the parameter register

IrDA_PARA.

19.3.4

Modulation unit

The modulation unit is active in the transmission state only, and it is responsible for the

modulation of the TX frames from the wrapper unit in order to generate the TX signal for the

off-chip IrDA transceiver. The actual modulation performed by this unit depends on the

infrared mode (SIR, MIR or FIR).

Note:

The POLTX bit in the IrDA_CONF register determines the polarity of the TX signal.

The TX signal is generated by means of the en_symb and en_pulse signals from the baud

rate generation unit (Section 19.3.5: Baud rate generation unit on page 250). If a frame is

completely transmitted, a frame transmitted interrupt (FT_INT, Section 19.4) is generated

and the FIrDA controller changes back to the listening state.

In case of FIR mode a 4PPM modulation is used. Additional preamble (PA), start flag (STA)

and stop flag (STO) are added. With a bit rate of 4 Mbps the resulting data symbol duration

is 500 ns and the chip duration is then 125 ns.

19.3.5

Baud rate generation unit

The baud rate generation unit creates the two enable signals which are used throughout the

FIrDA controller, namely:

●

en_symb, which determines the symbol rate at which the synchronized inverted RX

signal from synchronization unit (Section 19.3.1) is sampled by the demodulation unit

(Section 19.3.2) in the reception state of SIR and MIR modes.

●

en_pulse, which creates the pulses of the TX signal during transmission.

The two signals are obtained from the same irda_clk clock signal by using cascaded clock

dividers, so the resulting frequencies are:

fen_pulse = firda_clk · K/L

fen_symb = fen_pulse / (N+1),

where the values of K, L and N parameters are determined by software setting the 8-bit field

INC, the 11-bit field DEC, and the 8-bit field N, respectively, of the divider register IrDA_DV.

Note:

The fractional divider causes jitter with a maximum of 1/(2·firda_clk), that is 10.417 ns at SIR

and MIR (being firda_clk = 48 MHz), which meets the IrPHY specification.

In case of SIR, for each SIR symbol one bit is transmitted, then the bit rate and the symbol

rate are equal. It follows that the baud rate generation unit has to create the following symbol

rates: fen_symb = 9.6 kHz, 19.2 kHz, 38.4 kHz, 57.6 kHz and 115.2 kHz. Besides, since a

pulse duration of 1.736 µs is used in SIR transmission, the baud rate generation unit has to

create a pulse rate of fen_pulse = 576 kHz.

Like SIR, for each MIR symbol one bit is transmitted only, then the bit rate and the symbol

rate are equal. The baud rate generation unit has to create the following symbol rates,

fen_symb = 576 kHz and 1.152 MHz. Moreover, since a pulse duration of a quarter of the

symbol duration is used for MIR transmission, the baud rate generation unit has to create a

pulse rate of fen_pulse = 4* fen_symb.

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