LS_Fast IrDA controller
RM0082
574/844
Doc ID 018672 Rev 1
26.3.4 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 26.3.5: Baud rate generation unit on page 574
). If a frame is
completely transmitted, a frame transmitted interrupt (FT_INT,
) 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.
26.3.5
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 26.3.1: Synchronization unit on page 572
sampled by the demodulation unit (
Section 26.3.2: Demodulation unit on page 573
) 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:
f
en_pulse
= f
irda_clk
· K/L
f
en_symb
= f
en_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·fi
rda_clk
), that is 10.417 ns at SIR
and MIR (being f
irda_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 f
en_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,
f
en_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* f
en_symb
.
At last, for each FIR symbol two bits are transmitted: the symbol rate is then one half of the
bit rate, and the
baud rate generation unit
has to create a unique symbol rate of f
en_symb
= 2
MHz (being a bit rate of 4 Mbps). Since the pulse duration is a quarter of the symbol