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and modulates it before it leaves USART0. In the same way, the input signal is demodulated before
it enters the actual USART module. The modulator is only available on USART0, and implements the
original Rev. 1.0 physical layer and one high speed extension which supports speeds from 2.4 kbps
to 1.152 Mbps.
The data from and to the USART is represented in a NRZ (Non Return to Zero) format, where the signal
value is at the same level through the entire bit period. For IrDA, the required format is RZI (Return to
Zero Inverted), a format where a “1” is signalled by holding the line low, and a “0” is signalled by a short
high pulse. An example is given in Figure 17.23 (p. 470) .
Figure 17.23. USART Example RZI Signal for a given Asynchronous USART Frame
Idle
Idle
USART
( NRZ )
S
0
1
2
3
4
5
6
7
P
St op
IrDA
( RZ I)
The IrDA module is enabled by setting IREN. The USART transmitter output and receiver input is then
routed through the IrDA modulator.
The width of the pulses generated by the IrDA modulator is set by configuring IRPW in
USARTn_IRCTRL. Four pulse widths are available, each defined relative to the configured bit period
as listed in Table 17.10 (p. 470) .
Table 17.10. USART IrDA Pulse Widths
IRPW
00
01
10
11
Pulse width OVS=0
1/16
2/16
3/16
4/16
Pulse width OVS=1
1/8
2/8
3/8
N/A
Pulse width OVS=2
1/6
2/6
N/A
N/A
Pulse width OVS=3
1/4
N/A
N/A
N/A
By default, no filter is enabled in the IrDA demodulator. A filter can be enabled by setting IRFILT in
USARTn_IRCTRL. When the filter is enabled, an incoming pulse has to last for 4 consecutive clock
cycles to be detected by the IrDA demodulator.
Note that by default, the idle value of the USART data signal is high. This means that the IrDA modulator
generates negative pulses, and the IrDA demodulator expects negative pulses. To make the IrDA module
use RZI signalling, both TXINV and RXINV in USARTn_CTRL must be set.
The IrDA module can also modulate a signal from the PRS system, and transmit a modulated signal to
the PRS system. To use a PRS channel as transmitter source instead of the USART, set IRPRSEN in
USARTn_IRCTRL high. The channel is selected by configuring IRPRSSEL in USARTn_IRCTRL.
2013-05-08 - Wonder Gecko Family - d0233_Rev0.50
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