UPSD3422_V01 datasheet(121/300 Pages) STMICROELECTRONICS

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IrDA interface

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22.2

Pulse width selection

The IrDA interface has two ways to modulate the standard UART1 serial stream:

An IrDA data pulse will have a constant pulse width for any bit time, regardless of the

selected baud rate.

An IrDA data pulse will have a pulse width that is proportional to the the bit time of the

selected baud rate. In this case, an IrDA data pulse width is 3/16 of its bit time, as

shown in Figure 39 on page 119.

The PULSE bit in the SFR named IRDACON determines which method above will be used.

According to the IrDA physical layer specification, for all baud rates at 115.2k bps and below,

the minimum data pulse width is 1.41µs. For a baud rate of 115.2k bps, the maximum pulse

width 2.23µs. If a constant pulse width is to be used for all baud rates (PULSE bit = 0), the

ideal general pulse width is 1.63µs, derived from the bit time of the fastest baud rate (8.68µs

bit time for 115.2k bps rate), multiplied by the proportion, 3/16.

To produce this fixed data pulse width when the PULSE bit = 0, a prescaler is needed to

generate an internal reference clock, SIRClk, shown in Figure 38 on page 119. SIRClk is

derived by dividing the oscillator clock frequency, fOSC, using the five bits CDIV[4:0] in the

SFR named IRDACON. A divisor must be chosen to produce a frequency for SIRClk that

lies between 1.34 MHz and 2.13 MHz, but it is best to choose a divisor value that produces

SIRClk frequency as close to 1.83MHz as possible, because SIRClk at 1.83MHz will

produce an fixed IrDA data pulse width of 1.63µs. Table 74 provides recommended values

for CDIV[4:0] based on several different values of fOSC.

For reference, SIRClk of 2.13MHz will generate a fixed IrDA data pulse width of 1.41 µs, and

SIRClk of 1.34MHz will generate a fixed data pulse width of 2.23 µs.

Table 73.

Baud rate of UART#1 for IrDA interface

BR3

BR2

BR1

BR0

Baud rate (kbps)

115.2

57.5

38.4

19.2

14.4

12.8

9.6

7.2

4.8

3.6

2.4

1.8

1.2

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