6

LT1328

The last IrDA encoding method is for 4Mbps and uses

pulse position modulation, thus its name 4ppm. Two bits

are encoded by the location of a 125ns wide pulse at one

of the four positions within a 500ns interval (2 bits •

1/500ns = 4Mbps). Range and input levels are the same as

for 1.152Mbps. Figure 4 shows the LT1328 reproduction

of this modulation.

1.152Mbps rate, for example, uses pulses of 217ns for its

bit time of 870ns. Light levels are 100mW/SR to 500mW/

SR over the 1cm to 1 meter range. A photo of a transmit-

ted input and LT1328 output is shown in Figure 3. The

LT1328 output pulse width will be less than 800ns wide

over all of the above conditions at 1.152Mbps. Pin 7

should be held low for these data rates and above.

TRANSMITTER

INPUT

RECEIVER

OUTPUT

RECEIVER

OUTPUT

TRANSMITTER

INPUT

IrDA-FIR (1m)

IrDA-FIR (1cm)

Figure 3. IrDA-FIR Modulation

RECEIVER

OUTPUT

TRANSMITTER

INPUT

IrDA-4ppm(1m)

IrDA-4ppm (1cm)

Figure 4. IrDA-4ppm

RECEIVER

OUTPUT

TRANSMITTER

INPUT

0

1

0

0

1

1

0

1

0

1328 F03a

NRZ DATA

IrDA-FIR HIGH = LIGHT TRANSMITTED

DATA RATES: FIXED AT 1.15Mbps

BIT INTERVAL: 870ns

PULSE WIDTH: 1/4 OF INTERVAL OR 217ns

0

1

0

0

1

1

0

1

0

1328 F04a

NRZ DATA

IrDA 4ppm HIGH = LIGHT TRANSMITTED

DATA RATES: FIXED AT 4Mbps

BIT INTERVAL: 500ns

PULSE WIDTH: 1/4 OF INTERVAL OR 125ns

DATA BIT

PAIR

00

01

10

11

4ppm DATA

SYMBOL

1000

0100

0010

0001