4Ppm Modulation; Ppm Modulation Encodings - Intel PXA255 Developer's Manual

Fast Infrared Communication Port
11.2.1

4PPM Modulation

Four-position pulse modulation (4PPM) is used to transmit data at the high-speed rate, 4.0 Mbps.
Data bits are encoded two at a time by placing a single 125 ns light pulse in one of four timeslots.
The four timeslots are collectively termed a chip. Bytes are encoded one at a time. They are divided
into four individual 2-bit pairings called nibbles. The least significant nibble is transmitted first.
Figure 11-1
an example of 4PPM modulation for the byte 0b10110001, which is constructed with four chips.
Bits within each nibble are not reordered, but nibble 0 (least significant) is transmitted first and
nibble 3 (most significant) is transmitted last.
Figure 11-1. 4PPM Modulation Encodings
Data = 0b00
Data = 0b01
Data = 0b10
Data = 0b11
Figure 11-2. 4PPM Modulation Example
Original
Byte Order
Reordered
Nibbles
Chips
Timeslots
4PPM
Data
Receive data sample counter frequency = 6/pulse width. Each timeslot is sampled on the third clock.
11-2
shows the 4PPM encoding for the possible 2-bit combinations and
1
Nibble 3
1 0
0 1
Nibble 0
1
1 2 3 4 1
Chip
Timeslots
2 3
Nibble 2 Nibble 1
1 1 0
0 0 1
Nibble 1 Nibble 2
3
2
2 3 4 1 2 3
Intel® PXA255 Processor Developer's Manual
Figure 11-2
4
Nibble 0
0 0 1
1 1 0
Nibble 3
4
4 1 2 3 4
125 ns
48 MHz
shows