SPRUIU1C July 2020 – February 2024 DRA821U , DRA821U-Q1
In FIR mode, data is transferred between the Host CPU and the peripheral devices at 4 Mbps. A FIR transmit frame starts with a preamble that is followed by a start flag, frame data, CRC-32, and ends with a stop flag.
Figure 12-395 shows the FIR transmit frame format.
Preamble (16x) | Start flag | Frame data | CRC-32 | Stop flag |
On transmit, the FIR transmit state-machine attaches the preamble, start flag, CRC-32, and stop flag. An abort sequence requires at least two transmissions of 0000. Back-to-back frames are allowed, but each frame must be complete.
The state-machine also encodes the transmit data into 4-PPM format (see Table 12-728) and generates the SIP (see Section 12.1.6.2.3.3.3.2, SIP Generation).
Data Bit Pair (Bin) | 4-PPM Data Symbol (Bin) |
---|---|
00 | 1000 |
01 | 0100 |
10 | 0010 |
11 | 0001 |
The four symbols described in Table 12-728 are the legal, encoded data symbols. All other combinations are illegal for encoding data. Some of these illegal symbols are used in the definition of the preamble, start flag, and stop flag because they are unambiguously not data (see Table 12-729).
Frame Part | Transmitted Frame (Bin) |
---|---|
Preamble | 1000 0000 1010 1000 (16 repeated transmissions) |
Start flag | 0000 1100 0000 1100 0110 0000 0110 0000 |
Stop flag | 0000 1100 0000 1100 0000 0110 0000 0110 |
All fields are transmitted LSBs of each byte first (see Table 12-730).
Data Byte (Hex) | Data Byte Pair (Bin) | 4-PPM Data Symbol (Bin) | Transmission Order |
---|---|---|---|
0x0B | 00 | 1000 | 4 |
00 | 1000 | 3 | |
10 | 0010 | 2 | |
11 | 0001 | 1 |
On receive, the FIR receive state-machine recovers the receive clock, removes the preamble and the start flag, decodes the 4-PPM incoming data, and determines the frame boundary with reception of the stop flag. The state-machine also checks for errors such as illegal symbol, CRC error, and frame-length error. At the end of a frame reception, the Host CPU reads the line status register (UART_LSR_IRDA) to detect errors of the received frame.
The module can transmit and receive data, but when the device is transmitting, the IR RX circuitry is automatically disabled by hardware.