S
C0
C1
C2
C3
C4
C5
C6
D0
D1
D2
D3
D4
S1
S2
T
A4
A3
A2
A1
A0
C5
C4
C3
C2
C1
C0
"0"
T
T
"1"
T
T
T
SIRC bit encoding
6ms
6ms
6ms
1.2ms
Functional Description
Figure 19-26. SIRC Bit Encoding
To construct comprehensive packets that constitute remote control commands, the host software must
combine a number of eight bit data characters in a sequence that follows one of the universally accepted
formats. For illustrative purposes, a standard RC5 frame is described below (the SIRC format follows this).
Each of the above fields in RC-5 can be considered as two T pulses (digital bits) from the TX and RX
FIFOs.
The standard RC5 format as seen by the UART_IrDA in CIR mode.
Figure 19-27. RC-5 Standard Packet Format
Where:
S1, S2: Start bits (always 1)
T: Toggle bit
A4–A0: Address (or system) bits
C5–C0: Command bits
The toggle bit T changes each time a new command is transmitted to allow detection of pressing the
same key twice (or effectively receiving the same data from the host consecutively). Since a code is being
sent as long as the CPU transmits characters to the UART for transmission, a brief delay in the
transmission of the same command would be detected by the use of the toggle bit. The address bits
define the machine or device that the Infrared transmission is intended for and the command defines the
operation.
To accommodate an extended RC5 format, the S2 bit is replaced by a further command bit (C6) that
allows the command range to increase to 7-bits. This format is known as the extended RC-5 format.
The SIRC encoding uses the duration of modulation for mark and space; hence the duration of data bits
inside the standard frame length will vary depending upon the logic 1 content. The packet format and bit
encoding is illustrated below. There is one start bit of two milliseconds and control codes followed by data
that constitute the whole frame.
Figure 19-28. SIRC Packet Format
It should be noted that the encoding must take a standard duration but the contents of the data may vary.
This implies that the control software for emitting and receiving data packets must exercise a scheme of
inter-packet delay, where the emission of successive packets can only be done after a real time delay has
expired.
3490
Universal Asynchronous Receiver/Transmitter (UART)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated