IR - Transmit IR NEC Raw Data and Decode

On the transmission side, the transmitter will send IR NEC raw data. The raw data can be seen as consecutive durations of “marks” and “spaces” (Figure 3) in microseconds (us).

• Mark: a specific period of sending pulses
• Space: a specific period of sending nothing

For more details, please refer to SB-Projects’ topic of IR Remote Control Theory to learn the theory of IR remote controls operation and a collection of IR protocol descriptions. In this example, we are going to use NEC (Now Renesas, also known as Japanese Format) as the transmission protocol.

NEC Features
• 8-bit address and 8-bit command length.
• Extended mode available, doubling the address size.
• Address and command are transmitted twice for reliability.
• Pulse distance modulation.
• The carrier frequency of 38kHz.
• Bit time of 1.125ms or 2.25ms.

Modulation
NEC protocol uses Pulse Distance Encoding of the bits for data communication (Figure 4). A logical “1” is represented by total duration of 2250us, with 560us of “marks” and (2250-560) us of “spaces”. While logical ”0” is represented by total duration of 1120us, with 560us “marks” and (1120-560) us of “spaces”.

Since a total number of 32-bit data together with the header and the end-bit will be transferred (Figure 5). If we separate the data in the time-frame (in us), there will be ( 2 + 32 ) x 2 + 1 = 69 “marks” / “spaces” to be transmitted (Figure 6), which forms the raw NEC data we would like to transmit in our Arduino “*.ino” file. This part of the code can be modified by users. Details of how to obtain raw data code for your remote devices, you may refer to Ken Shirriff’s blog, where it provides multiple libraries provided online.