The NEC IR transmission protocol uses pulse distance encoding of the message bits. Each pulse burst (mark – RC transmitter ON) is 562.5µs in length, at a carrier frequency of 38kHz (26.3µs). Logical bits are transmitted as follows:
When transmitting or receiving remote control codes using the NEC IR transmission protocol, the WB_IRRC performs optimally when the carrier frequency (used for modulation/demodulation) is set to 38.222kHz.
When a key is pressed on the remote controller, the message transmitted consists of the following, in order:
The four bytes of data bits are each sent least significant bit first. Figure 1 illustrates the format of an NEC IR transmission frame, for an address of
00000000b) and a command of
Figure 1. Example message frame using the NEC IR transmission protocol.
Notice from Figure 1 that it takes:
If the key on the remote controller is kept depressed, a repeat code will be issued, typically around 40ms after the pulse burst that signified the end of the message. A repeat code will continue to be sent out at 108ms intervals, until the key is finally released. The repeat code consists of the following, in order:
Figure 2 illustrates the transmission of two repeat codes after an initial message frame is sent.
Figure 2. Example repeat codes sent for a key held down on the transmitting remote controller.