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The start code is the most important part of the packet. This identifies the start of the packet and allows the receiving side to pick it out of the random noise that is always present in the RF environment. There are many kinds of codes, but there are just a few basic concepts to understand prior to defining a start code. First comes a wake-up sequence. This is a method of preparing the receiver for the data to come. It consists of nothing more that a alternating series of 1s and 0s. It is typically at least 1 byte, but can be longer depending on the radio that is used. This centers the internal voltages and sometimes allows the receiver to extract the clock from the data. A noise filter is a long pulse that is what the receiver is looking for. The receiver times the length of the pulse and accepts the following bits only if the pulse is within the correct timing window. The length of the pulse is up to the designer and does somewhat depend on the radio. A common method is to use one byte with the first nibble 1s and the second nibble 0s. Alternatively, this can be a timed pulse not tied to a specific data rate, common values for this time are between 800µs and 1.5ms. After the noise filter it is recommended to use a digital logic filter. Sometimes called a sync byte, it is a byte with a specific value that is the same in every packet in the system. This identifies the system components to each other in case there are other systems with the same noise filter in the same area. The value and length are up to the designer.
