There are various methods for sending data between multiple MCUs. Of these the most common methods are custom UART-based protocol, Manchester code and time slotting protocol. In time slotting protocol, 1s and 0s are sent as pulses with variable on time. Logic one is represented with 2ms high followed by 1ms low while logic zero is represented as 1ms high followed by 1ms low. Thus data encoded and decoded using these infos.

When idle, most low cost 315/433MHz RF modules receive ambient RF noise and so constant on/off signals are never recognized by them. However they seem to work properly with high frequency on-off signals. To reliably send data, bursts of relatively high frequency pulses are send out before broadcasting actual data and its CRC. These trigger the receiver RF module to reduce its gain and get ready to catch up data. After responding to these sync bits actual data and CRC are retrieved. The CRC is simply the logical AND of the actual 8 bit data and 0xAA.

Time slotting techniques just like Manchester code doesn't depend on whether a MCU has a UART or not. Any I/O pin can be used and the coding for such is not too large and complex.