A transmitter and receiver must be programmed with identical 32 bit radio IDs. With those IDs, the transmitter and receiver select a set of 23 channels using a pseudorandom number generator.
23 channels are selected. Their selection and order is determined as follows:
The pseudorandom number function is:
prn = (prn * 0x0019660D) + 0x3C6EF35F
A potential channel is prn % 125. Channels are evaluated in order such that:
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The prn is evaluated once, then modulo 125 to generate a candidate channel
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No channel is repeated. If the candidate channel has already been selected, the candidate is discarded.
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No more than a certain number of channels can be chosen from each of the different bands. If a candidate channel would exceed this limit, it is discarded.
- 0-31: 6
- 32-63: 6
- 64-95: 6
- 96-125: 5
The transmitter and receiver use the nrf24l01 Enhanced ShockBurst with the following parameters:
- 5 byte address field, the channel ID mapped into 5 bytes as below
- Dynamic payload length
- 2 byte CRC
- No automatic retransmission
- Auto acknowledgement
- 1Mbps data rate
The transmitter sends a new packet every 20ms, waiting 1ms for an acknowledgement with data on the same channel. Each transmitted packet advances to the next channel in the channel selected list, wrapping around.
To initially lock onto a transmitter, the receiver picks a random channel from the list and listens for 20 channel change times (0.4s). If no packet is received, it then moves to the next channel in the list and waits another 20 time periods, continuing this process until a packet is received. Once a packet is received it switches to the following procedure.
In normal reception, immediately after receiving and acknowledging a packet, the receiver switches to the next channel in the list. It waits at least 22ms for the next packet to be received. If no packet is received, then the channel is switched to the next one and listening recommences. If 5 packets in a row are missed, then the receiver returns to the initial lock procedure.
The ShockBurst address is derived from the 32 bit ID as follows.
1100bbbb bbbbbbbH bbbbbbbH bbbbbbbH bbbbbbH
b is a bit from the 32 bit ID.
The H bits are selected to be the opposite of whatever the
preceeding bit was.
Rationale: The highest bits are defined to be distinct from the preamble, which consists of alternating bits. The H bits are present to reduce the likelihood that random noise will cause the address to be identified by the baseband receiver.
The transmitter and receiver have 15 different "slots" to hold outgoing data. Each can be configured for a different priority or transmission rate and each can hold up to 16 bytes of data.
The transmission rate/priority is configured by providing a 32 bit bitmask denoting in which timeslots this slot should be sent. Configuring an infeasible set of slots will result in unspecified data being sent at a lower rate.
A priority of 0 is the default, which signifies that the slot should not be transmitted. 0xffffffff implies it should be sent every timeslot. 0x55555555 would be every other timeslot, etc.
The data in a slot will continue to be transmitted at the specified frequency whether or not it has been updated by the client recently.