Tuesday, April 2, 2013

Improve RF24 Radio Performance With Proper Addressing Schemes

RF24 Addressing Gotchas

The NRF24L01(+) radios require addresses be configured on each of its Multiciever pipelines. Without much thought, people assign addresses to these pipelines. This can be problematic because of RF decoding requirements as dictated by the preamble (data transmitted in front) of each message. Read 7.3.1 and 7.3.2 of the data sheet for more information.

A longer addressing scheme allows for more opportunity to disambiguate from the preamble. As such, in my opinion, the maximum available address length should be preferred. 40-bit addressing is most effective when proper addressing is provided to each pipeline. Per the data sheet, addresses which use single bit transitions should be avoided as they are more likely to confuse preamble message detection logic should the preamble become distorted from noise.

As such, addresses with the following octets and/or nibbles (half byte) should be avoided.

Octet values to avoid:
'0xaa' - '0b10101010'
'0x55' - '0b01010101'
'0x2a' - '0b00101010'
'0x15' - '0b00010101'

Nibble values to avoid:
'0x0a' - '0b00001010'
'0x05' - '0b00000101'
'0x02' - '0b00000010'
'0x01' - '0b00000001'

This in turn means addresses with those nibbles should not be used. Remember, a nibble is half an octet, so two of those nibbles can be used to create an octet. The octets of 0xaa and 0x55 are very bad. Always avoid them. Notice these appear in the octet list to avoid and that they are made up of a pair of nibbles from our nibble list to avoid. This is because both nibbles of both octets (both halves of the byte) are 0x0a or 0x05; making for an octet of 0xaa or 0x55. Accordingly, octets 0xaa and 0x55 represent the worst possible octet to use for an address because they exactly mirror the preamble of messages.

Other nibble combinations to avoid, for example, would be 0xa1, 0x52, x12, 0x25, so on and so on. But even simple octets, such as 0x01, 0x02, and 0x05, should also be avoided.

In general, use of values contained above is more likely to cause the loss of packets and in turn, require additional retransmissions if using the auto-acknowledgement hardware features.

But Wait, There's More

The list above is hardly the exclusive list of bit patterns to avoid. In section 7.3.2, the data sheet says, "Addresses where the level shifts only one time (that is, 000FFFFFFF) can often be detected in noise and can give a false detection, which may give a raised Packet Error Rate [(PER)]. Addresses as a continuation of the preamble (hi-low toggle; [single bit transitions, as referred above]) also raises the Packet Error Rate.

As such, an address of 0xFFFFFFFFFF should never be used. And in general, octet sequences of all bits on or off should be avoided. Which means, nibbles of 0x0F and 0x00 should be frowned upon.

Please note addressing is a little more complex and that the tips provided here. These tips should be regarded as rules of thumb rather than absolutes. Regardless, if you follow the advice here, your reliability is generally improved (PER is reduced).

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