RF24 Performance Improvement w/SPI

The Up Side

The RF24 (nrf24l01, nrf24l01p, nrf24l01+) radios use the SPI protocol to communicate with its master device. The speed at which the devices communicate is controlled by the master's clock speed. Here, the Arduino is the master device. Accordingly, we can control the Arduino's SPI bus speed. On the Arduino, this clock speed is set by means of a divisor based on the speed on the CPU. For a standard Arduino, the CPU runs at 16Mhz.

To reduce the chance of communication errors, a compromise was made in the RF24 driver to not run at maximum bus speed. By default of the driver, the divisor value is SPI_CLOCK_DIV4. This divides the 16Mhz clock by 4. Which drives the SPI bus at (16/4) 4Mhz.

To further improve performance, it is possible to use a divisor of 2 (SPI_CLOCK_DIV2), allowing for an 8Mhz SPI bus speed. That's twice the performance of the default setting. This translates into half the latency for SPI bus communication. If you're pinched for maximum performance, this may well be a change worthy of consideration.

To make this change, look at RF24.cpp. Find the line containing "SPI_CLOCK_DIV4". Change that value to one of the other supported constants to adjust the SPI bus speed accordingly. Just remember, its a divisor, so the larger the number, the slower the SPI bus speed. Inversely the lower the number, the higher SPI bus speed.

The Down Side

There are down sides to running at faster speeds. And this is why a lower value was picked for a default. At faster speeds noise becomes much more noticeable and communication between devices becomes more easily corrupted.  Corrupted communications means packet loss, an increase in communication errors, and/or corrupted packets at the receiving end of things. This is important because, should a packet become corrupted during SPI transfer, it will be delivered in its corrupted form at the receiving radio. In spite of corruption, it will still pass a CRC check because it was received exactly as was transmitted - corrupted. That's not to say these things will happen. But they can happen. So caution is advised.

For breadboard use, when jumper leads are long and noise is likely, I strongly encourage you to maintain a slower SPI bus speed of 4Mhz. But, once you're ready for production use, where you have everything connected properly with decoupling capacitors and short leads, a clock divisor of 2 (SPI_CLOCK_DIV2), providing for an (16/2) 8Mhz SPI bus speed, may well give your performance a tiny, extra boost.

Customization

As happens with many Arduino projects, you may find you've simplified things by converting to a simple AVR project. Many users who do this deviate from the 16Mhz clock speed to something slower or faster, as dictated for the given project. As such, if you find you've increased your clock speed to 20Mhz or even decreased to 1Mhz, don't forget you can tweak these settings as needed. Always remember, SPI bus speed is determined by the speed of your CPU. If you change the speed of your CPU, you might re-evaluate your SPI bus divisor.

History & Summary

When the original SPI bus speed was selected in Maniacbug's code, I did observe errors when running at 8Mhz on a breadboard. Infrequent yet present. This is why I picked a 4Mhz speed as a default value. That, however, doesn't mean you can't try faster.

Be mindful of your decision should you decide to run at a faster SPI bus speed. Use leads as short as possible. Use decoupling capacitors as close to the radio as possible. If you're unsure, leave it where its at. Worst case, you now know you have some options to squeak every last bit of performance out of your project.

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).