Double standards Raspberry Pi 5

It’s now been a little over two months since announced Raspberry Pi 5, and the time has flown by.  We knew it built something quite special, but it’ve been overwhelmed by the positive response from the community.

The production ramp has been steeper than for any previous flagship product: It’ve been producing 70,000 units a week for the last few weeks, and this rate is set to increase to 90,000 units a week by the end of January.

Once people had recovered from the shock of seeing both a power button and a real-time clock on a Raspberry Pi, one of the most commented-on features of the new platform was the small, vertical, 16-way FFC (Flat Flexible Cable) connector on the left-hand side of the board, which exposes a single-lane PCI Express interface.

PIP PIP

Peripheral Component Interconnect Express (PCI Express or PCIe) is, as the name suggests, a board-level interconnect that allows high-speed data transfer between a processor chip (in our case BCM2712) and external peripherals such as NVMe SSDs, Ethernet cards, or more exotic things like AI/ML accelerators.

PCIe works by serialising data transfers and sending one bit at a time down a single channel.  Each channel sends data down one or more ‘differential pairs’ on the PCB, which are basically controlled waveguides made by two closely spaced wires (with a ground plane beneath) that can carry very fast signals with high noise immunity and low signal loss and distortion.  For a single-lane (×1) PCIe interface, we have a single transmit pair, a single receive pair, and a clock pair.  This means that three differential pairs (and six wires) are required.  Higher-capacity PCIe interfaces have more lanes (×2, ×4, ×8, ×16);  on Raspberry Pi 5, BCM2712 is connected to our RP1 I/O controller via an ×4 interface.

Each lane run at 5Gbits/s for PCIe 2.0 (the fastest mode that we officially support on Raspberry Pi 5);  after coding overhead, this translates into a capacity of 4Gbits/s.  Even taking into account other protocol overheads, you’re likely to see ~450MBytes/sec to and from a good NVMe SSD.  Pretty fast!

Alongside the data and clock channels, the PCIe specification requires some sideband signals like reset, clock request (which does double duty as a power state signal), and wakeup.

Our 16-way connector provides all these signals.  We also have two pins that allow us to control board power, and to ensure that an appropriately designed PIP (our new word for a PCIe Peripheral) is automatically detected by the Raspberry Pi firmware.

Not an M.2

Why didn’t we add an M.2 connector to the Raspberry Pi 5?  The M.2 connector is large, relatively expensive, and would require us to provide a 3.3V, 3A power supply.  Together these preclude us offering it in the standard Raspberry Pi form factor.

Using a small, low-cost FFC connector allowed us to provide a PCIe interface without growing the board, or imposing the cost of an M.2 connector and its supporting power-supply circuitry on every Raspberry Pi user.

Specification the first

One thing we did not have ready at Raspberry Pi 5 launch was a specification for how to build peripherals that attach to the 16-way PCIe connector.  The interaction of PCIe peripherals with Raspberry Pi power states and firmware required detailed consideration, and we wanted to make sure we had done extensive testing of our own prototype product to make sure everything was working as expected.

Today, we’re releasing the first revision of that specification.  Our own M.2 M Key HAT+ is in the final stage of prototyping, and will be launched early next year.