It’s been over four years since Raspberry Pi 4 was released, but parts of Raspberry Pi 5 have been in development for far longer. In the latest issue of The MagPi, James Adams, CTO (Hardware) at Raspberry Pi, talks about the new technologies on the board and what they bring to Raspberry Pi.
RP1 – an in-house southbridge
Raspberry Pi 5 is coming out in October 2023, yet one of its most important new components is a chip that has been in development since 2015; RP1. Technically, it’s the first bit of Raspberry Pi silicon (hence the number) as it entered development long before RP2040. Over the years, we’ve heard inside Raspberry Pi of this ‘Project Y’ device being planned for inclusion first in for Raspberry Pi 3B+, and then in Raspberry Pi 4.
“This is our longest running chip development program,” says James Adams. “I guess it’s the reason that we built ourselves a chip team in the first place… the idea is that we separate out Raspberry Pi I/O away from the main processor. That allows the main processor to be much simpler and you can iterate it more quickly, as it’s almost a purely digital design.”
You may have noticed that the layout of the USB and Ethernet ports have swapped back to the original Model B+ arrangement after they were reordered on Raspberry Pi 4. We wanted to know how RP1 had informed this, along with the layout of the board in general.
“This is a result of our having, for the first time, total freedom in specifying the pinout of the various silicon devices on the board. So, the Model B+ board layout came first. That then informed the pinout of RP1, and then the pinouts of the other device,” continues James. “In the middle of the board you’ll see a big sort of multi-lane highway, and that’s a four-lane PCI Express (PCIe) bus.
“If you had to swap any of those lanes over, it would be a disaster. So, we designed the pinouts of RP1 and BCM2712 (with Broadcom) to match. It’s the same story with the power supplies: we’ve designed the [power supply] chip with Dialog (now Renesas) to make sure the various voltage rails come out in the right order relative to the devices that consume them.”
The initial concept for a Raspberry Pi southbridge, back in 2015, was to build a device which could bridge the spare MIPI camera and display channels on BCM2837 to Gigabit Ethernet and other fast interfaces. BCM2711 integrated these interfaces, rendering this concept obsolete, but the project lived on as today’s RP1.
“The MIPI bridge was the original idea for the RP1 chip, but we very quickly settled on the idea that actually, we should use something more standard like PCI Express,” explains James. “So, that RP1 chip has gone through two prototype silicon steps (A and B) and this C-step – that is, the third iteration – is the production version. It’s had fairly long gestation and I don’t believe it’s suffered for it: I think we’ve improved on it at each stage, and have had the time to make it all it could be.”
PCI Express – more speed on board
One of the things we’ve seen users request for Raspberry Pi over the years is eSATA connectors, so hopefully it will come as a pleasant surprise to most that Raspberry Pi 5 is jumping right past that to PCIe. “We’re seeing SATA as a legacy port these days,” James tells us.
The Raspberry Pi connector for PCIe looks a lot like the DSI and CSI ports, using a similar flat flexible cable to connect to a HAT. Why that, and not a standard connector, like M.2?
“M.2 is the smallest standard connector, but it’s still quite large,” says James. “Also, it has fairly chunky power requirements, hence the little custom connector there. We are doing a HAT that will allow you to put an M.2 on it though.”
When connected to the HAT, smaller devices in the M.2 form factor should fit neatly on top of the Raspberry Pi itself.
“The 2242 and 2230 form factors [Ed note: 42 and 30 mm long respectively] should fit fine,” James says. “For longer devices, currently, I’ve got some mounting holes at the edge of the board. We may be able to build some kind of bracket that fits onto those holes and restrains the device, but we’re still working on that one.”
Apart from the obvious opportunity to add NVMe storage, other uses for the PCIe port include Ethernet bridges, SATA bridges, and even a TPU for machine learning applications.
PMIC – pushing USB C to the limit
The ‘fairly chunky power requirements’ that James mentioned are only possible thanks to a new PMIC (power management integrated circuit) that’s been developed specifically for Raspberry Pi 5.
“This Dialog (now Renesas) PMIC is a real beast,” James says. “It’s a brilliant bit of engineering. I spec’d it to add everything that I wanted on there – despite not being really super-sure that we’d be able to deliver everything – and, of course, so it fitted on the board. Actually, working with Dialog, we managed to just squeeze it all into the space available at the right kind of cost. It’s just been a lovely bit of engineering and it looks really neat and tidy… It even has supply voltage and current sensing. So, you can get on the fly voltages and current readings for each rail which is useful, and it’s also got a real-time clock on there.”
And because of this custom work, a feature that’s been much desired since the launch of the original product in 2012 has finally been added to Raspberry Pi 5: a power button. James has wanted to add a button to a Raspberry Pi for a long time. “The PMIC reads the state of the power button, and handles ‘hard’ startups and shutdowns, itself,” James explains to us. “For ‘soft’ startups and shutdowns, it sends the button state to the main processor, which manages graceful transitions in and out of shutdown.
The souped-up PMIC allows for the gruntier (James’ wording) main SoC to consume more power, and it does this in tandem with a new USB power supply.
“We’ll be launching a new Raspberry Pi USB power supply, which can supply five volts at five amps. We’re using the USB-C connector at its maximum current, rather than going to a higher voltage.”
“Both the PMIC and the power supply support the USB Power Delivery (USB PD) standard. With USB PD there’s a little serial interface running over the power cable. You talk to the power supply, it starts off at five volts, but then you ask it: ‘what voltages do you support?’. And it says: ‘I support all these voltages.’ Usually it’s five, nine, 15, 20 volts if you’ve got a PC one. We could have just used a standard PD supply, get nine volts at three amps, but then you have to do the voltage conversion on the board. That costs you area, it costs you silicon, it costs you efficiency. So, we decided to do something a little bit non-standard, which is what we often do in search of performance, and create a five-volt, five-amp profile for our power supply. That means you don’t need all that conversion, but it can still support the board, and if you’re using the new supply we can support extra USB current downstream compared to Raspberry Pi 4. Or more HAT current.”
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