lizofaustin
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Raspberry Pi 3 B+ - Southbridge

Tue Jan 08, 2019 11:07 pm

I'm not quite sure which hardware forum to post to. If this is not the right one, please let me know.

Looking at the Raspberry Pi 3 B+ schematic here: https://www.raspberrypi.org/documentati ... duced.pdf
I'm trying to understand what part of that diagram represents the southbridge (and northbridge) and where that would be if I'm looking at the physical board.

A nudge in the right direction would be appreciated. Thanks!

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rpdom
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Re: Raspberry Pi 3 B+ - Southbridge

Wed Jan 09, 2019 8:19 am

That diagram is only a partial one. It just shows some of the peripheral circuitry of the Pi

If the Pi actually had a Southbridge and Northbridge (terms that don't really apply here), they would be inside the main Broadcom chip along with the CPUs, GPU and pretty much everything else.

PhilE
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Re: Raspberry Pi 3 B+ - Southbridge

Wed Jan 09, 2019 8:54 am

Yes, as an SoC (System-on-Chip), all of the peripherals directly addressable by the CPU are present on the BCM2837 die. The other peripherals (LAN7515 Ethernet and USB hub, power management, GPIO expander) are addressed over the relevant peripheral buses.

lizofaustin
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Re: Raspberry Pi 3 B+ - Southbridge

Fri Jan 11, 2019 2:48 am

Thanks for the help!
rpdom wrote:
Wed Jan 09, 2019 8:19 am
That diagram is only a partial one. It just shows some of the peripheral circuitry of the Pi

If the Pi actually had a Southbridge and Northbridge (terms that don't really apply here), they would be inside the main Broadcom chip along with the CPUs, GPU and pretty much everything else.
rpdom, when you say that the Southbridge and Northbridge terms don't apply... can you please explain? I ask because at 2:41 on this video https://www.youtube.com/watch?v=zQZxdiz ... aspberryPi, Roger Thornton says that they've changed the southbridge to another product from Microchip. I was trying to understand exactly what he's talking about. That sent me off in search of what is a southbridge....and then I was trying to relate that back to the diagram and then to the physical Pi in my hand.

PhilE, from your response and another article that says For Raspberry Pi 3B+, Microchip have supported us with an upgraded version, LAN7515, which supports Gigabit Ethernet. it sounds like Roger is equating that LAN7515 chip as the southbridge? Or is it really "(LAN7515 Ethernet and USB hub, power management, GPIO expander)"... all of those though he only mentioned the Microchip's LAN7515?

PhilE
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Re: Raspberry Pi 3 B+ - Southbridge

Fri Jan 11, 2019 6:41 am

Roger is well aware of what LAN7515 is - a combined USB hub and USB Ethernet controller. The input to the device is USB, which is why it does not count as a Southbridge, and why he would not describe it as such.

plugwash
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Re: Raspberry Pi 3 B+ - Southbridge

Fri Jan 11, 2019 8:56 am

For many years most PC chipsets consisted of two main chips (and a bunch of support stuff round them), called the Northbridge and the Southbridge. The Northbridge contained the memory controller, controllers for high speed IO interfaces and (if the chipset supported it) integrated graphics. The southbridge handled a bunch of lower speed stuff (storage interfaces, slower busses, USB etc).

Exactly what was handled by the northbridge and the southrbridge evolved over time, in the early days the northbridge would have hosted the PCI bus while the southbridge was a client on it. Later a PCIe like interface was used between the northbridge and the southbridge with the southrbridge acting as host for the legacy PCI bus.

Eventually Intel moved away from the Northbridge/Southbridge architecture, putting the memory controller and parts of the integrated graphics functionality onto the processor and integrating the remainder of the Northbridge and Southbridge functionality into a chip they called the "PCH".

The LAN951x/LAN7515 USB hub with Ethernet chips fill a role in the Pis architecture that if you squint a bit could be considered similar to the southbridge in a PC. I would guess this, along with the fact that saying "USB hub with Ethernet chip" gets awkward is why Rodger reffered to it as a southbridge in the video.

lizofaustin
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Re: Raspberry Pi 3 B+ - Southbridge

Fri Jan 11, 2019 10:22 am

Plugwash and philE - Thank you so very much for the additional history and background. Those were definitely the missing pieces in my understanding. Have a great day!

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