W. H. Heydt
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BCM2711 inplications for the A series

Mon Jun 24, 2019 9:52 pm

(Kind of a back formation for those who aren't following Pi4B developments in detail the '2711" is the SoC used on the Pi4B.)

In the announcement blog post, Eben Upton indicated a *possible*, *future* interest in making a Pi4A. This post is NOT a wish list, nor is a plea for him to do so in the immediate future. Perhaps in a year or two we might see such a beast... This post is my take on what certain features of the 2711 suggest might be new Pi4A features.

First, the USB-C power connector on the Pi4B doubles as a USB OTG port (it is separate from the internal USB port that goes to the hub for the main USB connectors). Using it that way could be kind of awkward. If one assumes as has been done on previous A series boards, and on the Pi0 series, the single USB port is directly exposed, then it might be possible make either USB port be the power connector, freeing the OTG port from "power duty".

Second, the GbE on the Pi4B does NOT run through the USB hub. It has its own port to the SoC. This implies that a "hub-less" Pi, i.e. a Pi4A, could have an ethernet jack in addition the normal, single USB connector.

Third, as everyone should know by now, the Pi4B has USB 3 ports, so one would expect a Pi4A to have one, too.

Edit to add...
If a Pi4A has an Ethernet jack, for reasons given above, then it should also have the PoE header. And that leads to...

One could power a Pi4-class machine through the PoE header, freeing the USB-C port for OTG duties. AND someone could design a simple PowerHAT that doesn't do the PoE extraction, but just has a plain power connector--USB-C would be best so as to work with the official PSU--but including the fan (cooling is anticipated to be an issue).

gtechn
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Re: BCM2711 inplications for the A series

Wed Jun 26, 2019 11:14 pm

This is not a wish list - I personally don't have any Model As - this is just my idea for the 4 Model A (because the RPF hasn't identified a suitable comprise of parts yet for the A series).

The Model A is used most frequently in embedded and industrial applications (remember the "digital signage" clearout that one December?). To save money and to benefit industrial customers, it would make most sense, in my humble opinion, to make the Pi 4 Model A in the exact same form factor of the Pi 3 Model A+.

This would mean a microUSB charging port, 1 full-size HDMI port, 1 USB 3.0 port (in the exact same position as the old one), 512MB of RAM - but with the new SoC. If there's room, offer a 1GB of RAM boost option. Ideally, just the Pi 3A+ with a SoC change. This makes more sense to me at least than trying to adopt a new form factor for the A with those microHDMI and USB-C ports (a big deal in industrial to flip form factors), for only minor differentiation from the 4B.

On the other hand, the Pi 4B is so tightly integrated (and I love that - I really do) that there might not be any sense at all to making a Model A this round. I can totally see the Model A becoming an every-other-release tradition. A on 1st gen, No A on 2nd gen, A on 3rd gen, No A on 4th gen, etc.

gtechn
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 12:59 am

On the other hand, let's say the 4A did come with some of the form factor changes of 4B. (NOT a wish list - I have never bought an A - Just thinking realistically about what to expect someday - No rush RPF, take a long vacation, 4B is awesome!) I also did some more research which you may find very interesting.

If the Model A did decide to adopt the USB-C power and dual microHDMI ports, I would expect the 4A to drop the RAM to about 512MB. It's cheaper than 1GB (as seen in the 3A+), helps maintain the $25 price, and is still enough for most embedded uses that the A series is known for. I could see the option to buy a 1GB variant, but I wouldn't hold my breath.

The USB hub is provided by a VLI VL805 chip, which provides a USB 3.0 hub over PCI Express. Obviously, on a 4A, this chip would be removed. This leaves a catch though: No USB 3.0. Again, the USB 3.0 on the 4B is provided using this hub chip over the 1-lane PCI Express connection. Without this hub chip, there is no way to connect a USB 3.0 port to the processor. This means that the USB port on a 4A will almost certainly be USB 2.0, over the old legacy connection which is currently connected to the USB-C power port on the 4B.

On the 4B, the Ethernet is handled by the Broadcom BCM54213PE. Obviously, on a future 4A, there is no room for this chip. I would strongly doubt the RPT adding an Ethernet jack for this reason.

So, with this setup, what does the theoretical Model A look like?
- 512MB RAM
- No Ethernet jack or chip
- USB 2.0 because of no USB 3.0 hub or ports
- Still 2 HDMI outs
- Still USB-C power
- Still 1.5GHz 4xA72 processor

I think that is a pretty solid Model A, in my humble opinion. Furthermore, it appears to balance the price. The 3A+ cut off $10 by removing the one hub chip, the Ethernet and USB2 ports, and dropping RAM to 512MB. This removes two hub chips, drops a USB2 and more expensive USB3 connectors, drops Ethernet, and also drops RAM to 512MB. This seems like a pretty reasonable setup, from what I know of Pi economics.

Of course, we might never see a 4A. Maybe in a few years.

Edit: Sources and an interesting read on the design: https://blog.hackster.io/meet-the-new-r ... 9b4698c284

W. H. Heydt
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 3:37 am

Thanks for those posts.

A couple of points...

The SoC for the Pi4 really can't be used as--effectively--a drop in replacement for the BCM2837 from the Pi3. Even if you just consider the number of external memory address lines, that's obvious.

On the RAM pricing... Prior to the Pi4B, all Pis used--if I understand it correctly--LPDDR2 while the Pi4B use LPDDR4. Did you take that into account in your pricing study?

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Gavinmc42
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 5:45 am

A Pi4A would be the same as the 3A+ except with the 2711 and faster DDR4.

Would there be a requirement for a faster 3A+?
Vision, robotics apps?
Only those bumping into the 3A+ limitations would know.
The USB-C is needed on the Pi4B because that needs extra current out the USB3 ports.
Not needed on a 4A.

So does the 4A need two HDMI?
Wireless dual monitors?

While plugging my new Pi4B1 in I noticed a USB hub on my Dell monitors :D
Now I know what to do with my spare 3A+ :lol:
Hmm maybe I need some 4A's too?

My Win10 work PC is dual screen, easy to add one 4A?
Wireless Pi's are easier to connect in my firewalled Uni Lan network.
But then I would want 4GB ram to use it as desktop :(
Another 4 model SKU A's?

How many monitors come with hubs these days?
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bensimmo
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 6:36 am

Two directions

Direct A replacement in the traditional cut bits off method,

One or two uUSB as they are everywhere now and also compact but traditional USB keeps with the current design of the 4B so just the one normal USB for adding hubs for setup (note I've not looked at how usb works without the onboard hub)
Add optional headers for additional USB if people want it or for compact setups.

Also the uHDMI is excellent too, best move (Zero should have used it ages ago at release)

Add the Raspberry stamp back to the WiFi module, I really liked that little touch now missing from the Pi4 :-(

--
But then I also see (in my head) a market for what is probably very niche really ;-)
A POE Ethernet streamlined Pi4A-e no need for a HAT.
The point of POE is it is not a user Desktop computer design, you don't need all the USB ports (you may need BT so that has to stay, see later)
It's for remote places that have no power (the good old weather system, similar remote.decixe hookup uses or for a compact client/sign etc.)

Keyboards/mice may demand BT to be used but then you wouldn't want wires to them anyway. This gives it a thin client use or tidy way to deploy. Hence no need for USB3 all files are remote or in the cloud anyway.
Unless USB3/C could provide the networking and POE and give it a nice compact connection (I really do dislike the bulk of Ethernet connectors, it's time for it to move on)

A Keyboard/Mouse+ market there too, can add that to the lineup. (I would buy a RPi BT keyboard/mouse, I don't like them having wires)
----

But hey ho, I'm off to explore and enjoy what they managed to do with the Pi4B, I have no rush for an A, almost no need for one any more. There is so much to do and see what the new gpio setup can bring.

alphanumeric
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 10:50 am

I currently have four Pi 3A+'s, all running headless as surveillance cameras. They run Motion Eye OS.
I would happily replace them all with 4A's with the new faster SOC. Assuming I get better streaming and a better FPS.
I'd be happy with the 4B layout minus the ethernet and just one USB port. Or two USB ports. One 2 and one 3. Having the ethernet jack wouldn't bother me, but I currently just use the WIFI on the 3A+. No ethernet Jack also makes for a slimmer package which may appeal to some embedded users.
I'm all for leaving what's already there, there. USB C power and the two Micro HDMI. It seems to me it would just be extra work to remove / replace them.
I'd be more than happy to pay more for more RAM. The 1 gig, 2 gig, or 4 gig option would be nice IMHO.

I also have two of the original A+'s on the go. They have Sense Hats attached and are setup as weather clocks. I prefer them over a Zero when attaching a full sized Hat. Everything just lines up nicely and the foot print is the same size as the Hat thats on top. It makes for a nice neat package.

Many thanks to the Pi Foundation for all the wonderful products that give us all kinds of options for what ever we are building. :D

gtechn
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 3:13 pm

W. H. Heydt wrote: Point 1: The SoC for the Pi4 really can't be used as--effectively--a drop in replacement for the BCM2837 from the Pi3. Even if you just consider the number of external memory address lines, that's obvious.

Point 2: On the RAM pricing... Prior to the Pi4B, all Pis used--if I understand it correctly--LPDDR2 while the Pi4B use LPDDR4. Did you take that into account in your pricing study?
Point 1: I agree. The board would need some redesigning (RAM moved to front, LPDDR4). I don't know what you mean by memory address lanes, unless you thought I meant keeping the 512MB of LPDDR2, which I don't.

Point 2: 4B uses much faster LPDDR4, whereas the earlier models use older, slower LPDDR2. There are several reasons I don't think this will be a problem.

1. In a statement I believe to the MagPi (though it could have been an engineer on here) a few years ago, RPT said they might have to upgrade RAM someday not just because of the speed, but as LPDDR2 becomes more niche, it actually becomes more expensive per chip, while as LPDDR4 becomes more widely used, it becomes cheaper. The difference between LPDDR2 and LPDDR4, in 2019, might be extremely close.

2. The 3A+ and 3B+ uses one hub chip for Ethernet and USB, whereas the 4B uses two chips (one for Ethernet, one for USB over PCIe). Thus, I would assume that having two hub chips on the 4B costs more than the old one hub chip on the 3A+ and 3B+ and earlier. The cost savings of removing the two hub chips, then, is much greater - and should help offset any added RAM costs. I don't know the official BOM pricing, I just think it's reasonable to assume that two hub chips is roughly twice the cost of one (even though they are different models).
Gavinmc42 wrote: A Pi4A would be the same as the 3A+ except with the 2711 and faster DDR4.

Would there be a requirement for a faster 3A+?
Vision, robotics apps?
Only those bumping into the 3A+ limitations would know.
The USB-C is needed on the Pi4B because that needs extra current out the USB3 ports.
Not needed on a 4A.

So does the 4A need two HDMI?
Wireless dual monitors?
The Model A is known heavily for its use in digital signage. A few years ago, there was one December where a "digital signage" company bought out the entire inventory - you couldn't buy an A for a few months after that. Eben Upton has described the dual-HDMI support as a much-requested feature by digital signage companies. I think it is very reasonable that they would try to satisfy the demands of a huge market for the A by adding two HDMI.

And if they are adopting two HDMI, they might as well adopt the USB-C power with it, even if it isn't quite necessary. It doesn't make sense to have a third I/O configuration combining microUSB + two HDMI. That would be just confusing and unnecessary. microUSB+1HDMI and USB-C+2HDMI is enough.
Gavinmc42 wrote: But then I would want 4GB ram to use it as desktop
Another 4 model SKU A's?
I would seriously doubt it, as adding multiple SKUs of anything is a logistics challenge. If you want 4GB of RAM, I would seriously not expect it to appear on an A model. Buy a 4B+.
bensimmo wrote: Also the uHDMI is excellent too, best move (Zero should have used it ages ago at release)
Agreed. I would have hoped that the 4B+ used miniHDMI because it is more stable and the Zero uses it, but what fits is what makes the decision. In this case, I would wish (my only wishlist here, the rest is just theories) that the RPF would switch the Zero from miniHDMI to microHDMI, for consistency's sake. But that might cost too much which would break the $5 price, in which case I say keep it. Two $5 dongles is nothing to cry about.
bensimmo wrote: Add the Raspberry stamp back to the WiFi module, I really liked that little touch now missing from the Pi4
If one of the Engineers knows why it is gone, I would love to know. I will miss that.
alphanumeric wrote: Many thanks to the Pi Foundation for all the wonderful products that give us all kinds of options for what ever we are building.
Yes, the RPF deserves our heartiest thanks!

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bensimmo
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 3:27 pm

the A+ /3A+ doesn't have a USB hub chip, that is the cost saving and why it only has one USB port like the Zero. (it a line from the SoC itself.

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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 3:30 pm

gtechn wrote:
Thu Jun 27, 2019 3:13 pm
bensimmo wrote: Add the Raspberry stamp back to the WiFi module, I really liked that little touch now missing from the Pi4
If one of the Engineers knows why it is gone, I would love to know. I will miss that.
Stamping tool wore out....and there is a logo right next to it on the board anyway, so not worth replacing the tool.
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hippy
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 5:06 pm

gtechn wrote:
Thu Jun 27, 2019 3:13 pm
2. The 3A+ and 3B+ uses one hub chip for Ethernet and USB, whereas the 4B uses two chips (one for Ethernet, one for USB over PCIe).
The 4B has the legacy USB connections going to the USB-C connector so the two hub chips could be removed and the legacy USB routed to a standard USB socket for a 4A just as it is on the 3A+.

gtechn
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Re: BCM2711 inplications for the A series

Thu Jun 27, 2019 9:49 pm

hippy wrote: The 4B has the legacy USB connections going to the USB-C connector so the two hub chips could be removed and the legacy USB routed to a standard USB socket for a 4A just as it is on the 3A+.
That's what I said earlier:
gtechn wrote: The USB hub is provided by a VLI VL805 chip, which provides a USB 3.0 hub over PCI Express. Obviously, on a 4A, this chip would be removed. This leaves a catch though: No USB 3.0. Again, the USB 3.0 on the 4B is provided using this hub chip over the 1-lane PCI Express connection. Without this hub chip, there is no way to connect a USB 3.0 port to the processor. This means that the USB port on a 4A will almost certainly be USB 2.0, over the old legacy connection which is currently connected to the USB-C power port on the 4B.

alphanumeric
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Re: BCM2711 inplications for the A series

Sat Jun 29, 2019 7:07 pm

One USB port, like what's on the A, 3A works for me. I have hubs with 3 USB ports and one ethernet jack that I plug in to do my setup with.
I use this with my 3A's https://shop.pimoroni.com/products/usb ... an-adaptor
This with my Zero's, https://shop.pimoroni.com/products/thr ... connector
And this with my old A+'s, https://www.adafruit.com/product/2937

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