The Future of Pyra's CPU


EvilDragon

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The heat spreader is only 0.2mm. And I checked today: We can stack about 0.8mm between the OMAP5 and the mainboard.
So with the heatspreader included, it means that an SoC could have a height of max 1mm.
 

EvilDragon

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Thats a bit of a bummer. With the iMX8M nano our battery life will likely go through the roof, but we'll likely be sacrificing a good amount of raw power limiting tasks like web browsing. Its hard to do a full comparison because there's a lot of differences between these core families, but a big one is the A53's are missing out-of-order execution which we see on the A15's. The Nano is also missing the VPU (so we lose multimedia decode/encode) and HDMI. Is there a reason the 8M is not an option in place of the 8M nano? I haven't looked at the physical size difference. I remember looking at the 8M and it was similar in size to the OMAP5.

All in all, it would be nice to get something with at least one A7x core for larger load tasks. Also a Mali GPU would be great as we are starting to get in-tree kernel support for Mali GPUs.

What are the restrictions on size? The RK3399 datasheet says the chip is 21mmx21mmx1.51mm. So may be a bit tall, but its smaller than 23mmx23mm.



I'm a bit hesitant to go with an A17. It's already just as obsolete than the A15. And to be honest, this is the only SOC I've actually seen sporting one, so I'm not very hopeful for good support for the A17 in the kernel.
For some reason, the 8M has a die in the middle which is 2mm high. The rest of the SoC would be fine, 0.8mm height would fit perfectly, but 2mm...?

The 8M Mini would fit, but that would be a step back. The 8M Plus might be a candidate. The physical size is different than the rest, but it's in preproduction and they don't tell us the height yet.
 

RZR

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For some reason, the 8M has a die in the middle which is 2mm high. The rest of the SoC would be fine, 0.8mm height would fit perfectly, but 2mm...?

The 8M Mini would fit, but that would be a step back. The 8M Plus might be a candidate. The physical size is different than the rest, but it's in preproduction and they don't tell us the height yet.
Isn't that the 8X?


The 8M appears as 17mmx17mmx0.65mm


EDIT: Ah it's true, page 79 shows a bump in the middle of almost 2mm...
 
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pyrat

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What are the restrictions on size? The RK3399 datasheet says the chip is 21mmx21mmx1.51mm. So may be a bit tall, but its smaller than 23mmx23mm.
Good question

And does it depend on other factors ?

I mean maybe we have all the ram chips but the OMAP cannot be got any more. Then we need a SOC the same size or smaller than the OMAP.
But if we have enough components secured for the Pyra orders, and it is just for a future cpu board, maybe it could carry different RAM chips and if they were smaller there could be some room
left for a bigger SOC.

I mean it sounds a little naive to me just looking at the size alone, because it's gonna have to be a new CPU board. I thought there was plenty of time for it, and I still hope it turns out so.

I'm a bit hesitant to go with an A17. It's already just as obsolete than the A15. And to be honest, this is the only SOC I've actually seen sporting one, so I'm not very hopeful for good support for the A17 in the kernel.
I think it might be a nomenclature thing. A12s were replaced with A17s. Anyway RK3288 was used in chromebooks, like veyron speedy (Asus C201) which someone liberated. I believe the support should be ok (or at least it was).
I believe it has uboot too, but if not, coreboot could work.
 

ToastBucket

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The 8M Mini would fit, but that would be a step back. The 8M Plus might be a candidate. The physical size is different than the rest, but it's in preproduction and they don't tell us the height yet.
A while back I picked up a Maaxboard which is based on the 8M to try and evaluate general performance of the part in comparison to the OMAP5. Unfortunately the support out the door for this board is pretty bad. Since there's a good amount of work required to test this out I dropped it for now since I don't have time to work on fixing the graphics before evaluation. I might try and borrow an official NXP devkit from work, I think we've got a few of the iMX8 platforms lying around.
 

EvilDragon

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Ah, sorry: The OMAP5432 is between 1.06mm and 1.29mm high.
So we have between 1.8 and 2mm space... which would be a VERY tight fit for the i.MX8M (which is 2.03 +/- 0.15mm high).
But we need to fit the 0.2mm heatspreader in as well (unless it doesn't run very hot).

The RK3399 would fit then - at least the height...

I'll check the datasheets of the connectors for the PCBs, they should have exact measurements for the total height as well.
 

ToastBucket

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The RK3399 would fit then - at least the height...

I'll check the datasheets of the connectors for the PCBs, they should have exact measurements for the total height as well.
That's good news. The RK3399 looks like an excellent candidate under the hood. Btw, the 1.51mm height includes the BGA connections. See pages 28-29 of the datasheet for package measurements.
 

ToastBucket

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What about the RAM that would be attached on top? We need to make sure to account for that.

-God Ginrai
RAM attached on top? You mean just the RAM on the SOC board? The height and width dimensions would affect that.

Memory isn't necessarily attached on top.
RAM is pretty much never attached on top? I've seen one weird custom spin of the i.MX6 which did this, but it was hugely problematic and eventually dropped due to yield issues.
 
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ToastBucket

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Pandora had, and OMAP5430 would've also had RAM popped on top.
But that's rare these days.
Ah interesting. Probably rare because it was really difficult and expensive. What seems a bit more successful is SIPs like those made by Octavo. The packages are definitely a bit larger, but its very convenient having RAM and PMIC and everything all buttoned up for you so long as you're happy with whats in the silicon.

 

EvilDragon

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Well, the good thing about PoP was: You didn't need to worry about the connections from the RAM chips to the SoC.
That was quite a deal with the 4GB RAM on the Pyra.
 

ToastBucket

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Ya it’s usually common practice to directly replicate the RAM part selection and layout from the EVK, but still a small mistake can cause lots of problems. At least we have OCRAM to help us debug
 

Swordfish II

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Yes, but you don't need that to boot. You can use the system without them, you would just be missing these features.
Wifi / BT could be done with a USB stick, for example, and not everyone needs the SGX.
Yea I just wanted people to be aware that it is very difficult to be blob free
 

Djoga'Ro

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Just a hypothetical question: Let's say an ideal SoC came along and it was a bit to high. Would it be feasible to "just" modify the CPU boards connector to get more room between it and the mainboard and modify the mold of the upper or lower half of the case to allow for that by making its wall taller? (As I understand it, adding to a molded part is the easier direction.)
 

levi

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I think the battery connector is on the main board, not the CPU board. If you allow space for the CPU board to be thicker, you need to move the battery away from the main board, which might cause problems connecting power to the system. I guess the simplest fix if you can't find a taller battery connector, would be to put the battery connector on a little mezzanine board made out of 1mm PCB.
 
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