An other CPU discussion. Rockchip RK3399 sounds good

hns

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That brings up an interesting point in these types of, "lets consider using X chip," discussions. What are the physical constraints of the Pyra's SoC daughter board?

Max physical dimensions in X,Y and Z axis?
Max current draw?
Max thermal output?
Thickness should be <1.5mm (incl. solder balls and some reserve etc.) or the board risks to be pulled by bending forces out of the B2B connectors.

Available area depends on what else is needed around the SoC chip.
There is also a limitation by one of the magnetic nub sensors on the mainboard so it is kind of a puzzle game.

Roughly we have for the OMAP5 board:
* 20x20mm for OMAP5 plus some circuits around (oscillator)
* 40x15mm for RAM chips
* 15x13mm for eMMC
* 16x15mm for Palmas power controller plus inductors and capacitors
* 12x12mm for twl6040 audio codec
There is also some random space which can be used for glue logic.

Max. current draw: less than ca. 2A @ 3.7V (battery voltage). Peak current can be higher (ca. 3A from battery), but then the system is discharged faster than it can be charged.

Thermal: not too hot :)

Another important factor is the number of available interfaces - here is some minimum:
* 3xSDIO (plus something internal for eMMC)
* 2xUART
* 4xI2C
* 1xMIPI
* 1xHDMI
* 3xUSB2 (the OMAP5 boards has an on-board 3-port hub for that purpose)
* 1xUSB3-OTG
* 1xSATA or USB3 (then 2xUSB2 is sufficient)
* Audio: 2x Handsfree speaker, Microphone, Line/Headset in/out, 1x vibramotor
* ~30-40 GPIOs

Also important: 1.8V I/O levels (unless defined otherwise by standard, e.g. SDIO, MIPI, HDMI, USB, SATA).
If a SoC does not provide them all, it must have some expanders on the board.

I did an evaluation ca. 2 years ago and a Z8550 would almost fit (except the power controller and the audio subsystem), but it is an Atom...
 

Askarus

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@hns
I have attached an older Datasheet.
The recent one I've seen some days ago is either taken down or I can't find it any more.

However, the Requirements should be fulfilled.

The only Issue might be the size. I could not find anything so far.
I hope there is a way to make it fit.
 

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Askarus

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EvilDragon

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Thickness should be <1.5mm (incl. solder balls and some reserve etc.) or the board risks to be pulled by bending forces out of the B2B connectors.
Well, it's between 1.41 and 1.61 for the Rockchip, so tolerances are a hit and miss.
Also, it would mean that the small heatpipe (0.2mm) wouldn't fit anymore.

Sent from my Moto Z using Tapatalk
 

Kuro

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@hns
I have attached an older Datasheet.
The recent one I've seen some days ago is either taken down or I can't find it any more.

However, the Requirements should be fulfilled.

The only Issue might be the size. I could not find anything so far.
I hope there is a way to make it fit.
The size is on page 28 and acording to it, the thickness is 1.41 to 1.61mm with solder balls included.

So it would be possible if we could get the minimum value, it seems unlikely but my knowledge about this stuff is almost none :(.

Edit: Looks like I took to long to answer...
 

levi

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Looks about the same size as the iMX6 in my SBC which I measured for heatsink purposes as 20mmx20mm. That may have an internal clock though - the only non-passive looking components directly nearby are the DRAM and a buffered QFP surrounded by a small collection of passives. Certainly nothing that looks like a traditional crystal or resonator, but then I'm not 100% what a surface mount one of those looks like at all.
 

Kuro

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Just remembered but should it not be possible to get a thinner battery heat shield and slightly higher connectors for the cpu board to connect to the mainboard?

How much space could be gained from such changes?
 

Askarus

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If that would hinder us to use that processor then I don't see much benefit in using a modular design.
If there are such strict limitations it will be even harder to get a new SoC.
 

vcoleiro1

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This video ED made shows the cooling system in the PYRA and the spaces available


 

hns

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If that would hinder us to use that processor then I don't see much benefit in using a modular design.
If there are such strict limitations it will be even harder to get a new SoC.
Well, the thickness limitations aren't that strict. AFAIR there is some space between the PCB and the plastics case. And I think there are variants of the B2B connector (Hirose DF40C) giving 0.5mm more distance. So it might be possible to get a "thicker" board into the Pyra. Someone should check that before we assume that it does not work.
 

Askarus

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Well, the thickness limitations aren't that strict. AFAIR there is some space between the PCB and the plastics case. And I think there are variants of the B2B connector (Hirose DF40C) giving 0.5mm more distance. So it might be possible to get a "thicker" board into the Pyra. Someone should check that before we assume that it does not work.
That sounds much better :)
Let's be positive.
You did a great job designing the board and making it modular.
Let's not loose hope because of 0.2mm ;)
 

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The space between the CPU board and the plastic is exactly 0.5mm and there is a 0.4mm aluminium plate cramped in there.

So yeah, thinner CPU board and higher Hirose connectors would probably be the only real solution.
 

Askarus

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The space between the CPU board and the plastic is exactly 0.5mm and there is a 0.4mm aluminium plate cramped in there.
Do we really have to use Aluminium?
Didn't we have much smaller tape for both, isolation and heat dissipation?
 

hns

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The space between the CPU board and the plastic is exactly 0.5mm and there is a 0.4mm aluminium plate cramped in there.

So yeah, thinner CPU board and higher Hirose connectors would probably be the only real solution.
Well, if a new SoC doesn't produce that much heat, it could work without the aluminium plate. But that isn't easy to predict.
 

levi

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0.4mm is already entering tin foil thinnesses (it's about double the standard gauge apparently). You could use thinner things and back them with plastics and things, but the crosssection of the transmissive surface is proportional to the rate of heat transfer I assume, so anything that isn't doing that is a bad thing. As far as I recall, copper is generally a better conductor of heat, but it's also a lot more costly than aluminium. Actually, if cost weren't a concern you could probably use pure gold.
 

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Do we really have to use Aluminium?
Didn't we have much smaller tape for both, isolation and heat dissipation?
That tape is being used to move the heat out of the way from the SoC.
It doesn't cool. You still need heatsinks and shield the battery.
 

rohezal

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It it said, that an improved case, which is a few mm thinner, could maybe be produced in the future. Would it be possible to use one or two of this mm to make room for a future sock?
 
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