Raspberry Pi - Hardware Flaws and Fixups?

Hans,

  It may seem to you that the RPi was designed with a ridiculously small RAM

in order to "make/force people to make/find more efficient solutions/programs".

But that isn't the case. 

It was designed the way it is because it was repurposed from the Roku design

as an educational computer rather than a media center, with no easy way to

enlarge the PoP memory.  As opportunities for enlarging the memory become

readily available, I'm sure the RPi will take advantage of them, even beyond 4GB.

If the RPi designers actually subscribed to the "hair shirt theory of programming

excellence", they would have reduced the cost by using 128MB instead of 256MB,

or maybe even left off the PoP RAM completely and used the 128KB L2 cache for

program memory.  That would still have been much more memory than the BBC micro.

Hynix 4Gb (=512MB) PoP RAM is now in mass production.

http://www.hynix.com/products/mobile/mobile.jsp?info.ramCategory=&info.ramKind=28&info.eol=NOT&posMap=MobileDDR2

previously this page showed the status as "customer samples".

A user on the new RPF forums has come up with what looks like a design bug --- http://www.raspberrypi.org/phpBB3/viewtopic.php?f=7&t=5830 , summarized as "voltage drop across miniSMDC014 can drop USB +5V way out of spec for common USB devices".

Morgaine.

Gert's reply over there seems to suggest that the device's pre-trigger resistance is non-linear and much lower below the rated 100mA than at the 140mA at which apparently the user measured the voltage drop while feeding the wifi dongle.

I'm having trouble finding a proper datasheet or application note showing the resistance curve of the polyfuse though, just this rather minimal spec --- http://www.farnell.com/datasheets/88959.pdf --- which I think says that the resistance varies between 1.5 and 6.0 ohms, without stating conditions.

Does anyone have something a bit more comprehensive on the miniSMDC014's electrical characteristics?

Morgaine.

There is a report of an expensive TV being zapped when connected

to the RPi hdmi port.

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=6&t=5632

perhaps prudent to use only inexpensive monitors until more is known

about this issue.

You know, I'm beginning to have my doubts about the whole power philosophy in the Pi's design.  Not for the next minor board update(s), but for the next major board design, I think the entire question of power should be reopened from scratch.

The concept of using cellphone chargers for power sounded like genius originally, but in practice it has turned out to have a very ugly underbelly.  These chargers are made for charging, not for delivering constant power to downstream USB peripherals, and they're made to such a rock-bottom price that a very high proportion of them has pretty dreadful electrical performance.  This has been a source of many problems for Pi users already, who commonly go through several chargers before finding a good one.

It's no surprise really, with the benefit of 20/20 hindsight.   Chargers bundled with cellphones are only expected to work with one specific load, their cellphone.

The polyfuse issue has exacerbated this.  It's bad enough using a wonky charger as a power supply, but when you then kill off voltage regulation almost completely by putting such high-resistance polyfuses in the supply line then you're just asking for trouble.  For all intents and purposes, the Pi is supplying an unregulated USB supply.

It's not quite a disaster just yet, but I'm pretty sure that it's possible to do far better than this and still remain within the price niche, particularly now that BOM costs have dropped hugely as a result of volume manufacturing.

Morgaine.

I've only had a little experience with polyfuses.  That was a product where part of the manufacturing process included tweaking the 5V power supplies to compensate for the voltage drop across the polyfuses.  Not a very nice way to manufacture products.  My suggestion (which was adopted) was to replace the polyfuses with automotive mini-fuses, which have very low resistance and are easily replaced.  Sure, self-resetting fuses sound wonderful and convenient, but we're seeing lots of problems with them in practice given the variability of power supply voltages, micro USB cables, and polyfuse characteristics.

I agree with Morgaine that the increased volume suggests that choices made to save pence should be revisited.  Personally, I like the idea of a built-in switching power supply with 3.5 mm jack and a bridge rectifier on the input so that you can hook up any 6V to 12V wall wart without worrying about polarity.

I agree 100%.

While I accepted it, I've got never convinced that using a charger and the USB connector for input power were the best approach.

I asked Pete more than once if they ever considered using a power management IC instead LDOs, Eben at some time talked about it, but never got/saw a response.

There are plenty of low cost/acceptable quality power supplies out there, and by the volumes/interest we are seeing with the Pi I'm sure the distributors can get a

very good price and bundle as an optional a power supply.

Get rid of the fuses, there are input voltage protection chips like the NCP349 used on the Beaglebone and USB power switches with protection can be used

in replacement of the fuses.

-J

Replacing polyfuses F1 and F2 by wires leading back to the power input point VIN is quite likely to become Raspberry Pi Hardware Hack #1.

Morgaine.

Coder

I can't seem to reply to that post. ( http://www.raspberrypi.org/phpBB3/viewtopic.php?f=6&t=5632 )

I suspect his TV decided to quit, and it seems thats not unusual for these models.

http://www.badcaps.net/forum/showthread.php?p=205335

The good news is the link above may help him fix the TV, but he may need to login to get the pics and cct diagrams.

I can't see how the Pi could cause the power issue he had.

It took out the mains supply to the house, rather than just stopping the TV from working.

Even if the Pi 5v was high (or low) the mains supply will not be effected in that way. The worst would be the TV fuse blowing, or the magic smoke escaping from a component, and therefore it stops working.

On the charger note, my son is waiting for his Pi, and we tested his charger supplied with his new smartphone, and it was 4.6v across TP1 and 2.

So I would suggest that someone develop a small 5v supply that takes a 9v to 12v Wallwart, and end this varying 5v situation once and for all.

Mark