Raspberry Pi - Hardware Flaws and Fixups?

Reviewing the rev 1.0 schematics and docs for the related parts, I found something that looks wrong.

The SMSC LAN9512 has two VDD18CORE pins that are internally connected between them and to the internal core 1.8V voltage regulator, those pins need to be connected externally as well and to a couple of filter caps.

The RasPi schematics shows those pins also connected to the +1V8 power line from RG1.

That looks wrong, the SMSC VDD18CORE pins should not be connected to +1V8.

-J

I fully agree! with your "VDD18CORE" remark.

Jamodio, have you found a "reset" pin on the 2835? The only thing I could find was "run". Would the CPU just pause when pulled low? I don't know.

While investigating this, I found that "D10" is not marked "(B)" as it should. (even though it is involved with the USB powersupply for the model A, the other side is not connected on the model A, so the component is redundant on model A).

Yes you are right D10 will not be needed for model A since as far as I know it does not include the SMSC LAN9512.

On the other hand even when the usb power control was with the poly fuses was implemented following the datasheet recommendations,

I'd have preferred a pair of USB switches instead. As we have seen from numerous reports the whole power strategy is questionable and

source of multiple problems.

About RESET I didn't check on the limited documentation if there is any reference about the the RUN pin works, I'd guess that probably

if we pull that pin down it may put the GPU in halt state but I'm not sure if it works as a complete reset.

-J

As there is a resistor on the "run" pin, I'm pretty sure the signal can be found on the "outside" of the PCB.... But getting a replacement 'pi is not yet easy and cheap enough for me to start testing this. Pete has the full datasheet and he can figure it out.

Very interesting observation about the VDD18CORE pins.

Can you tell what symptoms this might cause?

It is likely that it doesn't have any bad consequences. However it goes against the intention of the chip maker, so you're effectively beta-testing the chip. There just MIGHT be weird effects that nobody had expected.

Some people claim that it is bad practise to connect multiple linear regulators in parallel. Which is what we're doing here. Some hobby-RC electronics (Electronic speed controllers) have four linear regulators on them and connected in parallel. Someone reported that these start to oscilate when unloaded. Everything was fine when a little current was used. He disconnected three of every four regulators and all was fine.

So: We risk oscilation. We risk injecting noise into the lan9512 chip where it isn't expected.... (or the other way around).

Unfortunatelly there is not much doc available for the BCM2835.

The electrical characteristics on the LAN9512 DS do not make any mention about those pins, then I'd infer that the only reason for their

existance is for the external caps for the internal regulator like many other devices. So those pins are not intended to be an input from

an external 1.8V source nor an output to become a 1.8V source.

Besides injecting noise on the core voltage, it really depends in which direction current is flowing at any given time, so extra current

draw from the internal regulator or additional current flowing through those pins will probably increase the chip temperature, and

since those pins are not intended to be connected to anything active, the silicon probably has not much protection, so potentially

you could damage the chip.

I don't know what consequences it may have on the BCM chip.

In summary, I don't believe it is something that will make the board blow up but MUST be corrected on future revisions, the way it is

right now it is simple wrong.

I'll see what happens if I pull down RUN.

Right now I'm testing the Arch Linux distro, Debian seems to have another issue on the kernel related probably to the USB/ethernet

driver.

My .02

-J

It's not connected to the BCM2835 1.8V regulator output: The BCM2835 doesn't have a 1.8V regulator.

The 1.8V comes from the RG1 regulator. (Ah! This means we'd be able to run 1.2V SDRAM!), but as things are right now, it'd be a funny experiment to remove the NCP1117 altogether and see what happens. :-)

When the 9512 has some "headroom" and/or the SDRAM interface doesn't use too much power on the 1.8V line, this would save a few components for the foundation/farnell/RS.

I though that the 1.8V regulator was the smallest of the three, so that the "design power" of the regulator was probably the lowest. Not true: RG1 is the middle one. So Pete problably thought a noticable current would flow there....

(The FT232BM had a 3.3V regulator on board, which could deliver 10mA of extra current. Useful, but quickly inadequate. The FT232RL has a 3.3V regulator with 50mA to spare! That's quite useful. You can easily power an AVR and a few leds from that!).

It's not connected to the BCM2835 1.8V regulator output: The BCM2835 doesn't have a 1.8V regulator.

The 1.8V comes from the RG1 regulator. (Ah! This means we'd be able to run 1.2V SDRAM!), but as things are right now, it'd be a funny experiment to remove the NCP1117 altogether and see what happens. :-)

When the 9512 has some "headroom" and/or the SDRAM interface doesn't use too much power on the 1.8V line, this would save a few components for the foundation/farnell/RS.

I though that the 1.8V regulator was the smallest of the three, so that the "design power" of the regulator was probably the lowest. Not true: RG1 is the middle one. So Pete problably thought a noticable current would flow there....

(The FT232BM had a 3.3V regulator on board, which could deliver 10mA of extra current. Useful, but quickly inadequate. The FT232RL has a 3.3V regulator with 50mA to spare! That's quite useful. You can easily power an AVR and a few leds from that!).

Yes afaik there is no internal regulators on the BCM2835.

The LAN9512 pins are connected to the main 1.8V fed by RG1 wich feeds the BCM2835 1.8V, several clocks and PLL circuits, the USB module, etc.

Those pins are not intended to be an OUTPUT just a connection point for the required filter caps.

I've no clue what is the total current draw by the BMC2835 through those pins.

I don't think it would be a good practice, even to try, sourcing the 1.8V from the LAN9512 pins. And for model A you will sill need the 1.8V regulator.

The NCP1117 can provide up to 1A (obviously not in this application), it shows about 95F so there is some current flowing there.

I guess Pete has the advantage of seeing all the BCM2835 documentation that is not available publically.

-J

jamodio wrote:

 

Yes afaik there is no internal regulators on the BCM2835.

not true... There is a switching regulator IN the BCM2835 that generates the 1.2V core power its own use!

C16 and C17 next to the SOC are involved (input/output capacitor) and directly opposite on the bottom side of the PCB, L5.

Suggestion for the future....

You know those pins that power the SD interface voltage? The one that was left floating in the alpha batch in january? That should become jumperable to 1.2V or 1.8V. That opens up a few new RAM chips.

In fact I think I have a 'pi that has a 1.2V RAM chip installed where this voltage is SUPPOSED to be 1.2V. Not 1.8V.... (On the other hand, the RAM chip that I have also needs 1.8V, and I don't know which is which..... In january I thought the missing connection was the one for the IO buffers on the chip, which I THINK are supposed to be 1.2V for my samsung K4P2G324ED. And from the schematics I cannot see what voltage would be present on SDRAM_VDD.) (*)

Oh, in the box "PWR1" at the top of page 1 of the schematics, you see several "voltage regulator outputs" on the BCM2835.... :-)

(*) On the other hand, the pins marked 1.8V are probably really 1.8V, and really power the SDRAM chips themselves. while the SDRAM_VDD might be the one that is the IO voltage for the SDRAM and be the 1.2V that the Samsung page mentions as the VDDQ, the IO voltage for the SDRAM chip.

You are absolutely right, didn't look in detail at those pins !!

I t would be nice if Broadcom at least releases the pinout documentation with a brief explanation of what each pin does.

-J