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Raspberry Pi 3 Current
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Raspberry Pi 3 Current

by netmech on Fri Jun 17, 2016 10:44 pm

well, this is a apparently a sore subject on the internet, and i'll add new post to the history of it for a clarification on a piece of it

From the various locations on the Rapsberry Pi site and other places, it seems clear that the 26pin headers have the 16mA per pin, an 50mA max for all 3.3V pins
However, it's not clear to me if that really does apply to the 40pin header, particularly on the RPi 3.

And, adding to that... I just picked up the 2nd edition of the Rapsberry Pi Cookbook by Simon Monk, and it states on page 199
"Do not draw more than 16mA per output and keep the total for all outputs below 50mA in total for an older 26pin Raspberry Pi, and below 100mA on a 40pin Raspberry Pi"

That would make sense since the 3.xmA per pin * 30 3.3V pins is right about 100mA.
But i've not been able to find any document that says what the 3.3V rail on the RPi3 can actually supply.

Anybody have that?
Does Mr. Monk frequent these forums per chance?
Anybody else know of a document giving some more details on the differences between the supplies to the 26 and 40pin headers?


So to summarize the power draws that may be in place on the RPi3, using his suggested max values of 100mA for 3.3V

2.4A Max
- typical board power draw: 400mA to 1.3A (depending on CPU usage and such)
-- that was using wifi.. granting it's quite a range. how much does ethernet add?
- 50mA for HDMI
- 100mA for 3.3V pins
- 250mA for camera
--------------------------------
1850mA

leaving (out of 2.4A) ~650mA for the 5V rail or USB devices (out of a maximum 1.2A)
- or allow more for these if the above items like the camera aren't in use

right?
(using values from https://www.raspberrypi.org/help/faqs/#powerReqs)

netmech
 
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Re: Raspberry Pi 3 Current

by adafruit_support_mike on Fri Jun 17, 2016 11:53 pm

Part of the confusion stems from the fact that microprocessors these days have different 'power domains'.. sometimes multiple silicon dice inside the package, each with its own VCC and GND pins.

If that wasn't complicated enough, the RasPi's GPIO pins are connected to configurable current sources that can limit each pin between 2mA and 16mA, with the total current through all pins limited to 51mA. As I understand it, the default configuration is for 8mA per pin.

The 3.3v regulator has an output limit of 50mA, and is connected to two pins in the GPIO header. The combined load on those pins shouldn't exceed 50mA.

The Pi3's microprocessor has four cores, each of which consumes about 175mA under full load. The nominal operating current is about 350mA to 400mA, but can go up to 750mA when all four cores are active.

When you start getting into the peripherals, that's heavily dependent on the kind of device and the way you use it. The most straightforward option is to hook everything up and track the power consumption over time.

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Re: Raspberry Pi 3 Current

by netmech on Sat Jun 18, 2016 1:19 am

As I understand it, the 2mA to 16mA per pin configurable drive strength is not a limit, as in something that will stop me from being unsafe and drawing more.
  • https://www.scribd.com/doc/101830961/GPIO-Pads-Control2
  • http://www.thebox.myzen.co.uk/Raspberry/Understanding_Outputs.html
That bit seems pretty straight forward.



So, from your comments, you'd be stating that the just released Raspberry Cookbook 2nd edition has an error on pg 199 when Simon says not to draw more than 100mA from the 3.3V pins on a 40pin header? (I did actually send Oreilly an email asking about it too)


If i'm looking at the schematic for the RPi3, the 3.3V and 1.8V regulator is the PAM2306AYPKE, right?
So, googling that.. it looks like it has a max output of 1A per channel, and is actually most efficient on 3.3A a bit over 100mA.
http://www.diodes.com/_files/datasheets/PAM2306.pdf

From
https://cdn-learn.adafruit.com/download ... odel-b.pdf
I see that the RPi3's 3.3V supply is beefier than the B+, and certainly more than the A and B.

What am i missing?
Are we sure that the RPi3's supply isn't able to safely supply more than 50mA total to the GPIO pins now?


Thanks for your time!

netmech
 
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Re: Raspberry Pi 3 Current

by adafruit_support_mike on Sun Jun 19, 2016 1:10 am

netmech wrote:As I understand it, the 2mA to 16mA per pin configurable drive strength is not a limit, as in something that will stop me from being unsafe and drawing more.

You can break anything if you try hard enough.

If you connect a circuit designed to draw 10mA to a pin configured for 8mA, you'll get 8mA through the circuit and some voltage drop on the pin. If you connect the positive rail of a 1A @ 3.3v power supply to a GPIO pin configured to sink 8mA, you'll probably kill the pin (and the whole microprocessor).

To kill a GPIO pin, you have to exceed the pin's limits far enough that it raises more questions about the external circuit than about the RasPi.

netmech wrote:So, from your comments, you'd be stating that the just released Raspberry Cookbook 2nd edition has an error on pg 199 when Simon says not to draw more than 100mA from the 3.3V pins on a 40pin header? (I did actually send Oreilly an email asking about it too)

I can say that the references I've seen apparently don't match the references he's seen. I'm not going to pick fights over who's right and who's wrong though. It's highly possible that we've stated two different simplified views of a more complex system.

If you want low-level design information about the Pi3, head over to the RasPi Foundation forums and ask its designers:

http://www.raspberrypi.org/forums/

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Re: Raspberry Pi 3 Current

by netmech on Sun Jun 19, 2016 2:12 am

Certainly not picking a fight! Just looking for info

Thanks for your insights

netmech
 
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Re: Raspberry Pi 3 Current

by medicdude on Wed Jun 22, 2016 1:40 am

adafruit support mike seems to be wrong on several counts of information. The pi3 3v3 voltage driver is 1Amp rated, and may be supplying up to 750mA to the device at 100% power, leaving around 250ma safe margin to the GPIO and 3.3V output rail. If you are intentionally limiting the onboard current draw, you *might* be able to get away with more than 250ma on the combined gpio pins.

Below is crossposted from Raspi Org Forums:
I found a link to this in some other thread, seems to be about the pi1 http://www.thebox.myzen.co.uk/Raspberry ... tputs.html
Also see pads document about pi1 here https://www.scribd.com/doc/101830961/GPIO-Pads-Control2

Empirical evidence seems to support what's contained in the pads document; The drive strength setting will guarantee a logical high voltage of at least 1.3V at the stated mA draw. Drawing more mA than this will result in a voltage lower than 1.3v.

The pads of the pi1 were only guaranteed rated at 16ma resistive current. Low ohm loads, direct shorts, and certain capacitive loads less than 16ma current, can and WILL exceed this 16ma limit and burn up the pad, and probably the rest of the pi with it. IT IS ENTIRELY POSSIBLE TO DRAW BEYOND 16ma AT ANY DRIVE SETTING PARTICULARLY IF A DIRECT SHORT TO GROUND OCCURS AND THIS COULD FRY THE Pi1.

Based on my calculations a 200ohm resistor should be enough to protect an output pin from a direct short even at drive 7 settings (16ma). A 200ohm resistor could also result in logic level problems with any downstream effective circuit resistance less than around 200 ohms, AFAIK almost everything will be way WAY above this. The fact that the design doesn't place a 200ohm thermistor (or at least resistor!) on the output of the current driver is so so stupid, but there you have it.

I have not found ANY empirical evidence of what the pi3 can actually support, it's all been for the pi1. If anyone knows of any actual testing or published documentation on what the per pad/pin limit is for the pi3, that's what OP asked for and noone has yet delivered.

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