bradburts
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sat Nov 19, 2011 12:14 pm

I would go with a hard power reset rather than SOC reset. There may be attached peripherals which would also benefit from the reset. Also I don't trust chip resets, have met a few chips which don't reset properly that way.
As mentioned PSU's have a reset pin. If you're battery operated then you could do something with a FET etc or, ging upmarket,then there are watchdog chips, say MAX696. Also allows you an orderly shutdown on power loss. Should be able to find good application notes which show you how to rig to PSU and hexplain the circuits required even if you end up using a GPIO port rather than the watchdog chip.

bradburts
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sat Nov 19, 2011 1:40 pm

PS
Sharing an SD card sounds interesting. Especially if you want to keep a cluster of RPIs in synch.
I read that you cannot boot from USB, have to boot from SD first. Do you know if there's an option to boot from ethernet? If not can you modify the SOC firmware?
Boot from ethernet would be great.

jacklang
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sat Nov 19, 2011 2:06 pm

Think of the SD card as holding the BIOS equivalent. You put a small loader on it that then does the main boot from internet, if that is what you want

hippy
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sat Nov 19, 2011 2:18 pm

Quote from bradburts on November 19, 2011, 13:40
Sharing an SD card sounds interesting. Especially if you want to keep a cluster of RPIs in synch.

And keeps costs down !

The theory is that reset will normally tri-state all I/O pins so if all R-Pi are connected to a single SD card, are all placed in reset, then released from reset one at a time, allowed to boot and then relinquish control of those lines ( tri-stating them again ) the next R-Pi can then be released from reset and do the same again.

The technique works with other micros so I'd expect it to work with the SoC here though it is only a theory that it will at present. The R-Pi won't be able to access the SD Card after booting and that may require changes in device drivers and the like.


I read that you cannot boot from USB, have to boot from SD first. Do you know if there's an option to boot from ethernet? If not can you modify the SOC firmware?
Boot from ethernet would be great.


I don't believe it's an option at present but it's certainly possible and I'm sure people will come up with a means to do the initial GPU load from SD Card then switch to booting the rest via USB or ethernet. There have been a few threads which have discussed that and I'd expect to see it as one of the early R-Pi developments. You won't be able to get rid of the SD Card(*) but it would only be needed for the first part of the boot process.

The 'binary blob' loaded into the GPU is apparently 18MB so we should be able to use cheap 32MB and upwards MMC cards to boot over USB or ethernet(**).

(*) One could replace SD Card with some compatible SPI Flash but that would be less convenient than having an actual SD Card. Having the 'BIOS' on SD Card is one of the things I feel is great about the R-Pi; impossible to brick it, no special purpose tools needed to rescue it.

(**) As MMC and small SD cards are so cheap it may be easier to have R-Pi clusters boot over ethernet from a master R-Pi than worry about sharing a single SD Card.

bradburts
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sat Nov 19, 2011 4:44 pm

Found 32MB SD for £1, £2 for 2GB!
I think that the 18MB 'blob' is the kernel. GRUB, for example, only takes a 1MB partition.
Forgot that SD is SPI. As a flight of fancy & getting back to topic....
The I/O microcontroller could host the bootstrap. The RPI could then boot from ethernet.
Not saving a lot of money but it keeps the SD slot free for data. That would save 2GB of OS image per RPI.
Which ever way the best scheme for me would be to boot from ethernet, the hardware to sequence the SD boot would scare me.

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abishur
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sun Nov 20, 2011 3:29 pm


However if you need such a signal, then it is easy to build into the power supply. If you fed +5V to the Pi via, say, the GPIO port (which I believe *is* possible), then you can simply choose to use a PSU regulator with a shutdown signal.


Max voltage for the GPIO is 3v3 which goes straight into the SoC, so I wouldn't recommend trying to send 5V down it ;)
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WizardOfOZ
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sun Nov 20, 2011 3:37 pm

Quote from abishur on November 20, 2011, 15:29
Max voltage for the GPIO is 3v3 which goes straight into the SoC, so I wouldn't recommend trying to send 5V down it ;)
I was hoping for the +5V PSU connector being mirrored on the GPIO port, as an alternative to the Micro USB connector for power. In fact I mentioned this point in the 'What have we missed?' thread some time ago. No-one suggests feeding board power to an I/O pin on the SoC. :D

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Re: Break out board (GPIO, SPI, I2C, and UART)

Sun Nov 20, 2011 4:12 pm

abishur: You mean it's been confirmed the 5V on the GPIO isn't in the final board version? (see Gert's post from a while back http://www.raspberrypi.org/for.....38;t=809.8)

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Re: Break out board (GPIO, SPI, I2C, and UART)

Sun Nov 20, 2011 8:41 pm

Quote from WizardofOz
No-one suggests feeding board power to an I/O pin on the SoC :D


Just making sure ;) I can see someone misunderstanding and trying to do it only to receive a shocking surprise

Quote from asb on November 20, 2011, 16:12
abishur: You mean it's been confirmed the 5V on the GPIO isn't in the final board version? (see Gert's post from a while back http://www.raspberrypi.org/for.....38;t=809.8)

Ah, no I haven't heard anything more on it, but until we get some hard specs for in production boards, I'm trying to avoid planning on certain "Cadillac" features lest I plan on them being there and then have to scrap it all when they're not :)
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Gert van Loo
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sun Nov 20, 2011 9:00 pm

As the layout is now more or less complete the GPIO connector will not change.
The GPIO connector has 2 pins which are directly connected to the 5V rail. There was a question from somebody if this was an input or output at which I explained that a 5V rail does not have an input or output. A power rail just 'is' there. It is up to you to 'source' or 'sink' it.

hippy
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Re: Break out board (GPIO, SPI, I2C, and UART)

Sun Nov 20, 2011 10:30 pm

Quote from Gert van Loo on November 20, 2011, 21:00
As the layout is now more or less complete the GPIO connector will not change.
The GPIO connector has 2 pins which are directly connected to the 5V rail. There was a question from somebody if this was an input or output at which I explained that a 5V rail does not have an input or output. A power rail just 'is' there. It is up to you to 'source' or 'sink' it.


With the R-Pi having a power supply fuse not all "5V" points are directly joined. We're going to need some accepted terminology to differentiate "5V" on each respective side of the fuse to avoid confusion and "input" and "output" is IMO appropriate; "input" the incoming PSU side of the fuse, "output" the 5V input passed through the fuse.

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abishur
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 7:44 am

Quote from hippy on November 20, 2011, 22:30
Quote from Gert van Loo on November 20, 2011, 21:00
As the layout is now more or less complete the GPIO connector will not change.
The GPIO connector has 2 pins which are directly connected to the 5V rail. There was a question from somebody if this was an input or output at which I explained that a 5V rail does not have an input or output. A power rail just 'is' there. It is up to you to 'source' or 'sink' it.


With the R-Pi having a power supply fuse not all "5V" points are directly joined. We're going to need some accepted terminology to differentiate "5V" on each respective side of the fuse to avoid confusion and "input" and "output" is IMO appropriate; "input" the incoming PSU side of the fuse, "output" the 5V input passed through the fuse.

Oh? I was getting the impression that the 5V took place before the fuse and rest of the power protection. That is to say that the power comes in and then before going through any of the regulators went off to the GPIO pins. That way if power was sent in over it, it would still pass through the protection.

I might be wrong, of course, that's just the impression I got when I saw gert's post :?
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Gert van Loo
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 9:13 am

"input" the incoming PSU side of the fuse, "output" the 5V input passed through the fuse.
According to that definition the GPIO 5V is on the 'output'.

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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 9:27 am

[ Deleted - cross posted with Gert ]

fdufnews
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 10:17 am

Quote from Gert van Loo on November 21, 2011, 09:13
"input" the incoming PSU side of the fuse, "output" the 5V input passed through the fuse.
According to that definition the GPIO 5V is on the 'output'.

So this will limit the current available on an extension board powered by the GPIO connector

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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 10:40 am

Quote from fdufnews on November 21, 2011, 10:17
So this will limit the current available on an extension board powered by the GPIO connector

That is correct. The fuse protects the PSU connected to the micro-USB from over-current drawn from the PSU through the R-Pi board.

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Burngate
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 12:36 pm

Quote from hippy on November 21, 2011, 10:40
The fuse protects the PSU connected to the micro-USB from over-current drawn from the PSU through the R-Pi board.
No. (Or rather I don't think so.)
The fuse in your (British 13A) plug protects the cable. In the same way, the PSU should have built-in protection to protect itself and its cable.
The fuse on the Pi is to protect the Pi, and (if I'm correct in my understanding) works in conjunction with the over-voltage device to protect it from excess voltage.

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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 1:11 pm

Quote from Burngate on November 21, 2011, 12:36
The fuse in your (British 13A) plug protects the cable.

I guess the correct answer is "the fuse protects from an over-current situation" and what it's actually protecting in that circumstance can vary.

For example the situation where a product using a 13A plug with a 13A-rated cable has a 5A fuse fitted it wouldn't be to protect the cable as up to 13A won't damage it so there's a different rationale for fitting a 5A fuse.

If a short developed at the mains plug end of the cable the fuse can be said to be protecting the ring from over-current ( that is the up-stream 'PSU' supplying the ring ). So perhaps the fuse protects the cabling on both side of the plug.

For an R-Pi there are two possible consequences in an over-current situation; the PCB malfunctions or the PSU and/or PSU cable malfunctions. IMO it doesn't really matter what the fuse primarily protects, just that it protects against damage in an over-current situation.

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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 1:36 pm

Quote from Gert van Loo on November 21, 2011, 09:13
"input" the incoming PSU side of the fuse, "output" the 5V input passed through the fuse.
According to that definition the GPIO 5V is on the 'output'.


Interesting... so if we use the GPIO pins to supply 5V, then we need to make sure to include our own voltage protection (since it's coming in after all the voltage protection)? Or am I have a mass fit of dyslexia?
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gbulmer
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 1:54 pm

Quote from abishur on November 21, 2011, 13:36
Quote from Gert van Loo on November 21, 2011, 09:13
"input" the incoming PSU side of the fuse, "output" the 5V input passed through the fuse.
According to that definition the GPIO 5V is on the 'output'.


Interesting... so if we use the GPIO pins to supply 5V, then we need to make sure to include our own voltage protection (since it's coming in after all the voltage protection)? Or am I have a mass fit of dyslexia?

The microcontroller GPIO pins can't supply 5V directly, they can only supply 3.3V. (don't quite do 3.3V, as there is usually a little voltage drop inside the chip)

I don't believe any of the GPIO's are 5V tolerant (can't see anything on the wiki), so the only way to 'supply' 5V will be via an external device, e.g. bipolar-transistor, MOSFET, or IC.

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TonyD
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 1:57 pm

Is the UART interface still planned for the I/O port?
Tony

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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 2:13 pm

Quote from gbulmer on November 21, 2011, 13:54
Quote from abishur on November 21, 2011, 13:36
Quote from Gert van Loo on November 21, 2011, 09:13
"input" the incoming PSU side of the fuse, "output" the 5V input passed through the fuse.
According to that definition the GPIO 5V is on the 'output'.


Interesting... so if we use the GPIO pins to supply 5V, then we need to make sure to include our own voltage protection (since it's coming in after all the voltage protection)? Or am I have a mass fit of dyslexia?

The microcontroller GPIO pins can't supply 5V directly, they can only supply 3.3V.


I think this is just a terminology issue. On the header connection that is often referred to collectively as "GPIO pins" there will be actual GPIO pins, 5V supply and 0V supply pins, and perhaps 3V3 and other voltage supply pins as well.

WizardOfOZ
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 2:31 pm

Quote from hippy on November 21, 2011, 14:13
I think this is just a terminology issue. On the header connection that is often referred to collectively as "GPIO pins" there will be actual GPIO pins, 5V supply and 0V supply pins, and perhaps 3V3 and other voltage supply pins as well.
Maybe we should try to remember calling the 26 pin connector for the GPIO port, to maybe prevent more confusion than necessary?

I am personally talking about supplying +5V DC (regulated and possibly suitably protected( as and when deemed required by yours truly)) to the Pi +5 VDC power rail, which is brought out to the 26 pin connector with 0.1" header spacing. The same connector may or may not contain one or more direct connections to the Broadcom SoC I/O pins. Those, if present, are irrelevant for the present discussion. We are (well, at least I am) talking about power supply issues, not digital I/O.

I positively don't care whether this connection is before or after any on board resettable fuse, and/or over voltage protection, if any, as I can deal with any related issue regardless of actual configuration. We now have confirmation that two of the pins on the 26 pin connector are somehow connected to the +5V Pi power rail, so I am happy. :)

Questions?

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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 3:17 pm

Quote from gbulmer on November 21, 2011, 13:54
Quote from abishur on November 21, 2011, 13:36

Interesting... so if we use the GPIO pins to supply 5V, then we need to make sure to include our own voltage protection (since it's coming in after all the voltage protection)? Or am I have a mass fit of dyslexia?

The microcontroller GPIO pins can't supply 5V directly, they can only supply 3.3V. (don't quite do 3.3V, as there is usually a little voltage drop inside the chip)

I don't believe any of the GPIO's are 5V tolerant (can't see anything on the wiki), so the only way to 'supply' 5V will be via an external device, e.g. bipolar-transistor, MOSFET, or IC.


Check the post from gert I'm referencing or the post from wizard above, two of the GPIO pins are attached to the 5V rail (though as Wizard says, I don't think they are also attached to the SoC, so it's a semantics issue)

Quote from WizardOfOZ on November 21, 2011, 14:31
I positively don't care whether this connection is before or after any on board resettable fuse, and/or over voltage protection,

Fair 'nuff, but I do care if it's before/after the protection devices. :P If it's after (which is what I *think* gert is saying) then that means we need to make sure to properly protect the r-pi board when we provide power to it through these pins. This would also mean that upping the available amperage to the USB ports would be most easily accomplished by providing power through these pins as we wouldn't have to worry about de-soldering and re-soldering anything on the r-pi itself.

One good thing regardless of where it lies on the protection devices is that it now provides some interesting options for a power switch (including turning it on via remote) :D
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Gert van Loo
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Re: Break out board (GPIO, SPI, I2C, and UART)

Mon Nov 21, 2011 7:15 pm

I'll try to be clear:
The board 5V rail is connected to two pins on the GPIO header.
You now have to options:
* You supply power to the board using the standard micro USB plug. The current will come from your external supply, through the fuse to your Raspberry Pi board. It will then also go through the GPIO pins to your extension board. In that case the fuse protects your power supply from a too have current drain.
* You can also build your own 5V supply on your extension board. You can then connect that 5V to the two pins on the GPIO connector. You should NOT connect a supply to the micro USB connector. You had also better make sure your 5V supply is correct otherwise you can blow up your Raspberry-Pi board, your connected USB equipment and possible your $1000 TV over the HDMI connector. You should also add a fuse or have current limit on your self-developed supply on the extension board to protect it from drawing too much current (e.g. from a connected USB device)

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