thexeno
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Electrical specifications BCM2835

Thu Sep 11, 2014 6:47 pm

Where can I find the electrical charactristics of the BCM ports? I've found, only barely mentioned, something about the Vil and Voh and nothing more, or maybe on the current. Otherwise some of my projects can not ad affordable as I want to design them. I just stepped in a problem of minumum voltage of a digital value today, solved just with some luck. I discovered that the Vil is around 0.8V, but for sure I can't keep discovering things in such a way.
Is that possible that the electrical considerations are not publicly available? If not, can anyone provide some link? I'm just referring to capacitances, open drain capability, currents, voltages, absolute values and the usual stuff related to that.

Thanks!! :)

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joan
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Re: Electrical specifications BCM2835

Thu Sep 11, 2014 6:56 pm

Gpios? I'm not sure there is an official position. Have a look at http://www.mosaic-industries.com/embedd ... ifications

thexeno
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Re: Electrical specifications BCM2835

Thu Sep 11, 2014 7:54 pm

Yeah, thanks, you got my point.
I opened this thread after reading that site: because they say something about some datasheet, but I didn't find nothing about that. For sure their reports are coherent.

Like the "officiality" of the missing of full datasheet/videocore infos, despite it is not a thing that is put in evidence, is said and written somewhere, and the user can buy an RPi and be aware of that lack. Quite fair. But here, at least, since I'm very disappointed about that lack/closure in such a board, I'd like to read/hear something "official" from the Foundation regarding the availability of these electrical characteristics. To be a fair situation, I hope that it is written somewhere and I've been simply inattentive.

W. H. Heydt
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Re: Electrical specifications BCM2835

Fri Sep 12, 2014 2:00 am

The characteristics of *board* are widely publicized: 5v at 700mA. Since you're buying a board and not just a BCM2835, why would the specifics of the BCM2835 matter to you?

thexeno
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Re: Electrical specifications BCM2835

Fri Sep 12, 2014 7:58 am

W. H. Heydt wrote:The characteristics of *board* are widely publicized: 5v at 700mA. Since you're buying a board and not just a BCM2835, why would the specifics of the BCM2835 matter to you?
It should be trivial that the GPIOs are connected to BCM, whichever the board. You can understand that current and voltages must be known in order to use other logic which is not only the 3v3 HC one, some cases of compatibility could be exploited or issues arises if unknown. Same for the current and the fan out, problem in which I stepped in. I should GUESS what is not working, exploiting the fact that i'm in this field, instead of make some calculus. For a newbie, this is not educational at all (for digital electronic circuits point of view).

If the GPIOs where inaccessible, that is the case in which your sentence make sense, because I can only use USB, which are known it's electrical specifications.

Once I asked about the L2 cache, the answer was "why do you ask this?". I can't stand to read those answers, expecially in a forum desinged to helping people learn around the Pi itself. But in a way, that was making more sense because I can't do nothing on the L2 cache (I think...). I'm not mad with you, but it's just to make the point.

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RaTTuS
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Re: Electrical specifications BCM2835

Fri Sep 12, 2014 8:24 am

the RPi GPIO runs at 3.3V
see
http://elinux.org/RPi_Low-level_periphe ... .28GPIO.29 [older not mentioning b+]
you max current draw is 50mA total , and 16mA for any one pin
How To ask Questions :- http://www.catb.org/esr/faqs/smart-questions.html
WARNING - some parts of this post may be erroneous YMMV

1QC43qbL5FySu2Pi51vGqKqxy3UiJgukSX
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thexeno
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Re: Electrical specifications BCM2835

Fri Sep 12, 2014 10:28 am

RaTTuS wrote:the RPi GPIO runs at 3.3V
see
http://elinux.org/RPi_Low-level_periphe ... .28GPIO.29 [older not mentioning b+]
you max current draw is 50mA total , and 16mA for any one pin
I already know that, but thanks for effort of course. But is not detailed, like, to do a reference with an MCU known by most here, the "Electrical parameters" (or "characteristics") index in a datasheet of the Atmega328. Creating the possibility to know very well what happens if something is wrong, creating well designed PCBs around it and so on.
I can't do that in RPi, making PCBs for the gpios connector has some costs, I can't just "guess" if it will work or not at the end of manufacturing. If I need to use the serial capability, I made some circuits that interconnect to USB by avoiding the need to know parameters of the chips, since the USB is known. But if I need somthing from other pins, situation is different. Especially the need is from the time domain paramters, i.e. rise and falling timing.

I don't ask the code of the firmware (despite I'd like to know how it works), I can undesrtand the "closure". But be closed about that parameters - I can't see the "money loss" from Broadcom if are available these infos. It's a huge "brake" to my projects. I think that this can became legally published, if the Foundation want it. I'd like to know if it is possible.

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AndrewS
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Re: Electrical specifications BCM2835

Fri Sep 12, 2014 3:58 pm

I think http://www.scribd.com/doc/101830961/GPIO-Pads-Control2 is the nearest there is (currently) to an official specification, but I don't know if it has all the info you're looking for.

OTOH, with the Pi being so cheap would it hurt to just buy one or two and actually do some experimentation to see if it works with the chips you're wanting to use with it? ;) Or you could always hook an MCU up to the Pi over TTL serial/i2c/SPI, and use the MCU to interface to the external hardware, and use the Pi to process everything else.

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Gert van Loo
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Re: Electrical specifications BCM2835

Fri Sep 12, 2014 4:09 pm

I just stepped in a problem of minumum voltage of a digital value today, solved just with some luck. I discovered that the Vil is around 0.8V, but for sure I can't keep discovering things in such a way.
When connecting devices there is no 'luck' involved.
All I/O of the BCM2835 are compliant with the standards.
Thus all digital I/O will be compliant with the CMOS I/O standard for 3.3V CMOS devices.
The same holds for all other interfaces like DSI, CSI (Mippi standard) HDMI (HDMI standard), SDRAM (JEDEC standard) etc.
If you want to connect the BCM28235 device to a product X you have to look at the spec of both and see if they match.
If not you have to add interface converter chips.

thexeno
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Re: Electrical specifications BCM2835

Fri Sep 12, 2014 4:54 pm

Gert van Loo wrote: All I/O of the BCM2835 are compliant with the standards.
Well, there are things that could differ from 3V3 to 3V3 cmos. But, I understood. Anyway, the improvement suggestion has been done, I tried at least.
AndrewS wrote:I think http://www.scribd.com/doc/101830961/GPIO-Pads-Control2 is the nearest there is (currently) to an official specification, but I don't know if it has all the info you're looking for.
Nice link, btw.

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