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GTR2Fan
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Wed Mar 09, 2016 9:06 am

gwideman wrote:Well, actually 5V TTL (as in 7400 and 74LS00 series) has a minimum VOH of 2.4V at 0.4mA, and typical VOH at 0.4mA of 3.4V. TTL has very low output current source capability, diminishing rapidly within 2 diode drops of VCC (ie: > 3.6V), and which can probably be handled by the RPi protection diodes, optionally aided by a pull-down resistor, or an external clamp diode.

If, as I suspect, RPi VIH < 2.4V, then it would be possible for RPi to read (non-buffer) TTL just fine, with no additional components, or just one. This is of some interest when using the RPi to retrofit additional functionality into old equipment.
You do realise that this is all utter nonsense from a design perspective? NOBODY who knows anything about digital circuit design relies on the correct interfacing of two different voltage logic families without using specifically designed level shifters which are ten-a-penny off-the-shelf devices.

Initially, I thought your request for detailed specs was serious, but I'm now beginning to wonder if this might actually be some kind of elaborate leg-pull. ;)
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Wed Mar 09, 2016 9:41 am

OK, please keep it civil everyone.

With regards timescales to getting this information (whether people think it's necessary or not), I have no idea. I have asked.
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Wed Mar 09, 2016 7:33 pm

rpdom wrote:Which was just an example, and you know it.
I did not know you intended this as just an example. Thanks for clarifying. I still think the 5mA is arbitrary.
I'm not arguing against having the full specs available.
Yay!
but it sounds like you want to know how close to the edge you can push things.
More to the point, knowing where the edge is lets one stay a safe distance from it.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Wed Mar 09, 2016 9:09 pm

GTR2Fan wrote:You do realise that this is all utter nonsense from a design perspective? NOBODY who knows anything about digital circuit design relies on the correct interfacing of two different voltage logic families without using specifically designed level shifters which are ten-a-penny off-the-shelf devices.
You might not like my example, but it's far from the case that only purpose-built level-shifters can be used to interface between logic families. (Though I would like to hear about your source for LVC245's at 0.1 cents!)

The resistor divider method mentioned by mahjongg is acceptable in some 5V-to-3.3V situations (but not the 7400 TTL case), the BSS138 FET is widely used for level shifting for I2C, and HCT chips like 74HCT125 are used to (up) translate 3.3 to 5V CMOS (VIL 30%, VIH 70%).

All of these are examples of why the electrical specs are useful, even when interfacing GPIO to logic, which was really the point I was making.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Wed Mar 09, 2016 10:29 pm

jamesh wrote:With regards timescales to getting this information (whether people think it's necessary or not), I have no idea. I have asked.
Thanks for your efforts.

tuxillo
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Thu Apr 14, 2016 2:08 pm

Hi all,

In my understanding after skimming through all the posts is that the foundation is trying to lift an NDA from a full-specs document for RPi2 and RPi3.

Apparently jamesh is the contact point here, so would you please give us an update?

Many thanks!
Antonio Huete

gwideman
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Tue Apr 26, 2016 12:22 am

jamesh wrote:With regards timescales to getting this information (whether people think it's necessary or not), I have no idea. I have asked.
Hi @jamesh,

It's coming up on two months since start of this thread, and I'm wondering if we could learn what progress has been made on this issue?

Thanks, Graham

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vaypay
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Sun Apr 01, 2018 6:14 pm

After 2 years without news and unless it was released on another channel, I don't think they will ever provide the info.
In case some meticulous engineer still wants to integrate the Pi in a project, an I/O expander chip like the MCP23017 that has well defined DC characteristics can be used. It's linked to the Pi with the I2C bus and very simple to use on the software level.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Sun Apr 01, 2018 6:24 pm

What specific information is required? AFAIK, thinks like max current etc are already published in the abridged datasheet.
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Sun Apr 01, 2018 8:06 pm

jamesh wrote:
Sun Apr 01, 2018 6:24 pm
What specific information is required? AFAIK, thinks like max current etc are already published in the abridged datasheet.
Where is this abridged datasheet?

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Sun Apr 01, 2018 9:46 pm

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Sun Apr 01, 2018 10:53 pm

jamesh wrote:
Sun Apr 01, 2018 9:46 pm
In the docs section

https://www.raspberrypi.org/documentati ... herals.pdf
I did not see any electrical specs in that doc, did I miss something?

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 6:54 am

gwideman wrote:
Sun Apr 01, 2018 10:53 pm
jamesh wrote:
Sun Apr 01, 2018 9:46 pm
In the docs section

https://www.raspberrypi.org/documentati ... herals.pdf
I did not see any electrical specs in that doc, did I miss something?
Probably not, what specific specs are you after?
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 7:16 am

jamesh wrote:
Mon Apr 02, 2018 6:54 am
gwideman wrote:
Sun Apr 01, 2018 10:53 pm
jamesh wrote:
Sun Apr 01, 2018 9:46 pm
In the docs section

https://www.raspberrypi.org/documentati ... herals.pdf
I did not see any electrical specs in that doc, did I miss something?
Probably not, what specific specs are you after?
If there are no electrical specs in that doc, why did you suggest it as a response to a request for electrical specs?

I am after the basic electrical specs that are customarily provided with every electronics component and module that is expected to be interfaced to in a robust and reliable way by engineers. This is Electrical Engineering 101. As I described in my original post of March 01 2016, and followup of March 02 2016, and which you yourself told us on March 08 2016 that Foundation acknowledged were needed and were taking steps to release.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 7:37 am

gwideman wrote:
Mon Apr 02, 2018 7:16 am
jamesh wrote:
Mon Apr 02, 2018 6:54 am
gwideman wrote:
Sun Apr 01, 2018 10:53 pm


I did not see any electrical specs in that doc, did I miss something?
Probably not, what specific specs are you after?
If there are no electrical specs in that doc, why did you suggest it as a response to a request for electrical specs?

I am after the basic electrical specs that are customarily provided with every electronics component and module that is expected to be interfaced to in a robust and reliable way by engineers. This is Electrical Engineering 101. As I described in my original post of March 01 2016, and followup of March 02 2016, and which you yourself told us on March 08 2016 that Foundation acknowledged were needed and were taking steps to release.
I'm a software engineer, not an electrical engineer. So humour me, define what you need. I'll talk to people, we'll see what i can find. Slipped off my radar tbh. We are very busy, and this is fairly low priority, but we are planning to improve docs for commercial users. In march 2016 I was a volunteer, now employee, so hopefully will be able get more information.

I know the current limits on the gpios are in one of the docs, thought it was that one.
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 8:17 am

jamesh wrote:
Mon Apr 02, 2018 7:37 am
...
I know the current limits on the gpios are in one of the docs, thought it was that one.
Gert provided a note on scribd (is that right?) which gave limited information about drive pads and being able to guarantee logic levels (e.g. how much could be sourced from a GPIO whilst the GPIO could maintain a logic 1 level).

I have seem nothing which gives absolute limits per GPIO or per GPIO bank other than an overall figure of 50mA. I think the 50mA figure is probably erroneous at it simply seemed to be based on the spare capacity available from the original Pi's 3V3 power supply chip.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 8:48 am

jamesh wrote:
Mon Apr 02, 2018 7:37 am
So humour me, define what you need. I'll talk to people, we'll see what i can find.
Well, I think it's fairly clearly spelled out earlier. But to recap:

This thread is about getting what are called the "DC Electrical Characteristics", (and also "Absolute Maximum Ratings")for GPIOs and any other signal pins that are on the interface. This may boil down to the specs for the GPIO and other pins on the SoC, but since there's no schematic we don't know for sure whether the SoC connects directly to the interface connector or via some other chip. I'm assuming it's direct.

As a concrete example of such specs, obtain a datasheet for virtually any digital chip, and look at the DC Electrical Characteristics section. As a case in point, look at this datasheet for Atmel microcontrollers:

http://www.atmel.com/images/atmel-8271- ... mplete.pdf

See chapter 30. Of greatest interest are:

1. The Absolute Maximum Ratings (section 30.1)

2. VIL, VIH, VOL and VOH; their respective values, and the conditions under which they apply. (See section 30.2).

Note that SoCs often have output and possibly input characteristics that are programmable. That is, by setting a register you can alter the VIL/VIH/VOL/VOH characteristics. A popularly adjustable feature is the output drive current (which in this datasheet is not adjustable and appears as a condition of the VOH and VOL specs rather than a separate value.)

So taking a second example, Qualcomm Snapdragon 410:
https://www.inforcecomputing.com/public ... cation.pdf

Here the DC Electrical Specs are in Chapter 3, and they are quite elaborate. But of most immediate interest are:

1. Absolute Maximum Ratings for signal pins, Table 3-1.

2. Section 3.6 Digital logic characteristics, which abbreviates the electrical characteristics of different sets of I/Os in Table 3-8 as "1.8V", "1.2V" and so on, cross referencing to more complete specs for VIL/VIH/VOL/VOH in Table 3-9 and Table 3-10, along with additional current specs. Further info on Programmable Drive Strength appears in Section 2.2.

I hope that provides enough detail of exactly the specs useful in applying GPIOs and other I/O pins, and which are virtually universally supplied.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 8:52 am

joan wrote:
Mon Apr 02, 2018 8:17 am
jamesh wrote:
Mon Apr 02, 2018 7:37 am
...
I know the current limits on the gpios are in one of the docs, thought it was that one.
Gert provided a note on scribd (is that right?) which gave limited information about drive pads and being able to guarantee logic levels (e.g. how much could be sourced from a GPIO whilst the GPIO could maintain a logic 1 level).

I have seem nothing which gives absolute limits per GPIO or per GPIO bank other than an overall figure of 50mA. I think the 50mA figure is probably erroneous at it simply seemed to be based on the spare capacity available from the original Pi's 3V3 power supply chip.
If you look back in this thread you will see where I surveyed the available would-be sources of the DC Electrical Characteristics, identified where Gert's doc doesn't make sense, and in general how multiple sources don't agree with each other.

There's no excuse why anyone has to guess about this, or do a bunch of testing to sleuth it out (which will only characterize the sample at hand). This is basic data that the manufacturer has, and which is virtually always made available. There is no possible proprietary reason to hide it, unlike, say, video driver code, or even some register maps.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 10:42 am

The GPIO are directly wired to the SoC on devices up to 3, on the 3 there is a GPIO expander for some pins, but I'm not sure what that covers.

I have no idea why the full electrical specs haven't been published. It's either getting permission from Broadcom, or a lack of time. As I said above it's low priority for a relatively small team. If it were vital, I sure it could be expedited, but clearly it's not since we still sell a considerable number of devices into industrial/commercial environments (probably in excess of 7M so far).

Just out of interest, what is the lack of information stopping you from doing? Huge numbers of people use the GPIO's successfully with the available documentation, so I'm just trying to see what particular circumstances you have that mean you are unable to proceed.
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 11:12 am

jamesh wrote: Just out of interest, what is the lack of information stopping you from doing? Huge numbers of people use the GPIO's successfully with the available documentation, so I'm just trying to see what particular circumstances you have that mean you are unable to proceed.
gwideman wrote:
Mon Apr 02, 2018 7:16 am
This is Electrical Engineering 101
Sounds like it's an undergrad hardware course, so presumably the aim is to teach students to design circuits properly (according to spec) rather than the one-off "make it work" approach.
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 11:36 am

davidcoton wrote:
Mon Apr 02, 2018 11:12 am
jamesh wrote: Just out of interest, what is the lack of information stopping you from doing? Huge numbers of people use the GPIO's successfully with the available documentation, so I'm just trying to see what particular circumstances you have that mean you are unable to proceed.
gwideman wrote:
Mon Apr 02, 2018 7:16 am
This is Electrical Engineering 101
Sounds like it's an undergrad hardware course, so presumably the aim is to teach students to design circuits properly (according to spec) rather than the one-off "make it work" approach.
Ah, OK, makes sense.

I think the waters a a bit muddied since the Pi wasn't really designed with that sort of teaching in mind, so we have never really concentrated on the more formal stuff like that. I'll have a chat with the HW chaps on Wednesday and se what I can knock up in the documentation.
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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 12:23 pm

Maybe I'm mistaken, but on some of the "I/O diagrams" I have seen there is talk of a (software switchable) schmitt-trigger option.
The option to enable schmitt-trigger functionality would change the definitions of ViH and ViL. but maybe these drawing were not actually for a RPI.

I have always understood that the electrical specifications of an RPI (GPIO) would be identical or very similar to 3V3 CMOS (the idea that they would be like TTL is simply laughable) so I have never had any need for electrical specs.

The output current specs (15mA per port, and 50mA in total) are just reasonable rule of the thumb estimates, the latter probably being defined by internal metal traces in the chip.

also it seems that the SoC has adjustable drive strengths, but no software seems to support it.

To me it seems the best person to ask is Gert van Loo, but its my understanding he is no longer with the RPF, he still hangs out here though. But perhaps he is not at liberty to answer these questions.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 12:43 pm

For what it's worth, yes the specs I have been describing would be essential for teaching Engineering 101. But I am not personally teaching such a class (though i do sometimes teach interest groups). My use of the phrase "Engineering 101" was to try to emphasize how fundamental and ubiquitous these specs are.

James, your info says "Engineer", though apparently not electrical. So whatever field you studied, think of the principles you learned first year, or perhaps already knew before uni, and those are the ones that you would not be able to apply. We're talking a level of "formality" that is at the level of the very first few things an interested person would know that a random person might not. Like F-stops on a camera. Or how to change the oil in a car.

Not having those specs prevents one from using even the basic first electrical principles one learns in high school, let alone EE 101. We're talking addition, subtraction, multiplication, division and ohm's law. Missing specs prevents determining if connected components will work without actually trying them out, and even that test only tells about those specific components, and not about a second unit.

I feel like I'm trying to make a case akin to "topics in science class are actually useful"... but plowing on...

I will give a couple of examples, but I emphasize that I am not asking for reports on whether these work. I am advocating that RPi docs facilitate performing the trivial calculations to make them work satisfactorily.

GPIO outputs:
1. Scenario: Use a GPIO output to drive an LED and a digital input. What is the max current (and hence series resistor) that the GPIO can supply to the LED, while maintaining a sufficient digital voltage level for the digital input? Does it make a difference whether the LED is connected so that the GPIO powers it high, or powers it low?

2. Scenario: Use a GPIO output to turn on a transistor that controls a 1A load. Will this work with a random transistor you have lying around, or do you need to pay attention to the minimum beta of the transistor so as to guarantee full turn-on?

GPIO inputs:
1. Various raw sensors can be used to detect simple presence/absence of some phenomenon. These sensors often do not work like switches, but instead they, for example, change their resistance. So they can be used with a resistor as a voltage divider, to produce a high-ish and low-ish voltage. To select values to work reliably into a GPIO, one needs to know VIL (Highest low threshold) and VIH (Lowest high threshold).

Again, of course you can assemble a test example and see if it "works". But (a) why does the user have to guess at this, and (b) a case that works tells nothing about whether a second unit will work, due to component variability. That variability is covered by the DC Electrical Characteristics.

To extrapolate these examples: a couple of years ago I investigated a poorly-conceived I/O adapter and level-shifter board that was produced as an accessory for a major-company SoC-based board that's loosely similar to RPi. The adapter was designed with receptacles to work with inexpensive Seeed Studio "Grove" series rudimentary sensors, actuators and peripherals, of which there were about 100, all of which have schematics. In short a sample of individual devices people might want to hook up to RPi. Because I had the DC electrical characteristics of the I/Os on the adapter board,, and for the modules, I was readily and quickly able to determine which modules would work reliably with the adapter board. Many would not.

Hopefully this gives you a sense of what those specs are useful for. And how out-of-step RPi is in not providing these specs for the I/Os that are touted as one of iRPi's most attractive features. Not to mention how dissonant this is with the educational mission.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 12:51 pm

mahjongg wrote:
Mon Apr 02, 2018 12:23 pm
the idea that they would be like TTL is simply laughable
Are you aware of LVTTL, which many 3.3V logic families abide by, and whose logic levels are identical to TTL?

For example: http://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf page 4.

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Re: Documentation bugs: RPi2&3 GPIO electrical specs

Mon Apr 02, 2018 2:04 pm

gwideman wrote:
Mon Apr 02, 2018 12:43 pm
For what it's worth, yes the specs I have been describing would be essential for teaching Engineering 101. But I am not personally teaching such a class (though i do sometimes teach interest groups). My use of the phrase "Engineering 101" was to try to emphasize how fundamental and ubiquitous these specs are.

James, your info says "Engineer", though apparently not electrical. So whatever field you studied, think of the principles you learned first year, or perhaps already knew before uni, and those are the ones that you would not be able to apply. We're talking a level of "formality" that is at the level of the very first few things an interested person would know that a random person might not. Like F-stops on a camera. Or how to change the oil in a car.

Not having those specs prevents one from using even the basic first electrical principles one learns in high school, let alone EE 101. We're talking addition, subtraction, multiplication, division and ohm's law. Missing specs prevents determining if connected components will work without actually trying them out, and even that test only tells about those specific components, and not about a second unit.

I feel like I'm trying to make a case akin to "topics in science class are actually useful"... but plowing on...

I will give a couple of examples, but I emphasize that I am not asking for reports on whether these work. I am advocating that RPi docs facilitate performing the trivial calculations to make them work satisfactorily.

GPIO outputs:
1. Scenario: Use a GPIO output to drive an LED and a digital input. What is the max current (and hence series resistor) that the GPIO can supply to the LED, while maintaining a sufficient digital voltage level for the digital input? Does it make a difference whether the LED is connected so that the GPIO powers it high, or powers it low?

2. Scenario: Use a GPIO output to turn on a transistor that controls a 1A load. Will this work with a random transistor you have lying around, or do you need to pay attention to the minimum beta of the transistor so as to guarantee full turn-on?

GPIO inputs:
1. Various raw sensors can be used to detect simple presence/absence of some phenomenon. These sensors often do not work like switches, but instead they, for example, change their resistance. So they can be used with a resistor as a voltage divider, to produce a high-ish and low-ish voltage. To select values to work reliably into a GPIO, one needs to know VIL (Highest low threshold) and VIH (Lowest high threshold).

Again, of course you can assemble a test example and see if it "works". But (a) why does the user have to guess at this, and (b) a case that works tells nothing about whether a second unit will work, due to component variability. That variability is covered by the DC Electrical Characteristics.

To extrapolate these examples: a couple of years ago I investigated a poorly-conceived I/O adapter and level-shifter board that was produced as an accessory for a major-company SoC-based board that's loosely similar to RPi. The adapter was designed with receptacles to work with inexpensive Seeed Studio "Grove" series rudimentary sensors, actuators and peripherals, of which there were about 100, all of which have schematics. In short a sample of individual devices people might want to hook up to RPi. Because I had the DC electrical characteristics of the I/Os on the adapter board,, and for the modules, I was readily and quickly able to determine which modules would work reliably with the adapter board. Many would not.

Hopefully this gives you a sense of what those specs are useful for. And how out-of-step RPi is in not providing these specs for the I/Os that are touted as one of iRPi's most attractive features. Not to mention how dissonant this is with the educational mission.
As I stated above, I am a software engineer.

What I don't understand then is how when something so apparently fundamental is missing, so many people have successfully used, and how so many have manufactured boards, that use the GPIO without actually having any problems. Like, millions of people.

That notwithstanding, if I can find any information, I will endeavour to add it to the existing GPIO docs.
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