Parallella 7010 no GPIO - no Samtec Connectors

Any technical questions about the Epiphany chip and Parallella HW Platform.

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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby FHuettig » Fri Jun 13, 2014 2:50 am

tnt wrote:I'm wondering if pulldowns are better actually (does the fpga support that).


The Zynq does support pulldowns. I think the preference for using pullups is a tradition going back to the early days of TTL when the drivers were much better at pulling down than up plus the inputs would supply current (they were natural pull-ups). Those concerns don't exist with CMOS, but I think the inertia is still there. If there's a concrete reason to go the other way I'm open to it.

The internal pull-up/down on the Zynq IOB moves around 100uA.
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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby tnt » Fri Jun 13, 2014 6:18 am

Yes, the inertia is definitely still there.

My thinking on this was that you can have custom board connected to those gpios and since the IO voltage is adjustable, it might not be configured correctly yet for that expansion board when the bitstream is loaded and so a pullup would be imposing a potentially too high voltage on the external device. OTOH, 100uA is pretty weak so it's doubtful this would cause any damage ...

Another thing that would be useful for the default GPIO config is that ability to configure state change interrupt on them. Not sure if the default EMIO GPIO module allows this (or maybe it's available already in the current config, I haven't checked).
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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby shodruk » Fri Jun 13, 2014 10:06 am

Is it possible to implement GPIO breakout by soldering lead wires?
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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby tnt » Fri Jun 13, 2014 11:16 am

Sure. There is nothing between the GPIO and the connectors so that board just doesn't the connectors but the signal get to the pad.
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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby shodruk » Fri Jun 13, 2014 12:12 pm

tnt wrote:Sure. There is nothing between the GPIO and the connectors so that board just doesn't the connectors but the signal get to the pad.


Great! So, there are GPIO, eLinks, power terminal! It just lacks Samtec connectors. :D
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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby FHuettig » Fri Jun 13, 2014 4:43 pm

tnt wrote:My thinking on this was that you can have custom board connected to those gpios and since the IO voltage is adjustable, it might not be configured correctly yet for that expansion board when the bitstream is loaded and so a pullup would be imposing a potentially too high voltage on the external device. OTOH, 100uA is pretty weak so it's doubtful this would cause any damage ...


You bring up an excellent point that we need to keep in mind, safety is important in both directions! The current setting of the VDD_GPIO regulator is 2.5V (actually it defaults to 2.9V on power-up and is set to 2.5V by our u-boot), and the Zynq specification (ds187) does not allow 3.3V input levels on a 2.5V bank. So anyone making a daughtercard should be careful not apply 3.3V signals directly to the GPIO inputs. After the system boots that voltage can be changed to 3.3V if needed (or 1.8V) through the I2C interface from linux or the external card, though this will increase (or decrease!) the power used by the HDMI interface a bit if using HDMI since that is a shared IO bank. The safest thing might be to use a level translator on the external board, powered by the VDD_GPIO outputs from the Parallella, but one might also get away with some series resistors.

tnt wrote:Another thing that would be useful for the default GPIO config is that ability to configure state change interrupt on them. Not sure if the default EMIO GPIO module allows this (or maybe it's available already in the current config, I haven't checked).


That is supported by the /dev/class/gpio interface, it's a bit clunky especially if you want to drive or sample a group of GPIOs as a bus, but it does work. I'll post an example to parallella-utils soon that shows how to drive the GPIO output.
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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby 9600 » Sat Jun 14, 2014 2:42 pm

FHuettig wrote:You bring up an excellent point that we need to keep in mind, safety is important in both directions! The current setting of the VDD_GPIO regulator is 2.5V (actually it defaults to 2.9V on power-up and is set to 2.5V by our u-boot), and the Zynq specification (ds187) does not allow 3.3V input levels on a 2.5V bank. So anyone making a daughtercard should be careful not apply 3.3V signals directly to the GPIO inputs. After the system boots that voltage can be changed to 3.3V if needed (or 1.8V) through the I2C interface from linux or the external card, though this will increase (or decrease!) the power used by the HDMI interface a bit if using HDMI since that is a shared IO bank. The safest thing might be to use a level translator on the external board, powered by the VDD_GPIO outputs from the Parallella, but one might also get away with some series resistors.


Wouldn't it be far safer to default VDD_GPIO to 3.3v? I can imagine novice hardware hackers jumping in and assuming that Raspberry Pi or worse still, Arduino, projects can be trivially re-purposed.

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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby tnt » Sat Jun 14, 2014 2:56 pm

Depends safer for who ... daughter card won't be safer.
And higher voltage means higher power consumption for the HDMI.
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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby 9600 » Sun Jun 15, 2014 9:58 am

Good points.

Cheers,

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Re: Parallella 7010 no GPIO - no Samtec Connectors

Postby FHuettig » Mon Jun 16, 2014 3:57 am

Yeah, I don't think there's an obvious answer. I haven't checked thoroughly but it seems that most existing add-on cards that people might want to re-purpose (pmods, arduino, rpi, etc.) are either 3.3V or 5V, is that wrong? I just looked at a bunch of pmods and they all worked at 3.3V with a few being 5V tolerant, in one case an older rev required 5V but the new rev works at 3.3. Are there 2.5 or even 1.8V boards out there?

I can do a quick experiment to see how much extra power it takes to run the HDMI interface at 3.3, probably not a lot. However the problem is that there are a lot of Parallellas out there now that all power-up and set VDD_GPIO to 2.5V, so anything we do really needs to accept that one way or another. I don't think many people will be comfortable re-flashing their boards to start at 3.3V because of the risk of bricking them, but it would be easy to provide a simple utility to set that Voltage as needed from Linux. Seems to me the best courses of action when building add-on boards for Parallella are:

  • use level translators or otherwise set the IO voltage from the VDD_GPIO output on the PEC
  • prevent the daughter cards from applying 3.3V signals to the GPIO inputs until the Parallella gives a go-ahead (dangerous if the Parallella mistakes this card for another)
  • require series resistors to keep an add-on cards 3.3V outputs from driving too much current into the Parallella's 2.5V inputs before VDD_GPIO is set to 3.3V. The pmod spec requires this, for example.
  • for higher-performance systems the daughter cards can use LVDS (use will have to load a parallella.bit.bin file that sets-up the GPIOs as LVDS first)

Other ideas?
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