[Mezzanine] Sensors board Rev B - call for review

Grant Likely grant.likely at linaro.org
Wed Oct 21 22:54:53 UTC 2015

On 21 Oct 2015 22:06, "King, Lawrence" <lking at qti.qualcomm.com> wrote:
> Hi Grant
> Your "stackable" connector rendering shows pins on the top of the board, but to me "stackable" means you should have a female socket on the top of the board so that you can stack another 96Boards mezzanine board above this one. Realistically it is unlikely anyone will be stacking anything above this board because anything stacked on top would block access to almost all of the onboard features. Do the pins interfere with Arduino Boards stacked above?

I think you're seeing the rendering taken at an unfortunate angle.
There is a socket block on the topside of the board, and the camera is
positioned so that you can see the pin contacts inside the holes.

It may be unlikely to stack another board on top of the sensors board,
but using a stackable header does make all the signals available which
is convenient for debugging. If I later decide it is unnecessary, I
can simply replace the stackable socket with a simple pin header.

> About the 3.3V/5V selection, I believe a lot of users will prefer jumper pins over soldering,  a switch is also good. If you need to have a wired 'default' I believe the 3.3V is used a lot more often than 5V and is less likely to cause things to blow up if the user didn't check the setting.
> Silkscreen for the connectors on the front edge says "I2C" and I2C0", I think it should something like be "I2C-96B" and "I2C-ARD" you also have a vertical I2C which should also be labeled "I2C-ARD".

There are actually three I2C busses and 6 connectors on the board. 2x
I2C0 (96B), 2x I2C1 (96B) and 2x I2C (Arduino). I get your point
though. I'll adjust the naming convention to reflect the connection.

> You have two different types of level shifters TXS and TXB, any reason for this?

The TXB shifter is a push-pull type, and it is used for the UART and
SPI. The TXS is an open-drain type that can be used with I2C. I could
have used the TXS shifter for SPI and UART, it does actually have
one-shot boosters to help with push-pull signals. However, the RevA
board already uses the TXB shifter for UART which is working fine. Not
having a strong need to change it, I left it alone.

I /could/ change it though. The two parts are pin for pin compatible.

> Lawrence King  lking at qti.qualcomm.com
> Engineer, Sr. Staff/Manager
> Qualcomm Canada Inc.
> (905)482-5403 desk (x25403)
> (416)627-7302 cell
> -----Original Message-----
> From: David Mandala [mailto:david.mandala at linaro.org]
> Sent: Wednesday, October 21, 2015 10:00 AM
> To: Grant Likely <grant.likely at linaro.org>; mezzanine at lists.96boards.org
> Cc: 96boards-team <96boards-team at linaro.org>; George Grey <george.grey at linaro.org>; Koen Kooi <koen.kooi at linaro.org>; Mark Brown <mark.brown at linaro.org>; King, Lawrence <lking at qti.qualcomm.com>; Gandhi, Ketal <ketalg at qti.qualcomm.com>
> Subject: Re: Sensors board Rev B - call for review
> Grant,
> Comment below:
> On 10/21/15 8:30 AM, Grant Likely wrote:
> >>  On Tue, Oct 20, 2015 at 9:11 PM, David Mandala <david.mandala at linaro.org> wrote:
> [snip]
> >>> 3) The board has a solder bridge jumper (QS1) for selecting between
> >>> running the IO at 3.3V and 5V. 5V is the default for compatibility,
> >>> but someone who knows what they are doing can switch the solder
> >>> bridge to run at 3.3V for everything. (Most sensor devices appear to
> >>> be 3.3V these days. The ATMEGA will happily run at either 3.3V or
> >>> 5V)
> >>>
> >>> Q: Is a solder jumper the best way to select the voltage level? Or
> >>> should I put a physical switch on the board?
> >>
> >>
> >> Personally I'd use a 3 pin jumper, pin 1-2 is 3.3VDC operations, pin
> >> 2-3 5VDC operations.  My second choice would be a switch.  Last
> >> choice would be a solder bridge, you might be surprised how many
> >> people shy away from a soldering iron.
> >
> > I have a concern here. A solder jumper I'm not too worried about, but
> > I don't like the idea of routing all of the supply current through a
> > shunt. I figured I'm not the first person to deal with this, so I went
> > and took a look at what others have done. This is what I found on the
> > Seeeduino design which can be run at both 3.3V and 5V:
> >
> > http://www.seeedstudio.com/wiki/images/c/ca/Seeeduino_v4.2_sch.pdf
> >
> > The 3.3/5V selection switch is merely a signal routed to a couple of
> > mosfets which gate the power. That removes the power supply trace from
> > the jumper or switch entirely. I assume it is done that way to reduce
> > noise on the supply rails. I'm wondering if I should do the same here.
> If you have space for the parts, that makes the most sense to me. I too was worried about a small circuit board switch carrying power load, Generally jumpers have bigger chunks of copper bridging the posts, but not shunting the power makes the most sense. That makes the idea of a small circuit board switch much more attractive.
> David
> >
> > g.
> >
> --
> David Mandala <david.mandala at linaro dot org> http://www.linaro.org/ Public Key id: 45B2D952 Murphy TX, 75094 +1.972.891.8436

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