[Mezzanine] Sensors board Rev B - call for review

Grant Likely grant.likely at linaro.org
Wed Oct 21 13:30:40 UTC 2015

On Tue, Oct 20, 2015 at 9:11 PM, David Mandala <david.mandala at linaro.org> wrote:
> Grant,
> Nice work. :-D Comments below.
> David
> On 10/20/15 2:28 PM, Grant Likely wrote:
>> I've completed rework, layout and routing of the Rev B Sensors board,
>> and it is now ready for review. I've attached the new schematic and
>> new component placement diagram. The design files have been pushed out
>> to the "rev-b" branch on github and git.linaro.org:
>> https://git.linaro.org/people/grant.likely/96boards-sensors.git/shortlog/refs/heads/rev-b
>> It is ready for review, and very close to being ready for
>> manufacturing. However, I have some questions that I would like some
>> feedback on. Help and suggestions are greatly appreciated.
>> First, here are the things to notice on the new design:
>> - Arduino connectors have been centered and lined up on the bottom edge
>> of the board, including the SPI header. This should make it compatible
>> with more Arduino shields
>> - More grove connectors have been added as well as more 96B IO.
>>    - Bottom edge uses right-angle SMD grove connectors to avoid
>> shorting against the USB ports on the baseboard.
>>    - GPIO A-F are all level shifted
>>    - SPI has been brought out to P7
>>    - Arduino Grove connectors match naming convention of Arduino Grove
>> shield (D3-D7, A0-A2, I2C). The Arduino Grove examples should now work
>> without any changes.
>> - Grove connectors are evenly spaced on either side of the Arduino headers
>> - A CBUS connector has been added for 1.8V IO controlled from the USB
>> port. This will be an undocumented feature allowing the FTDI to be
>> used to control boot select pins on the base board, but requires the
>> signals to be manually wired up (hence I'm not going to document it -
>> It will just cause confusion)
>> - J1 added to support manufacturing test. It allows the Arduino to be
>> reset from the FTDI
>> - I2C0 & I2C1 have stronger pull-ups on the data and clock lines.
>> Questions:
>> 1) Are the Grove connectors too close together. I tried to give lots
>> of space so that labels can be easily read, but there isn't much room
>> between the mounting holes. I could have more space if I dropped two
>> Grove connectors and put more space between P14,P15,P17 and
>> P11,P10,P9.
> Space wise it looks good just as it is.  All of my grove cables fit
> internally to the connectors, and I can read the labels with no issues.

Okay. I'll go with it.

>> 2) The LS expansion connector is currently an SMD pin header on the
>> bottom side. Seeed has sourced a through-hole stackable connector that
>> I could use instead
>> Q: Is it worth replacing the SMD pin header with a 2x40 stackable header?
> Yes it's worth it, stack-able gives a Maker or Developer access to all the
> signals and power out not just what you bring out and there are 5 pins you
> don't bring out.  The cost is comparatively minimal but allows full access
> or stack-ability if needed.


> We should also make sure those through-hole stackable connectors are
> available for sale at Seeed Studios. I have at times added a stackable
> connecter with an rPi just to increase the spacing between the rPi and an
> expansion board.

I will ask Seeed about that. Shouldn't be a problem.

>> 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:


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.


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