As promised, here's the exciting conclusion:
Arduino Uno uses A4 and A5 for SCL and SCA. On new Arduino Megas (since
R3, I think), SCL and SDA come out on separate pins; not A4 and A5. So
on the Mega, A4 and A5 should be available. Without really admitting it,
the Adafruit MotorShield *connects* the new SCL and SDA pins to A4 and
A5. Since SDA and SCL are both pulled up to 5V, whenever I connected pot
wipers to A4 or A5 (which then went to ground), the SDA or SCL lines
were no longer pulled up properly and so the I2C interface failed.
The documentation from Adafruit should make this routing clear! Instead
they hem and haw around it so you can't really tell what's happening.
Even their schematic just shows a dotted line connecting A4 and A5 to
SDA and SCL.
Beware the undocumented "feature"!
On 3/10/2015 3:17 PM, Jim Larson wrote:
Thanks for looking at this Nathan! You have the correct driver board.
However, the "A0" thru "A5" on the schematic refers to the pins that
set the I2C Address for the board, not to Analog pins on the Arduino.
Just a bit confusing! The only connection to Arduino (besides power
and ground) is the I2C bus on SCA and SCL. (It is also possible to
drive two servos, but I'm not doing that.)
On 3/10/2015 2:15 PM, Nathan McCorkle wrote:
On Mon, Mar 9, 2015 at 1:24 PM, Jim Larson <[email protected]> wrote:
Following is a description of a problem I'm having in a motor
control system
I'm working on down here in Sunny San Diego. Any thoughts or
insights would
be most appreciated.
I have 8, 10K linear pots. The wiper of each pot is tied to an
analog input
of an Arduino Mega (Analog pins 0 to 7).
Hmm, I think I'm looking at the correct motor shield schematic here:
https://learn.adafruit.com/assets/9536
If so, it seems that has 5 of the analog pins tied up... even if they
aren't being used on the motor shield in your setup (I can't tell on
cursory glance) maybe they aren't turned off (if they can even be
disabled).
If I'm right and that is the correct schematic... you could either
destructively cut the traces coming from the analog pins, or get some
headers between the Mega and the motor shield stack, and for the
analog pins instead of upright headers, use 90-degree angle headers so
the analog are completely diverted from the motor shields.
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