Thanks for your thoughts as always!
Antenna effect is possible, I'll try the caps. The response can't be too
slow since the pots are driven by DC motors which position the sliders.
The slider positions are monitored and the motors are stopped when the
correct position is reached. Still, response on the order of 100msec
would be ok.
The I2c has a 10K pullup on each motor driver. So with 6 boards, the
pull up should be about 1.5K. I can try a stronger pull up, but I'm not
sure it will help.
On 3/9/2015 2:10 PM, Jerry Biehler wrote:
If they relied on the internal pull up in the uC it may not be good. There are
some white paper out there and it basically said the internal pullup are not
good for i2c, FWIW.
-Jerry
On Mar 9, 2015, at 2:07 PM, Greg Peek <[email protected]> wrote:
Have you looked at the signal quality of the I2C (working with 6 boards) with a
~100MHz scope? 6+ loads on i2c needs a stronger pull-up than 10K.
I like the antenna idea. Maybe try shielded cables to the POTs, or "big" caps.
Who cares how slow the pot voltage changes, right?
Input leakage would explain the unconnected analog pins going to Vcc. Does a
100K to ground pull them low (just as a test).
On Mon, Mar 9, 2015 at 1:32 PM, Jerry Biehler <[email protected]> wrote:
Might put some small caps between the pot signal wire and the ground to kill
noise. They might be acting like giant antennas.
You also could try stronger pull-ups on the i2c.
-Jerry
On 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). One end of each pot is tied to the 5V
rail; the other to ground. The pots are located 3 to 8 feet from the Arduino. I
have a header for the power connections and another for the grounds. A wire
from each header connects them to the 5V and GND connections on the Mega. There
are 6 motor driver shields (ADFruit Motor Shield V2) stacked on the Mega and
the header connections actually plug into the top one of these. My problem is
that connecting the headers to the Mega with all the pots connected to the
headers causes the motor shields to fail initialization. When I connect the
headers with only 6 (maybe 7) pots connected, works fine. If I connect the last
one or two pots, it fails. If the two headers are not connected, all is well.
Motor boards initialize just fine. If I connect even the ground header(!), the
initialization fails. By fails, I mean that somewher
e
in the initialization of the very first board, the driver code hangs. I have
not yet tried to find exactly where the driver code hangs - I don't think
that's where the problem lurks.
Voltages measure just fine. That is, the Arduino rails measure 4.99V between
them with the headers connected or not connected. Using an external supply to
drive the Arduino doesn't change anything. I'm considering an independent 5V
supply for the pots, but I'm not convinced that will solve the problem (since I
don't know what the problem is.)
Resistance between the headers measures about 1.3K - what you'd expect for 8,
10K resistors in series.
One possible clue: the motor shields use I2C for all control. That could be my
"canary".
Something that bothers me: There is a measurable voltage on the headers when
they are not connected to power and ground. (4.99V). Since the wires only
connect to each end of the pots and should not be connected to the circuitry
otherwise, I don't know why this voltage should exist. The wipers of the pots
are connected to the Analog pins of the Arduino Mega. I am assuming that these
are configured as inputs by Arduino and should not be sourcing any voltage,
right? So where is the voltage coming from? Is it induced by voltages elsewhere
in the system? The resistance measurement does not indicate a short anywhere.
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