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Topic: I2C bus being "jammed" on external PSU? (on USB OK) (Read 958 times) previous topic - next topic


Hi all,

I have been working on our next detection system quite long. Overall have great experience with Arduino for about three years almost without any bigger problems.

Now I am modernising some very old precise measurement equipment.
Basic scheme of operation - reading analog sensor with voltage divider and doing some mathematical stuff with it (and printing to serial port, lcd display, and SD card).

But, even my project is almost finished (from software point of view), I have faced following huge problem:

This is NOT a software issue, when powering  from USB everything works fine !!!

When arduino is connected to +12V power supply (I have tested 3 different), all serial SPI devices doesnt work as they should.  Arduino onboard regulator seems to heat a lot, but there is very low ambient temperature (about 10 Celsius degrees) so it shouldnt affect this.

The colour SPI LCD (ST7735) displays a random data when it has to display bitmap from SD:

Otherwise when I write black to full screen it works fine:

Also rewriting small areas with lines and text works fine:

And the main problem - when trying to read/write data to SD card causes havoc on card (BTW file RSSlog.TXT created by Arduino is empty):

Here is a image how the components are wired (for illustration only, sorry fro mess around):

Any ideas what could cause this?
I am thinking about some kind of interference or jamming from pulse converters (detector needs negative 12V and negative 300V). I also added a ferrite core filter on data wires, but it seems not having some remarkable effect....

Thanks in advance ! ;)


You'll be pulling too much current I think - with 12V in the small on-board regulator
has to drop 7V for all that current and is overheating.  These devices normally cutout
but they also limit max current pulses so that when things are marginal the 5V will be
dipping when some device pulls extra current.  The LCD backlight and SDcard
together will be demanding significant current.

Suggest using a USB phone charger to power the board - that way you get 5V
directly without needing a computer or a DC-DC converter.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Might be huge current...

But, Vin pin is connected to onboard regulator - isnt it? So i have to power lcd backlight out of arduino?
I will try to use DC-DC step-down, cause instrument should be powered from 12V battery. Also I have High Voltage converter for 12V and it consumes huge current.

How much current does SD card consumes? I have a converter that provides 5V 500mA from 12V, would it be enough for MCU and SD?


How much current does SD card consumes?

There's no exact figure (it depends on the card) but it can go up to 300mA.

I have a converter that provides 5V 500mA from 12V, would it be enough for MCU and SD?

That's not really much better than the Arduino regulator.

There's a simple way to find out if current is the problem - measure it!

Watch the voltage on the +5V pin, see if it drops under load.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)


There's a simple way to find out if current is the problem - measure it!

Watch the voltage on the +5V pin, see if it drops under load.

I thought about it since first reply. So I have measured it and doesnt seems to be problem - voltage is constant at 4,94 V.
But idea inspired me that there might be peak voltage drops in very short times - e.g. in 1 microsecond or so - this will definitely affect hi-speed SPI that runs in 8 Mhz. I also looked at 5V voltage under oscilloscope, but seems linear.

I will try adding 1000 mikroF capacitor at input, maybe it will help.


OK, so revisited all components.

Added 1000 microF capacitor at Arduino power input pin. Also added 10R resistor to "lighten" the Arduino line regulator.
Still no effects.

I am slowly getting out of ideas ... :smiley-roll:

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