Hi,
Hope someone can help me. I've designed a small PCB that drives a Max7219 from an arduino/AVR 329p The arduino gets commands via I2c and is powered using the same cable, and it all works just fine. But before anyone asks for a schematic, I've checked this with an Uno and Max7219 using a bread board, ie signal pins connected to Max as required.(but no connection to vcc of max, just GND)

This issue is this. I wanted to power the Max7219 via an external 5v source.However if any of the serial lines are raised, ie CLK, LOAD etc, the VCC pin on the Max, shows 3.9v output, but its supposed to be an input! What I think I'm seeing is the power from the ardiuno signal pins is connecting to what should be VCC in.

Do I need to put some sort of diode in place to prevent this from happening? I'm hoping when I supply 5v to the VCC pin it doesn't find its way back to the signalling pins and that's my concern.

Any ideas why I'm seeing these voltages?

regards

 

The '7219 will have input protection diodes and if there are input voltages without the chip being powered then the input voltages will be seen on the chip VCC.

 

So it's to be expected and I have nothing to worry about?

regards

 

It happens with most chips but it isn't a good idea to have inputs without chip power. From the datasheet, the DIN, CLK, LOAD, and CS pins can be taken up to 6V without chip power but the other pins should not be higher than VCC.

 

Thanks, appreciate your input.

regards