I want to experiment with a MAX7219 LED matrix driven by an UNO. Since the display could well have all LEDs on at once I plan to run it off of an external PS so I don't kill the UNO board - or underpower the display. I have one of those add-on breadboard power supplies for this purpose.
I've done some searching on this parasitic power issue and haven't found anything definitive, to me, as to how to deal with this.
Referring to the schematic, is putting resistors in the LOAD/CLK/DIN lines enough? Something else?
That is a common approach, and will certainly help. 1K to 10K are often used. The higher the resistance, the better electrical safety, but decreased performance at high signal bit rates.
Never going to happen with a LED matrix chip.
You might see all LEDs on, but only 1/8th of the LEDs is on at a given time.
Don't use resistors in series with the LEDs. That chip has internal LED current regulation.
You can reduce overall current with the a single resistor on the Iset pin (see datasheet).
From memory..., the chip will never draw more than 200mA with all LEDs on, so no problem to power it from the 5volt pin of the Arduino on USB supply.
Leo..
I went with this for the single module and it's working. I've ordered a double male USB cable so when two additional displays are added all three and the UNO can power up/down together.
I'm sorry I said the backwards.
When you turn everything on, turn on the MAX7219 first then the Arduino.
When you turn everything off, then turn the Arduino off first then the MAX7219.
Since the MAX7219 connections to the Arduino are all inputs, they won't source current back to the Arduino, so it's safe to turn it on first and off last.
You don't need the resistors if you do it this way.
The resistors will slow down the rise and fall times of the signals. This may become an issue at high clock (CLK) frequencies causing it to stop working.
