I planning on having 8x seven segment displays driving off a single 7219. The vcc is 5v. I was wondering how does the 7219 divide the power amongst the displays? Not decided yet but I probably only want 5 ma for each segment so 40ma for each seven segment display. The datasheet doesn't explain it in a way I understand.
Is it just usual way of working out a resistor for an led ? if so how is it able to guarantee the same amount for every one of them
The MAX7219 is a constant current source.
It uses the resistor connected between \$V_{cc}\$ and \$I_{set}\$ to set the current delivered to each LED.
It also multiplexes the 8 digits, so only 1 digit is "alive" at any one time.
Thus, the maximum current draw by a display, no matter how many digits you have, will be \$I_s \times 8\$ (8 is 7 segments plus the decimal point, \$I_s\$ is the current of a single segment).
The datasheet gives a very handy table with typical values for the resistor depending on the forward current and the voltage of a single segment. This all depends on the displays you are using - check their data sheet for this information.
So for a display that has a 2.5V voltage and 20mA per segment, you would use a 28KΩ resistor.
I have come across a page on the Arduino site which gives some good information about the RSET resistor.
