MAX7219, Arudino Uno, and 42 LEDs

I'm making a digital clock (HH MM SS). Each digit will have 7 LEDs, rather than an actual 7-segment display. I have a few novice questions. First of all, take a look at my circuit diagram and see if everything looks okay.

Questions: - What size should the decoupling capacitors be? - From the MAX7219 datasheet, Rset should be between 22-24 kohms. (The LED's are 3.2V, 20mA) Can I alter Rset to affect the LED brightness? - Since Rset controls the current to the LEDs, do I even need a resistor on each LED? - I'm using the Arduino Uno. Which output pins should I use for CLK, Din, and LOAD(CS)? - If I wanted to put 2 LEDs on each segment (instead of 1), how would that affect Rset? And would the Arduino and the 7219 be able to handle it?

Thanks for your help!

What size should the decoupling capacitors be?

0.1 uF ceramic

Can I alter Rset to affect the LED brightness?

Yes

do I even need a resistor on each LED?

No

Which output pins should I use for CLK, Din, and LOAD(CS)?

There is a library for driving this chip so look at that and it will tell you. Hint, it's the SPI lines.

If I wanted to put 2 LEDs on each segment

Could be tricky if they are not red, best bet is to put them in series so Rset would be the same.

Thanks for the response Mike!

Could be tricky if they are not red, best bet is to put them in series so Rset would be the same.

I'm using white LEDs. Ultra bright. I'll do some experimenting and see how well it works with multiples.

I'm using white LEDs

The problem with this is that white LEDs need about 4V to turn them on. So two in series will not work if you are only powering them off 5V. You need to up the drive voltage. Wiring in parallel is tricky because LEDs will not share the current. If you do it this way then it is back to putting a resistor in series with each LED.

Thanks again Mike, you're a great resource!

The 7219's maximum operating voltage is 5.5V. So if I'm understanding correctly, the only way to make two LEDs work is to wire them in parallel, each with a resistor.

So hopefully just one will be sufficient.

I have a MAX7221 doing something similar - I have 8 digits, with each segment made of 3 high brightness LEDs wired in parallel - no resistors at all. The MAX7221 drives it just fine. Because they are so bright, I have the current set pretty low, my notes say 150K but I can’t get at the part to see what I actually have installed without some disassembly.
I think the difference between the 2 is just the serial interface.
The part is multiplexing the segments turned on at a pretty high rate.