What is the cheapest way to light up 600 RGB LEDs with PWM?

That is usually too dim. You will also need high and low side drivers with that mux ratio (high current needed). The MAX6974/6975 will do 16 RGB LED with 1/2 mux.

A 40 pin micro could do 12 RGB LED with 1/2 mux and software PWM. Use a group of 5 pins for a pair of LEDs.No drivers needed - just resistors. Using 1/3 mux would allow 15 RGB LED, but may be too dim.

PIC16F724 would be my recommendation. The internal osc is good enough for async serial (AVR internal osc usually is not). ~$80 for 50 chips.

There are some nice drivers you can use for this: AS1119 can drive 144 LEDs with 8bit PWM on each LED austriamicrosystems.com/eng/Products/Lig... AS1115-18 can drive 64 with 16steps PWM per 8 LEDs austriamicrosystems.com/eng/Products/Lig... If you already got the software written for MAX7219/21 you can take either AS1100 AS1106 or AS1107 from austriamicrosystems. Those parts are fully dropin & software compatible but lower priced. http://www.austriamicrosystems.com/eng/Products/Lighting-Management/LED-Driver-ICs/AS1106 If you can handle the AS1119 package I think this is the best cost/performance part you can get at the moment, else on of the AS1115-18 parts 600LEDs: 10x AS1115 =40USD 5x AS1119 = 49USD 10xAS1116 = 60USD BTW: You get 3 samples for free there.

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