Extra microcontroller to pulse the PWM up and down in sinus

Hi. I am “New” here.
I have been investigated long time how to make the MEGA2560 to be fast without interrupt the other I/O to become slower. I have found that I need an very small microcontroller that just read which button I have pressed With Logic functions. I need just one PWM output. This microcontroller shall just run constant to read inputs if it is activated to make the output pin to give the PWM signal.

One PWM, 4 output to detect which LED shall blink in wave, just one at each time.

I have solved my issues with programming. In the first test, I will buy much of shift registers and connect stepper motors to determine how many stepper motor it can run. With for example 200 shift registers divded by ( 54 pins in MEGA2560 / 3 pins ) thus 11 shift register each 3 pins, but I will priority such as some stepper motor needs to update faster and faster update rate in some situations and some isn’t in use in specific situations; so, in some situations I will add logic or something in 2 pins to switch which shift register I will use in specific situations.

I will trying to building home cockpit simulator.

I might do not need help; I found an sketch example in the Arduino 1.6.6 which with a little modification in this code, I can achieve it!

Thankful to those who clicked in this topic.