"The PCA takes time to switch"? How so? What cpu are you using? How does your code operate? Do you write in the correct sequence to the PCA registers?

You need to get some hard evidence of what is happening. Does it happen at the same point everytime? Have you timed your code?

There are many things that maybe causing the problem you mention - unfortunately, unless someone is doing the exact same thing as you are - how are we to know?

You can use your sound card to capture the signal - find the program cooledit2000 it will be able to record the whole sequence and allow you to look carefully at what is happening.