How to choose the capacitors of the Crystal
Have you ever experienced something like this? Once you design MCU circuit and then you test, it happens that it doesn’t work! It doesn’t work even you design with correct voltage. It has no responses. Of course, I experienced it before. At that time, I was absolutely confused – didn’t know why it didn’t work (why the circuit didn’t oscillate frequency). I was thinking in my head “Why? Why? Why? Why it didn’t work even I designed based on reference.” Finally, I found out that the capacitor that I chose was not correct due to I used new crystal of which load capacitance figure was not the same.
Below equation will show you primary basis of how to calculate crystal figure.
Ccrystal = ( (Cx x Cy) / (Cx + Cy) ) + Cs
C stray (Cs) = 2-5 pF
Ccrystal Load capacitance of crystal
Cx and Cy are designed to be equal. Thus, the equation is as following.
Cx = Cy = 2(Ccrystal – Cs)
See! The crystal figure calculation is not that difficult at all. So, to choose crystal properly, we have to consider not only frequency but also load capacitance of that crystal too.