In PSoC 3, maximum CPU speed is 67MHz. At 3MHz, IMO has accuracy of +-1%. So, to guarantee the reliable operation, with the accuracy maximum speed should be 67MHz or less. Maximum nominal frequency based upon the PLL dividers when IMO is set to 3Mhz is 66MHz. It will give you maximum frequency of 66.66MHz when clock is +1% accurate. However, accuracy is not in user’s hand until external clock source is used.