IC PIC16F74
When Timer1 is running in Asynchronous mode and then disabled, data in the Timer1 register (TMR1) may become corrupted. Corruption occurs when the timer enable is turned off at the same instant that a ripple carry occurs in the timer This issue only occurs in asynchronous operation. In synchronous operation, the relevant signals are latched with the CPU clock and the problem condition does not arise.
When Timer1 is configured to operate as an asynchronous counter, care must be taken that there is no incoming pulse while the module is being turned off. If an incoming pulse arrives while Timer1 is being turned off, the value of register TMR1 may become corrupted.
Diagram Block PIC16F74
When resonators above 2 MHz are used, the HS mode oscillator is required to ensure reliable operation. HS mode oscillator drive at frequencies from 2 MHz to 4 MHz is often excessive, resulting in the amplitude of the oscillator waveform exceeding VDD and VSS. In such cases, the waveform may experience distortion as ESD protection devices begin to operate on the OSC1 and OSC2 pins. This distortion appears as a non-sinusoidal waveform or clipping, and can generate substantial harmonics that may create excessive noise in the application.