Generating an accurate PWM signal using the device internal oscillator is an important feature for many cost-sensitive applications in which an external crystal is not desired. The MSP430FR4133 uses an on-chip 32-kHz RC oscillator (REFO) combined with the 16-MHz digitally controlled oscillator (DCO) with frequency-locked loop (FLL) to provide the clock source for the timer peripheral to generate the PWM. The REFO frequency may change across different temperatures. To achieve improved PWM accuracy, application software may periodically measure the device temperature and compute an appropriate timer capture/compare correction value to offset for REFO temperature drift. For more information on how to implement this algorithm refer to How to Achieve Higher Accuracy Timer with Internal Oscillator on MSP430 . Figure 10-6 shows the absolute value of a typical error percentage for a 44-kHz PWM signal over the temperature range.

The absolute value error percentages shown below can be interpreted as either positive or negative resulting in a slightly faster or slower PWM frequency.

Figure 10-6 Calibrated 44-kHz Timer PWM Error Magnitude