VBAT and 32.76kHz oscillator doubt

[gabrielemarocco]

I don't understand why while the TWR-K20D50 board is powered by USB (J36 is open) and no battery is inserted, the demo program works only if J35 is closed.

From the datasheet of the microcontoller I have understood that Vbat powers only the RTC clock but the test on the evaluation board shows the opposite: is Vbat necessary for the system (and not only for the RTC module) in order to work properly?

What about the 32.768kHz crystal? Is it necessary if I don't want to power the RTC oscillator and if a 8MHz resonator feeds the system clock?

[luispuebla · Accepted solution]

Hello Gabriele,

      K20D50 does not require VBAT to operate and you can use only the 8MHz crystal/resonator when RTC is not used,  but TWR-K20D50M Demo software is using the RTC module,  and any attempt to access an RTC registers  when VBAT is powered down,  will result in a bus error and causing the MCU to reset.  If you have a terminal connected to the TWR-K20D50M you will see this message when VBAT is disconnected ****default_isr entered on vector 3*****.