22.5.8 Waveform Generation

Waveforms can be generated by the RTC in order to take advantage of the RTC inherent prescalers while the RTC is the only powered circuitry (Low-power mode of operation, Backup mode) or in any active mode. Going into Backup or Low-power operating modes does not affect the waveform generation outputs.

The RTC outputs (RTCOUT0 and RTCOUT1) have a source driver selected among seven possibilities.

The first selection choice sticks the associated output at 0. This is the reset value and it can be used at any time to disable the waveform generation.

Selection choices 1 to 4 select 1 Hz, 32 Hz, 64 Hz and 512 Hz, respectively.

64 Hz can drive, for example, a twisted nematic (TN) LCD backplane signal while 1 Hz can be used to drive a blinking character like “:” for basic time display (hour, minute) on TN LCDs.

Selection choice 5 provides a toggling signal when the RTC alarm is reached.

Selection choice 6 provides a copy of the alarm flag, so the associated output is set high (logical 1) when an alarm occurs and immediately cleared when software clears the alarm interrupt source.

Selection choice 7 provides a duty cycle programmable pulse that can be used to drive external devices for power consumption reduction or any other purpose.

PIO lines associated to RTC outputs automatically select these waveforms as soon as RTC_MR corresponding fields RTCOUT0 and RTCOUT1 differ from 0 .