24.6.8.6 Active Layer Protection

The RTC provides a means of detecting broken traces on the PCB, also known as Active Layer Protection. In this mode an RTC output signal is routed over critical components on the board and fed back to one of the RTC inputs. The input and output signals are compared and a tamper condition is detected when they do not match.

Enabling active layer protection requires the following steps:

Enable the RTC prescaler output by writing a one to the RTC Out bit in the Control B register (CTRLB.RTCOUT). The I/O pins must also be configured to correctly route the signal to the external pins.
Select the frequency of the output signal by configuring the RTC Active Layer Frequency field in the Control B register (CTRLB.ACTF).
$GCLK_RTC_OUT = \frac{CLK_RTC}{2^{CTRLB.ACTF + 1}}$
Enable one of the tamper inputs (INn) in active layer mode by writing 3 to the corresponding Input Action field in the Tamper Control register (TAMPCTRL.INnACT). When active layer protection is enabled, the value of INn is sampled on the falling edge of CLK_RTC and compared to the expected value of OUT. Therefore up to one half of a CLK_RTC period is available for propagation delay through the trace.