on page 332
25.6.8.5. 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 =
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.
Related Links
on page 340
Atmel SAM L22G / L22J / L22N [DATASHEET]
Atmel-42402E-SAM L22G / L22J / L22N_Datasheet_Complete-07/2016
344