21.6.8.5.1 Timestamp

As part of tamper detection the RTC can capture the counter value (COUNT/CLOCK) into the TIMESTAMP register. Three CLK_RTC periods are required to detect the tampering condition and capture the value. The TIMESTAMP value can be read once the Tamper flag in the Interrupt Flag register (INTFLAG.TAMPER) is set. If the DMA Enable bit in the Control B register (CTRLB.DMAEN) is ‘1’, a DMA request will be triggered by the timestamp. In order to determine which tamper source caused a capture, the Tamper ID register (TAMPID) provides the detection status of each tamper channel and the tamper input event. A DMA transfer can then read both TIMESTAMP and TAMPID in succession.

A new timestamp value cannot be captured until the Tamper flag is cleared, either by reading the timestamp or by writing a ‘1’ to INTFLAG.TAMPER. If several tamper conditions occur in a short window before the flag is cleared, only the first timestamp may be logged. However, the detection of each tamper will still be recorded in TAMPID.

The Tamper Input Event (TAMPEVT) will always perform a timestamp capture. To capture on the external inputs (INn), the corresponding Input Action field in the Tamper Control register (TAMPCTRL.INnACT) must be written to ‘1’. If an input is set for wake functionality it does not capture the timestamp; however the Tamper flag and TAMPID will still be updated.

Note: Once the value from the TIMESTAMP register is read, the INTFLAG.TAMPER bit must be cleared. The next value from this register must be read only after the INTFLAG.TAMPER bit is set again.