27.6.3 External Pin Processing

Each external pin can be configured to generate an interrupt/event on edge detection (rising, falling or both edges) or level detection (high or low). The sense of external interrupt pins is configured by writing the Input Sense x bits in the Config n register (CONFIG.SENSEx). The corresponding interrupt flag (INTFLAG.EXTINT[x]) in the Interrupt Flag Status and Clear register (INTFLAG) is set when the interrupt condition is met.

When the interrupt flag has been cleared in edge-sensitive mode, INTFLAG.EXTINT[x] will only be set if a new interrupt condition is met.

In level-sensitive mode, when the interrupt has been cleared, INTFLAG.EXTINT[x] will be set immediately if the EXTINTx pin still matches the interrupt condition.

Each external pin can be filtered by a majority vote filtering, clocked by GCLK_EIC or 32KHz_LPCLK. Filtering is enabled if the bit Filter Enable x in the Configuration n register (CONFIG.FILTENx) is written to ‘1’. The majority vote filter samples the external pin three times with GCLK_EIC or 32KHz_LPCLK and outputs the value when two or more samples are equal.

When an external interrupt is configured for level detection and when filtering is disabled, detection is done asynchronously. Level detection and asynchronous edge detection does not require GCLK_EIC or 32KHz_LPCLK, but interrupt and events can still be generated.

If filtering or synchronous edge detection or debouncing is enabled, the EIC automatically requests GCLK_EIC or 32KHz_LPCLK to operate. The selection between these two clocks is done by writing the Clock Selection bits in the Control A register (CTRLA.CKSEL). GCLK_EIC must be enabled in the CRU. In these modes the external pin is sampled at the EIC clock rate, thus pulses with duration lower than two EIC clock periods may not be properly detected.

The detection latency depends on the detection mode.