• http://www.farnell.com/datasheets/2014285.pdf

The Event System consists of several channels which route the internal events from peripherals (generators) to other internal peripherals or IO pins (users).

Initialisation

The datasheet suggests the following sequence:

1. Enable output in event generator peripheral. E.g. RTC.EVCTRL: the RTC can generate events on overflow, comparison, and periodic intervals.

2. Then confgure EVSYS: from the above diagram we see we need to configure CHANNEL[0-11].EVGEN and USER[0-41].CHANNEL.

   The CHANNEL[0-11] register can be used to set things like whether the channel runs in sleep mode, edge detection selection, path selection, event generator etc. The event generator bits choose which event generator to connect to the selected channel. For example, where we to use the RTC overflow to channel 0 we would set CHANNEL0.EVGEN to the value 0x03.

   USER[0-14].CHANNEL tells us to which event user the channel output should go. I.e. setting USER0.CHANNEL dictates which channel is sent to user 0.

3. Configure the event user to use the event via the EVCTRL.EVACT in said user's registers. E.g., could count events by getting TC to be an event user via its TC.EVCTRL.EVACT register.
4. Enable the event in the user: TC.EVCTRL.STARTEI.

Channel Paths

Asynchronous:

• Event from generator sent to user without event system interaction.
• No clock latency. Routing latency device dependent.
• Cannot generate interrupts.

Synchronous:

• Event generator and channel must share same generator for generic clock.
• Can generate interrupts.
• Routing latency is one GCLK_EVSYS_CHANNEL_n clock cycle.

Resynchonized:

• Event generator and channel do not share same generator for generic clock.
• Routing latency is three GCLK_EVSYS_CHANNEL_n clock cycles.