Workflow

1.
Initialize AST to generate periodic event 0. see sam/drivers/events/example1 for more detail:
```
init_ast();
```
2.
Initialize the event module and enable it:
```
init_events();
```
3.
Initialize PDCA channel 0 to transfer data to USART. see sam/drivers/events/example1 for more detail:
```
init_pdca();
```