Product Specs

15 Peripheral interface
Page
70
15.6 Events
Events are used to notify peripherals and the CPU about events that have happened, for example, a state
change in a peripheral. A peripheral may generate multiple events with each event having a separate
register in that peripheral’s event register group.
An event is generated when the peripheral itself toggles the corresponding event signal, and the event
register is updated to reflect that the event has been generated. See Figure 10: Tasks, events, shortcuts,
and interrupts on page 68. An event register is only cleared when firmware writes a '0' to it.
Events can be generated by the peripheral even when the event register is set to '1'.
15.7 Shortcuts
A shortcut is a direct connection between an event and a task within the same peripheral. If a shortcut is
enabled, its associated task is automatically triggered when its associated event is generated.
Using a shortcut is the equivalent to making the same connection outside the peripheral and through the
PPI. However, the propagation delay through the shortcut is usually shorter than the propagation delay
through the PPI.
Shortcuts are predefined, which means their connections cannot be configured by firmware. Each shortcut
can be individually enabled or disabled through the shortcut register, one bit per shortcut, giving a maximum
of 32 shortcuts for each peripheral.
15.8 Interrupts
All peripherals support interrupts. Interrupts are generated by events.
A peripheral only occupies one interrupt, and the interrupt number follows the peripheral ID. For example, the
peripheral with ID=4 is connected to interrupt number 4 in the Nested Vectored Interrupt Controller (NVIC).
Using the INTEN, INTENSET and INTENCLR registers, every event generated by a peripheral can be
configured to generate that peripheral’s interrupt. Multiple events can be enabled to generate interrupts
simultaneously. To resolve the correct interrupt source, the event registers in the event group of peripheral
registers will indicate the source.
Some peripherals implement only INTENSET and INTENCLR, and the INTEN register is not available
on those peripherals. Refer to the individual chapters for details. In all cases, however, reading back the
INTENSET or INTENCLR register returns the same information as in INTEN.
Each event implemented in the peripheral is associated with a specific bit position in the INTEN, INTENSET
and INTENCLR registers.
The relationship between tasks, events, shortcuts, and interrupts is shown in Figure 10: Tasks, events,
shortcuts, and interrupts on page 68.
15.8.1 Interrupt clearing
When clearing an interrupt by writing "0" to an event register, or disabling an interrupt using the INTENCLR
register, it can take up to four CPU clock cycles to take effect. This means that an interrupt may reoccur
immediatelly even if a new event has not come, if the program exits an interrupt handler after the interrupt is
cleared or disabled, but before four clock cycles have passed.
Important: To avoid an interrupt reoccurring before a new event has come, the program should
perform a read from one of the peripheral registers, for example, the event register that has been
cleared, or the INTENCLR register that has been used to disable the interrupt.
This will cause a one to three-cycle delay and ensure the interrupt is cleared before exiting the interrupt
handler. Care should be taken to ensure the compiler does not remove the read operation as an
optimization. If the program can guarantee a four-cycle delay after event clear or interrupt disable another
way, then a read of a register is not required.