Datasheet

184
32133D–11/2011
UC3D
Because of synchronization, the transfer of the INSEL value will not happen immediately. When
changing/setting the INSEL value, the user must make sure that the prescaler bit number INSEL
will not have a 0-to-1 transition before the INSEL value is transferred to the register. In that case,
the first periodic interrupt after the change will not be triggered.
14.5.3.2 Alarm interrupt
The AST can also generate alarm interrupts. If the ALARMn bit in IMR is one, the AST will gen-
erate an interrupt request when the counter value matches the selected alarm value, when the
AST is enabled. The alarm value is selected by writing the value to the VALUE field in the corre-
sponding Alarm Register (ARn.VALUE).
The corresponding ALARMn bit in SR will be set when the counter reaches the selected alarm
value.
Because of synchronization, the transfer of the alarm value will not happen immediately. When
changing/setting the alarm value, the user must make sure that the counter will not count the
selected alarm value before the value is transferred to the register. In that case, the first alarm
interrupt after the change will not be triggered.
If the Clear on Alarm bit in the Control Register (CR.CAn) is one, the corresponding alarm inter-
rupt will clear the counter and set the OVF bit in the Status Register. This will generate an
overflow interrupt if the OVF bit in IMR is set.
14.5.4 AST wakeup
The AST can wake up the CPU directly, without the need to trigger an interrupt. A wakeup can
be generated when the counter overflows, when the counter reaches the selected alarm value,
or when the selected prescaler bit has a 0-to-1 transition. In this case, the CPU will continue
executing from the instruction following the sleep instruction.
The AST wakeup is enabled by writing a one to the corresponding bit in the Wake Enable Regis-
ter (WER). When the CPU wakes from sleep, the wake signal must be cleared by writing a one
to the corresponding bit in SCR to clear the internal wake signal to the sleep controller. If the
wake signal is not cleared after waking from sleep, the next sleep instruction will have no effect
because the CPU will wake immediately after this sleep instruction.
The AST wakeup can wake the CPU from any sleep mode where the source clock is active. The
AST wakeup can be configured independently of the interrupt masking.
14.5.5 Synchronization
As the prescaler and counter operate asynchronously from the user interface, the AST needs a
few clock cycles to synchronize the values written to the CR, CV, SCR, WER, PIRn and ARn
registers. The Busy bit in the Status Register (SR.BUSY) indicates that the synchronization is
ongoing. During this time, writes to these registers will be discarded and reading will return a
zero value.
Note that synchronization takes place also if the prescaler is clocked from CLK_AST.