Datasheet

158
7766E–AVR–04/10
ATmega16U4/ATmega32U4
15.10.1 Fault Protection Trigger Source
The main trigger source for the Fault Protection unit is the external interrupt pin (INT0). Alterna-
tively the Analog Comparator output can be used as trigger source for the Fault Protection unit.
The Analog Comparator is selected as trigger source by setting the Fault Protection Analog
Comparator (FPAC4) bit in the Timer/Counter4 Control Register (TCCR4D). Be aware that
changing trigger source can trigger a Fault Protection mode. Therefore it is recommended to
clear the FPF4 flag after changing trigger source, setting edge detector or enabling the Fault
Protection.
Both the external interrupt pin (INT0) and the Analog Comparator output (ACO) inputs are sam-
pled using the same technique as for the T0 pin (Figure 12-1 on page 89). The edge detector is
also identical. However, when the noise canceler is enabled, additional logic is inserted before
the edge detector, which increases the delay by four system clock cycles. An Input Capture can
also be triggered by software by controlling the port of the INT0 pin.
15.10.2 Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The
noise canceler input is monitored over four samples, and all four must be equal for changing the
output that in turn is used by the edge detector.
The noise canceler is enabled by setting the Fault Protection Noise Canceler (FPNC4) bit in
Timer/Counter4 Control Register D (TCCR4D). When enabled the noise canceler introduces
additional four system clock cycles of delay from a change applied to the input. The noise can-
celer uses the system clock and is therefore not affected by the prescaler.