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Figure 23.2. 20 mV Hysteresis Selected
In POS
In NEG + 20m V
In NEG
Tim e
In NEG -20m V
ACMPOUT without hysteresis
ACMPOUT with hysteresis
23.3.4 Input Selection
The POSSEL and NEGSEL fields in ACMPn_INPUTSEL controls which signals are connected to the
two inputs of the comparator. 8 external pins are available for both the negative and positive input. For
the negative input, 3 additional internal reference sources are available; 1.25 V bandgap, 2.5V bandgap
and V DD . The V DD reference can be scaled by a configurable factor, which is set in VDDLEVEL (in
ACMPn_INPUTSEL) according to the following formula:
V DD Scaled
V DD_SCALED = V DD ×VDDLEVEL/63
(23.1)
A low power reference mode can be enabled by setting the LPREF bit in ACMPn_INPUTSEL. In this
mode, the power consumption in the reference buffer (V DD and bandgap) is lowered at the cost of
accuracy. Low power mode will only save power if V DD with VDDLEVEL higher than 0 or a bandgap
reference is selected.
Normally the analog comparator input mux is disabled when the EN (in ACMPn_CTRL) bit is set low.
However if the MUXEN bit in ACMPn_CTRL is set, the mux is enabled regardless of the EN bit. This will
minimize kickback noise on the mux inputs when the EN bit is toggled.
23.3.5 Capacitive Sense Mode
The analog comparator includes specialized hardware for capacitive sensing of passive push buttons.
Such buttons are traces on PCB laid out in a way that creates a parasitic capacitor between the button
and the ground node. Because a human finger will have a small intrinsic capacitance to ground, the
capacitance of the button will increase when the button is touched. The capacitance is measured by
including the capacitor in a free-running RC oscillator (see Figure 23.3 (p. 326) ). The frequency
produced will decrease when the button is touched compared to when it is not touched. By measuring
the output frequency with a timer (e.g. through PRS), the change in capacitance can be calculated.
The analog comparator contains a complete feedback loop including an optional internal resistor.
This resistor is enabled by setting the CSRESEN bit in ACMPn_INPUTSEL. The resistance can
be set to one of four values by configuring the CSRESSEL bits in ACMPn_INPUTSEL. If the
internal resistor is not enabled, the circuit will be open. The capacitive sense mode is enabled by
setting the NEGSEL field in ACMPn_INPUTSEL to 1011. The input pin is selected through the
POSSEL bits in ACMPn_INPUTSEL. The scaled V DD in Figure 23.3 (p. 326) can be altered by
configuring the VDDLEVEL in ACMPn_INPUTSEL. It is recommended to set the hysteresis (HYSTSEL
in ACMPn_CTRL) higher than the lowest level when using the analog comparator in capacitive sense
mode.
2011-04-12 - d0001_Rev1.10
325
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