Microchip Technology AC244055 Data Sheet

 2011-2013 Microchip Technology Inc.

DS40001574C-page 317

The CPS module can monitor up to 16 inputs. The
capacitive sensing inputs are defined as CPS<15:0>.
To determine if a frequency change has occurred the
user must:
• Select the appropriate CPS pin by setting the 

CPSCH<3:0> bits of the CPSCON1 register.

• Set the corresponding ANSEL bit.
• Set the corresponding TRIS bit.
• Run the software algorithm.
Selection of the CPSx pin while the module is enabled
will cause the capacitive sensing oscillator to be on the
CPSx pin. Failure to set the corresponding ANSEL and
TRIS bits can cause the capacitive sensing oscillator to
stop, leading to false frequency readings.

The capacitive sensing oscillator consists of a constant
current source and a constant current sink, to produce
a triangle waveform. The CPSOUT bit of the
CPSCON0 register shows the status of the capacitive
sensing oscillator, whether it is a sinking or sourcing
current. The oscillator is designed to drive a capacitive
load (single PCB pad) and at the same time, be a clock
source to either Timer0 or Timer1. The oscillator has
several different current settings as defined by
CPSRNG<1:0> of the CPSCON0 register. The different
current settings for the oscillator serve two purposes: 
• Maximize the number of counts in a timer for a 

fixed time base.

• Maximize the count differential in the timer during 

a change in frequency.

The capacitive sensing oscillator uses voltage refer-
ences to provide two voltage thresholds for oscillation.
The upper voltage threshold is referred to as Ref+ and
the lower voltage threshold is referred to as Ref-.
The user can elect to use fixed voltage references,
which are internal to the capacitive sensing oscillator,
or variable voltage references, which are supplied by
the Fixed Voltage Reference (FVR) module and the
Digital-to-Analog Converter (DAC) module.
When the fixed voltage references are used, the V

SS

voltage determines the lower threshold level (Ref-) and
the V

DD

 voltage determines the upper threshold level

(Ref+).
When the variable voltage references are used, the
DAC voltage determines the lower threshold level
(Ref-) and the FVR voltage determines the upper
threshold level (Ref+). An advantage of using these ref-
erence sources is that oscillation frequency remains
constant with changes in V

DD

.

Different oscillation frequencies can be obtained
through the use of these variable voltage references.
The more the upper voltage reference level is lowered
and the more the lower voltage reference level is
raised, the higher the capacitive sensing oscillator fre-
quency becomes.
Selection between the voltage references is controlled
by the CPSRM bit of the CPSCON0 register. Setting
this bit selects the variable voltage references and
clearing this bit selects the fixed voltage references.
Please see 

 for more information on config-

uring the variable voltage levels.