CapSense Design Tools
CY8C20xx7/S CapSense
®
Design Guide
Doc. No. 001-78329 Rev. *E
30
3.4.2.4 Scanning Speed
This parameter sets the sensor scanning speed by setting the clock input to the
Although a
faster scanning speed provides a good response time, slower scanning speeds give the following advantages:
Improved SNR
Better immunity to power supply and temperature changes
Less demand for system interrupt latency; you can handle longer interrupts
Possible values are Ultra Fast, Fast, Normal, and Slow.
3.4.2.5 Shield Electrode Out
A shield electrode is used to reduce parasitic capacitance and achieve liquid tolerance. Refer to
in this document and the
“Shield Electrode and Guard Sensor” section in
for more details on shield electrode. This parameter selects where to route the output of the shield
electrode.
Possible values are P0[0], P1[2], P0[2], P2[2], and P2[4].
3.4.2.6 Precharge Source
This parameter selects the clock source for precharge switches, referred to as precharge clock in the
Possible values are PRS and Prescaler. The selection of Prescaler sets the switching clock frequency as
IMO/prescaler. The selection of PRS passes the divided IMO clock through a pseudo-random generator, providing a
spread-spectrum clock. Use the PRS source in most cases to get better EMI immunity and lower emissions as PRS
averages the switching frequency over a wide range.
3.4.2.7 Prescaler
This parameter sets the prescaler ratio and determines the precharge switch output frequency. This parameter also
affects the PRS output frequency.
Possible values are 1, 2, 4, 8, 16, 32, 64, 128, and 256.
3.4.2.8 PRS Resolution
This parameter changes the PRS sequence length.
When very short scan times are needed, an 8-bit PRS must be used to avoid excessive noise. The scan time is
determined by the
(not to be confused with PRS resolution) parameter. For scan times less than or equal
to
380 μs, PRS resolution should be set to 8 bits; for scan times of greater than 380 μs, PRS resolution should be set
to 12 bits. The default setting is 8 bits.
3.4.2.9 Autocalibration
When Autocalibration is enabled, the modulation and compensation I
DACs
are automatically calibrated to establish a
raw count baseline at approximately 85 percent of 2
N
setting of the sensor. Enabling
autocalibration overrides the device editor settings of I
DAC
and I
COMP
.
When autocalibration is disabled, the raw count value depends on I
DAC
range, I
DAC
value, resolution, sensor
capacitance, IMO frequency, prescaler, precharge source, and V
REF
parameters set in the device editor.
Autocalibration consumes ROM and RAM resources and increases start time. Autocalibration does not automatically
select the I
DAC
range value. If the raw count value after calibration is less than half of the resolution range, you should
increase the I
DAC
range or reduce the precharge frequency. Autocalibration works to improve marginally functional
configurations.
3.4.2.10 I
DAC
Range
The I
DAC
Range parameter scales the I
DAC
current output. For example, selecting 8x will scale the I
DAC
output to twice
the 4x range.
Possible values are 4x and 8x.
