CapSense Performance Tuning with User Modules
CY8C20xx7/S CapSense
®
Design Guide
Doc. No. 001-78329 Rev. *E
37
4.2.2 I
DAC
Range
For projects where the maximum sensor C
P
is less than 45 pF, use 4X; otherwise, use 8X.
4.2.3 Autocalibration
Autocalibration should always be set to
„Enabled‟ in CY8C20xx7/S CSD/CSDPLUS designs. The ability of the
autocalibration algorithm to successfully set the I
DAC
relies on the prescaler being set properly and C
MOD
being the
recommended size.
4.2.4 I
DAC
Value
This parameter determines the current output of I
DAC
when autocalibration is disabled. When autocalibration is
enabled, as recommended, this parameter is overridden and has no effect. When autocalibration is disabled, raising
this parameter lowers the raw count baseline and vice versa. Also, as recommended in
in
CSD/CSDPLUS User Module Low-Level Parameters
, I
DAC
value should be chosen such that raw count is 85 percent
of 2
N
.
4.2.5 Compensation I
DAC
Value
This parameter determines the current output of the Compensation I
DAC
when autocalibration is disabled. This
parameter is applicable only for the CSDPLUS UM. When autocalibration is enabled, as recommended, this
parameter is overridden and has no effect. When autocalibration is disabled, raising this parameter lowers the raw
count baseline and vice versa. If auto-calibration is disabled, use the following method to tune compensation I
DAC
value.
Initially, tune the raw counts to 85 percent of the maximum raw counts by only using the modulation I
DAC
(keep the
compensation I
DAC
at zero). Then, increase the compensation I
DAC
to 10 percent of the modulation I
DAC
value, and re-
tune the modulation I
DAC
to get 85 percent of the maximum raw counts. Note down the SNR. Repeat this procedure
with a higher value of compensation I
DAC
, until maximum SNR is achieved. Note, that a higher compensation I
DAC
value results in higher signal (see
in
CSD/CSDPLUS User Module Low-Level Parameters
for more details). Also, note that the best SNR is generally achieved with I
DAC
value = I
COMP.
value. Enabling
autocalibration also sets the I
DAC
and I
COMP
such that I
DAC
= I
COMP
and raw counts are near 85 percent of 2
N
.
4.2.6 Precharge Source
This parameter selects the sensor switching clock source. The available options are Prescaler, which uses the IMO
through a divider, or PRS, which passes the divided IMO clock through a pseudo random generator, providing a
spread-spectrum clock. PRS provides superior noise immunity and lower noise emissions and is therefore the
recommended default setting for Precharge Source. In some instances, the prescaler precharge source can provide
higher SNR. However, when using copper circuitry, this SNR improvement is usually marginal and rarely justifies
foregoing the benefits of PRS.
4.2.7 Prescaler
Prescaler is the divider applied to the IMO to develop the precharge clock. This is the most critical hardware UM
parameter for properly tuning a CSD design. Prescaler depends on the selected precharge source, IMO, and the C
P
of the sensors being scanned.
gives recommended prescaler settings based on these parameters.
Table 4-1. Prescaler Setting Based on Precharge Source, IMO, and C
P
C
P
(pF)
Precharge Source = PRS
Precharge Source = Prescaler
Prescaler
IMO = 24 MHz
Prescaler
IMO = 12 MHz
Prescaler
IMO = 6 MHz
Prescaler
IMO = 24 MHz
Prescaler
IMO = 12 MHz
Prescaler
IMO = 6 MHz
<6
1
2
1
1
7
–11
2
1
4
2
1
12
–15
2
1
4
2
1
16
–19
4
2
1
8
4
2
20
–22
4
2
1
8
4
2
23
–26
4
2
1
8
4
2
27
–30
4
2
1
8
4
2
31
–34
4
2
1
8
4
2
35
–37
8
4
2
16
8
4
