Datasheet
CY8C21x34/B CapSense
®
Design Guide Document No. 001-66271 Rev. *B 13
2.2 CapSense Methods in CY8C21x34/B
CY8C21x34/B devices support several CapSense methods for converting sensor capacitance (C
X
) into a digital code.
These are CapSense Sigma Delta (CSD), CSD with ADC (CSDADC), and SmartSense. These methods are
implemented in the form of PSoC Designer User Modules and are described in Section 3 of this design guide.
2.2.1 CapSense with Sigma Delta Modulator (CSD)
The CapSense Sigma-Delta (CSD) method in CY8C21x34/B devices incorporates C
X
into a switched-capacitor circuit
as shown in Figure 2-3. The sensor (C
X
) is alternately connected to V
DD
and the Analog MUX (AMUX) bus by the
underlapped switches Sw1 and Sw2, respectively. Sw1 and Sw2 are driven by a Precharge clock to generate a
current (I
SENSOR
) into the AMUX bus. The magnitude of I
SENSOR
is directly proportional to the magnitude of C
X
. The
Sigma-Delta converter samples AMUX bus voltage and generates a modulating bit stream that controls the constant
current source (I
DAC
), which charges AMUX such that the average AMUX bus voltage is maintained at V
REF
. The
sensor bleeds off the charge I
SENSOR
from the modulating capacitor (C
MOD
). C
MOD
in combination with Rbus forms a
low-pass filter that attenuates Precharge switching transients at the Sigma-Delta converter input.
Figure 2-3. CSD Block Diagram
Cx
Vdd
i
sensor
Sigma-Delta
Converter
Precharge
Clock
Cmod
Rb
High-Z
input
Sw1
Sw2
CY8C21x34/B
Gnd
i
bleed
= External Connection
AMUX Bus
Vref
Rbus
R
inline
In maintaining the average AMUX voltage at a steady state value (V
REF
), the Sigma-Delta converter matches the
average bleed current (I
BLEED
) to I
SENSOR
by controlling the bit stream duty cycle. The Sigma-Delta converter stores
the bit stream over the duration of a sensor scan and the accumulated result is a digital output value, known as raw
counts, that is directly proportional to C
x
. This raw count is interpreted by high-level algorithms to resolve sensor
state. Figure 2-4 plots the CSD raw counts from a number of consecutive scans during which the sensor is touched
and then released by a finger. As explained in CapSense Fundamentals, the finger touch causes C
x
to increase by
C
F,
which in turn causes raw counts to increase proportionally. By comparing the shift in steady state raw count level
to a predetermined threshold, the high-level algorithms can determine whether the sensor is in an On (Touch) or Off
(No Touch) state.