Cypress PSoC 4000 Series - 17.5 CapSense CSD Shielding

PSoC 4000 Family: PSoC 4 Architecture TRM, Document No. 001-89309 Rev. *D 133
CapSense
17.5 CapSense CSD Shielding
PSoC 4 CapSense supports shield electrodes for waterproofing and proximity sensing. For waterproofing, the shield elec-
trode is always kept at the same potential as the sensors. PSoC 4 CapSense has a shielding circuit that drives the shield
electrode with a replica of the sensor switching signal (see GPIO Cell Capacitance to Current Converter on page 129) to nul-
lify the potential difference between sensors and the shield electrode. See the PSoC 4 CapSense Design Guide to under-
stand the basics of shielding.
In the sensing circuit, the sigma delta converter keeps the AMUXBUS A at V
REF
(see Sigma Delta Converter on page 131).
The GPIO cells generate the sensor waveforms by switching the sensor between AMUXBUS A and a supply rail (either V
DD
or ground, depending on the configuration). The shielding circuit works in a similar way; AMUXBUS B is always kept at V
REF
.
The GPIO cell switches the shield between AMUXBUS B and a supply rail (either V
DDD
or ground, the same configuration as
the sensor). This process generates a replica of the sensor switching waveform on the shield electrode.
Depending on how AMUXBUS B is maintained at V
REF
, two different configurations are possible.
■ Shield driving using V
REF
buffer: In this configuration, a voltage buffer is used to drive AMUXBUS B to V
REF
, as
Figure 17-9 shows. An external C
SH_TANK
capacitor is recommended to reduce switching transients. Setting the
REBUF_OUTSEL bit in the CSD_CONFIG register connects the buffer output to AMUXBUS B. The REFBUF_DRV bit
field in the same register can be used to set the drive strength of the buffer. Writing a '0' to this field disables the buffer;
writing 1, 2, and 3 selects the low, mid, and high-current drive modes respectively.
Figure 17-9. Shield Driving Using V
REF
Buffer
■ Shield driving using GPIO cell precharge: This configuration requires an external C
SH_TANK
capacitor, as Figure 17-10
shows. A special GPIO cell and a reference comparator is used to charge the C
SH_TANK
capacitor and hence the AMUX-
BUS B to V
REF
. The reference comparator always monitors the voltage on the CSH_TANK capacitor and controls the
GPIO cell switch to keep the voltage at V
REF
. The reference comparator connects to the C
SH_TANK
capacitor using a ded-
icated sense line known as Channel 2 sensing line, as Figure 17-10 shows.
GPIO
Cell
GPIO Pin
GPIO Pin
V
REF
Shield Tank
Capacitor
(optional)
Shield Electrode
(1.2V)
AMUXBUS B
(Always kept at V
REF
)
V
REF
Buffer
C
SH_TANK
C
SHIELD
Shield
electrode
capacitance