Cypress PSoC 4000 Series - CMOD Precharge

134 PSoC 4000 Family: PSoC 4 Architecture TRM, Document No. 001-89309 Rev. *D
CapSense
Figure 17-10. Shield Driving Using GPIO Precharge
This GPIO cell precharge capability is available only on a
fixed C
SH_TANK
pin. See the device pinout in the device
datasheet for details.
COMP_MODE bit in the CSD_CONFIG register selects
between the reference buffer precharge and GPIO pre-
charge; 0: reference buffer precharge, 1: GPIO precharge.
17.5.1 C
MOD
Precharge
When the CapSense hardware is enabled for the first time,
the voltage across C
MOD
starts at zero. Then the sigma
delta converter slowly charges the C
MOD
to V
REF
. The
charging current is supplied by the IDACs in the IDAC sourc-
ing mode and by the sensor switched capacitance circuit in
the IDAC sinking mode. However, this is a slow process
because C
MOD
is a relatively large capacitor.
Precharging of C
MOD
is the process of quickly initializing the
voltage across C
MOD
to V
REF
. Precharging reduces the time
required for the sigma delta converter to start its operation.
There are two options for precharging C
MOD
.
■ Precharge using V
REF
buffer: When the shield is
enabled, the V
REF
buffer output is always connected to
AMUXBUS B (Figure 17-9). To precharge using the
V
REF
buffer, C
MOD
is initially connected to AMUXBUS B.
After the precharging process, C
MOD
is connected to
AMUXBUS A for normal sigma delta operation. When
the shield is disabled, the V
REF
buffer output is always
connected to AMUXBUS A for precharging and discon-
nected afterwards.
■ Precharge using GPIO cell: In this configuration, a spe-
cial GPIO cell and a reference comparator is used to
charge the C
MOD
capacitor to V
REF
. This GPIO cell pre-
charge capability is available only on a fixed C
MOD
pin.
See the pinout in the device datasheet for details. The
comparator used for this purpose is the same reference
comparator used for CSH_TANK precharge.
COMP_PIN bit in the CSD_CONFIG register is used to
select which capacitor is connected to the reference
comparator. If this bit is 0, the sense line designated as
"Channel 1" is used to connect C
MOD
to the reference
comparator as Figure 17-11 shows; if this bit is 1, Chan-
nel 2 sense line is used to connect CSH_TANK to the
reference comparator, as Figure 17-10 shows. Note that
the GPIO cells must be configured properly for the GPIO
cell precharge to work.
Figure 17-11. GPIO Cell Precharge
Precharge using a GPIO cell is faster than using the
V
REF
buffer. Therefore, GPIO precharge is the recom-
mended precharge configuration. However, if you do not
need a fast initialization of CapSense, use the V
REF
buf-
fer precharge.
The Channel 1 sense line can also be used to connect
C
MOD
to the sensing comparator in the sigma delta mod-
ulator. Setting the SENSE_INSEL bit in the
CSD_CONFIG register to '1' enables this option. Clear-
ing this bit connects C
MOD
to the sensing comparator
using AMUXBUS A.
GPIO
Cell
GPIO Pin
C
SH_TANK
Pin
Shield Tank
Capacitor
Shield Electrode
C
SH_TANK
C
SHIELD
Shield
electrode
capacitance
AMUXBUS B
(Always kept at V
REF
)
GPIO cell switch
V
DD
V
REF
(1.2V)
Reference
Comparator
Channel 2 sensing line
C
MOD
Pin
C
MOD
AMUXBUS A
(Always kept at V
REF
)
GPIO cell switch
V
REF
(1.2V)
Reference
Comparator
Channel 1 sensing line
V
DD