NXP Semiconductors KE17Z Series, Руководство пользователя, Страница: 21 / 37

• Discharge phase: The switch ph2 controls the discharge phase, when ph1 turns off then ph2 turns on, the transmit
channel changes the voltage from V
DD5V
to -V
DD5V
, as TX drive mode is selected to output -5 - +5 V by configuring
M_MOD to 1. Receiver converts the received voltage change (Vpre-∆V) to the current I
discharge
through Rs.
As the integrated C
i
is charged/discharged by the mirrored/amplified current from the receiver, the voltage V
ci
ramps on C
i
.
When V
ci
becomes larger than the pre-setting V
p
, the comparator stops this TSI scan round. TSI can scan multiple rounds by
configuring the digital SINC filter (TSIx_SINC) to filter out the low frequency noise. Accumulated sample results are recorded the
as TSIx_DATA[TSICNT].
There are two drive modes of Transmitter.
Figure 19 shows the difference. In one switching clock cycle, when M_MODE = 1,
the voltage change value of RX terminal is ΔV. When M_MODE = 0, the voltage change value of RX terminal is 2 * ΔV, and the
charging efficiency of Ci is also twice M_MODE = 1. Setting M_MODE to 0 is recommended as it is more energy efficient.
Tx
Rx
Tx
Rx
M_MODE=0
M_MODE=1
V
pre
+
∆V
V
pre
+5V
-5
-5V
0
V
pre
+
∆V
V
pre
- ∆V
V
pre
Figure 19. TX drive mode
3.3.1 Single scan in mutual-cap mode
The digital process in mutual mode is same as self-cap mode, as shown in Figure 11 and Figure 12 . C
i
ramp up steps in
per cycle is detected and accumulated by digital SINC filter. Digital SINC filter outputs totally counter which can be read
from TSIx_DATA[TSICNT].
NSTEP is the result of TSI single scan in mutual-cap mode, as shown in
Equation 11 .
NSTEP = Ci × Vp − Vm × Rs
∆ V
× M_PMIRRORL
M_PMIRRORR ×
1
t3
Equation 11.
• Ci: Fixed 90 pF, the integrated capacitance inside TSI module.
• V
p
, V
m
: Configurable, dual reference voltage which can be configured by DVOLT<1:0>.
• R
s
: Configurable, parameter of analog front end which can be configured by M_SEN_RES<3:0>.
• M_PMIRRORL, M_PMIRRORR: Configurable, the current multiplier.
• t3: Configurable, SSC output low period.
• ΔV: Signal voltage RX received, decided by VDD5V × C
m
/(C
m
+C
s
)
C
m
is the mutual capacitance between the TX and RX touch electrode. C
s
is the parasitic capacitance of the touch electrodes.
When the mutual-cap electrode is touched, C
m
is decreased, C
s
is increased, and ΔV is reduced. The value of NSTEP is
increased and the accumulated sample results TSICNT also increase.
Equation 12 is the basic equation of the scan time. T
nstep
is the time cost of TSI single scan.
NXP Semiconductors
TSI Mutual-cap mode
KE17Z Dual TSI User Guide, Rev. 0, 05 May 2022
User Guide 21 / 37