Button Sampling Window (ms)
Ma
ximu
m
Sensor Frequency (MH
z)
0
1
2
3
4
5
6
7
8
9
0
5
10
15
20
25
30
D008
LCDIV=0
LCDIV=1
LCDIV=2
LCDIV=3
LCDIV=4
LCDIV=5
LCDIV=6
LCDIV=7
Figure 8-7. LCDIV as a Function of Sensor Frequency and Button Sampling Window
8.1.7 Scaling Frequency Counter Output
The LDC3114 requires this internal frequency counter scaler to be set based on the button sampling window to
avoid data overflow. Use
to set the scaler in
Register CNTSC (Address 0x1E)
:
n
n
n
n
f
2
SENSOR
0.0861
SENCYC
1
CNTSC
LCDIV
ceiling log
,
0, 1, 2, or 3
§
·
u
¨
¸
¨
¸
©
¹
(8)
where:
• CNTSC
n
is the internal frequency counter scaler
• SENCYC
n
and LCDIV are the linear and exponential scalers that set the number of sensor oscillation cycles
•
f
SENSOR
n
is the sensor frequency in MHz
8.1.8 Setting Button Triggering Threshold
Every material shows some hysteresis when the material deforms then returns to the original state. The amount
of hysteresis is a function of material properties and physical parameters, such as size and thickness. This
feature modifies the hysteresis of the button signal threshold according to different materials and various button
shapes and sizes. Hysteresis can be programmed in
Register HYST (Address 0x18)
. By default, the button
triggering hysteresis is set to 32. The nominal button triggering threshold is 128. With hysteresis, the effective
on-threshold is 128 + 32 = 160. This means if the DATA
n
(
n
= 0, 1, 2, or 3) reaches 160, the LDC3114 considers
that as a button press. When the DATA
n
decreases to 128 – 32 = 96, the LDC considers the button to be
released.
ON
Threshold
128 Hysteresis
(9)
OFF
Threshold
128 Hysteresis
(10)
SNOSDD0 – DECEMBER 2021
42
Copyright © 2021 Texas Instruments Incorporated