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10
CapTIvate™ Touch Getting Started Manual
Copyright © 2020, Texas Instruments Incorporated
2.3
TI's capacitive touch technology
TI's capacitive touch sensing technology CapTIvate™ is based on charge transfer collection. The principle includes:
1) Charging the sensor input capacitor
𝐶
𝑒𝑞𝑢𝑎𝑙
; 2) Transferring the accumulated charge to the internal sampling
capacitor
𝐶
𝑠𝑎𝑚𝑝𝑙𝑒
. This process will continue to repeat until the voltage on both sides of
𝐶
𝑠𝑎𝑚𝑝𝑙𝑒
reaches the
trigger voltage
𝑉
𝑡𝑟𝑖𝑝
of the internal comparator. The number of charge transfers required to reach the threshold
directly characterizes the size of
𝐶
𝑒𝑞𝑢𝑎𝑙
. When the capacitive sensor is touched by a human hand,
𝐶
𝑒𝑞𝑢𝑎𝑙
and
charge transfer number will change. The MCU senses the occurrence of a touch event by comparing the numbers
of different charge transfer cycles. MSP430 uses a current mirror to control the proportional relationship between
the input current of
𝐶
𝑠𝑎𝑚𝑝𝑙𝑒
and the discharge current of
𝐶
𝑒𝑞𝑢𝑎𝑙
, so as to equivalently amplify
𝐶
𝑠𝑎𝑚𝑝𝑙𝑒
and have a
larger range.
