CTMU peripherals for capacitive touch sensing applications are implemented using the formula I × T = C × V. Among them: I is the constant current source of CTMU, T is the fixed period of CTMU charging capacitive touch sensor, C is the capacitance of capacitive touch sensor, and V is the voltage of capacitive touch sensor (read by ADC).
By rearranging the formula into the form of C = (I × T) / V, the relative change in capacitance can be detected by observing the voltage change. According to the previous formula, the following steps are involved in detecting the touch process: connecting a capacitive touch sensor (as a capacitor) to a channel multiplexed with the CTMU peripheral and the ADC;Initially, the constant current source charges for the touch sensor in a fixed period of time (T), and the voltage (V) on the sensor is measured by the ADC, as the picture shows; as long as the capacitance due to the touch sensor pad does not change, The voltage does not change during the continuous measurement of the charge multiple times.
The constant current source present in the CTMU peripheral, combined with a multi-channel ADC, provides an effective platform for interfacing with capacitive touch sensors. Connect the CTMU peripheral directly to the input of the ADC so that it can be connected to any pin via an analog multiple switch。With this configuration, the number of sensors that a single CTMU peripheral can measure will be equal to the number of ADC channels.
The trimming associated with the current source facilitates calibration，this makes it possible to cope with external disturbances and transmission losses.
