Firmware implementation

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3

Firmware implementation

This section describes the implementation of the RC acquisition principle in the ST touch

sensing firmware library.

3.1

Charge Time measurement principle

The charge time measurement must be accurate enough to ensure a robust capacitive

sensing application. There are two common ways to measure the charge time:

1.

The first solution is using an input capture (IC) timer which is triggered when the

voltage reaches the threshold level. This approach offers a time measurement

resolution which is directly linked to the timer counter frequency. However, as one input

capture channel is required per electrode, standard MCUs are not compatible with this

type of capacitive sensing application.

2.

The second solution uses both a simple timer (without IC capability) and a simple

software sequence which regularly checks if the voltage on the acquisition I/O has

reached the threshold level. In this case, the time resolution measurement is equal to

the number of CPU cycles used to execute a complete loop of the software sequence.

This measurement approach introduces some jitter which is not compatible with the

capacitive sensing acquisition. However, a large number of electrodes can be

implemented as there are no hardware constraints.

A variant of the second solution obtains a fixed measurement resolution equal to the CPU

ST touch sensing firmware library and is described below.

3.1.1

Basic measurement

A general purpose timer performs the charge time measurement. The timer counter start

value is saved (or reset) at the beginning of the capacitance charge. When the voltage on

counter value is saved. The difference of the two time counter values gives the charge or

discharge time.

Figure 5.

Timer counter values

Counter = start_value

Counter = stop_value