Low cost solution for capacitive touch control
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Low cost solution for capacitive touch control

Posted Date: 2024-02-01

In this application note, we will discuss how to implement touch keys using IO ports. We will also show how to connect the touch key directly to the LCD using a few IO port wires. This article will describe and discuss two approaches for low-cost solutions for capacitive touch control.

Most handheld or wall-mounted instruments require a display and some buttons; these often represent a large portion of the product's cost. An economical display is a glass LCD, which is driven directly by a microcontroller and therefore does not require a display controller. The disadvantage is that the number of segments on the display is limited by the number of free I/O port lines available. The cost of the keys is a touch key made from copper pads on a PCB, so the cost is negligible. The disadvantage is that each key must be the size of a fingertip, and it is difficult to implement in a matrix format. However, for small 7-segment or 14-segment displays with few keys, this method of operation will be economical.

Glass LCD driving signal

Consider a simple LCD display. It has only one segment. In order to operate this segment, it must have a backplane, often called a COM plane. A voltage is applied between the segment (SEG) and COM to open the segment. In order to stop the DC voltage building up on this segment, the voltage needs to be switched so that the effective DC level is 0 and the voltage on the segment remains constant. The switching rate is called the refresh rate and should be between 30 Hz and 75 Hz. Lower frequencies may cause flickering, but higher frequencies may create a "ghosting" effect where segments may take longer to shut down. Higher frequencies may also consume more power, so choose the lowest frequency possible without causing the display to flicker.

To turn off the segment, apply voltage of the same polarity to the SEG and COM pins, to turn the segment on, apply the opposite polarity,

Half Vcc method

A two-segment display can be arranged in two ways: two SEG pins and one COM pin, or one SEG pin and two COM pins. In this method, the COM pin is toggled and the SEG pin is set to opposite polarity to turn on or same polarity to turn off. This is the simple approach, but requires N + 1 pins for N segments. The second method is difficult to implement because there are two backplanes and they must operate in multiplexed mode. The number of phases depends on the number of backplanes. In this example there will be two stages. During the phase, the COM1 pin is set high and the SEG pin is set high or low depending on whether seg1 is on or off. COM2 pin remains on? Vcc, then the COM1 pin remains low, followed by the SEG pin. During the next phase, the COM1 pin remains at ? Vcc, while the COM2 pin switches in-phase or out-of-phase with the SEG pin. This method is called the Half VCC method. In this example, there is no advantage to either approach since both methods require three pins, but as the number of segments increases, the advantages become clear. Since each pin is driven by an I/O port line, the number of port lines required for N segments is (N/C) + C, where C is the number of COM planes. Therefore, for 128 segments, the design in Method 1 requires 129 pins, while Method 2 only requires 36 pins (with 4 COM planes). The number of port lines required for N segments is (N/C) + C, where C is the number of COM planes. Therefore, for 128 segments, the design in Method 1 requires 129 pins, while Method 2 only requires 36 pins (with 4 COM planes). The number of port lines required for N segments is (N/C) + C, where C is the number of COM planes. Therefore, for 128 segments, the design in Method 1 requires 129 pins, while Method 2 only requires 36 pins (with 4 COM planes).

A 1/2 Vcc voltage can be obtained by connecting two equal resistors (100 kΩ) on the COM signal and then programming the I/O port as an input.

Compared

If the above design is actually implemented, this segment will most likely remain open all the time. The reason is that it has some DC voltage coming from Vcc. To reduce the DC bias, some delay must be introduced. This delay acts as a contrast. The delay period can then be changed to increase or decrease the contrast level. A delay period is added after each pulse where both mid and COM lines are held at Vcc.

The software must generate twice the number of pulses in the same time period and change the time period based on activity or contrast cycles. The delay period can be implemented with two timers or, as in this application, with one timer.


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