S3C2440 bare metal --- touch screen _ resistive touch screen hardware principle

1. Principle of resistive touch screen

2. Resistive touch screen equivalent circuit

2.1 Touch screen idle state

When the touch screen is not pressed, the equivalent circuit is shown in Figure 14.5. S4 and S5 are closed, S1, S2, and S3 are open, that is, YM is grounded, XP is pulled up, XP is used as analog input (for the CPU), YP is used as analog input, and XM is high impedance.

Normally, when the touch screen is not pressed, due to the pull-up resistor, Y_ADC is at a high level. When the x-axis and y-axis are squeezed and contacted, the voltage of Y_ADC becomes a low level because the y-axis of the connecting channel is grounded. This low level can be used as an interrupt trigger signal to notify the CPU of a "Pen Down" event.

2.2 Sampling X_ADC voltage

Sample the X_ADC voltage to obtain the x-coordinate. The equivalent circuit is shown in Figure 14.6.

S1 and S3 are closed, S2, S4, and S5 are open, that is, XP is connected to the power supply, XM is grounded, YP is used as analog input, YM is high impedance, and XP is prohibited from pulling up. At this time, YP, namely X_ADC, is the voltage divider point of the x-axis, and the x-coordinate is obtained after A/D conversion.

2.3 Sampling Y_ADC voltage

Sample the Y_ADC voltage to obtain the y coordinate. The equivalent circuit is shown in Figure 14.7.

S2 and S4 are closed, and S1, S3, and S5 are disconnected, that is, YP is connected to the power supply, YM is grounded, XP is used as the analog input, XM is high impedance, and XP is prohibited from pulling up. At this time, XP, namely Y_ADC, is the voltage divider point of the y-axis, and the y coordinate is obtained after A/D conversion.