Near Field Imaging
Capacitive touch screens are curved or flat glass substrates coated with a transparent metal oxide. A voltage is applied to the corners of the overlay creating a minute uniform electric field. A bare finger draws current from each corner of the electric field, creating a voltage drop that is measured to determine touch location.
Projected Capacitive Touch Controller – PCAP7200
This projected capacitive touch controller (PCAP7200) is designed for small scale touch panel applications in portable devices. The system block diagram is as shown in Fig. 1.
PCAP7200 has up to 22 channels to connect the touch screen and three communication pins connected between CPU and PCAP7200, which are including external interrupt INT, I2C pins SCL and SDA. The INT is active low while the touch state is calculated by PCAP7200 and the touch information can be translated via I2C communication interface. The PCAP7200 specifications are as shown in Table 1. The I2C data format, protocol, report packet and sample code are described as following.