STM32 Series 26 - Capacitive Touch Screen

Touch screen classification:

1. Resistive: accurate positioning, single-point touch

2. Capacitive induction: Capacitive induction: supports multi-touch, but is more expensive. Most widely used in industry

3. Infrared: Low price, but the frame is fragile, prone to light interference, and distortion on curved surfaces

4. Surface acoustic wave type: solves various shortcomings, but if there are water droplets and dust on the screen surface, the touch screen will become dull

Note: The touch screen is essentially separated from the LCD. The touch screen is responsible for detecting touch points, while the LCD is responsible for display. Capacitive touch screens generally require a driver IC to detect capacitive touch, and generally output touch data through the IIC interface.

Common driver ICs:

1. GT9147: 17*10 drive structure (10 sensing channels, 17 drive channels)

2. OTT2001A: 13*8 drive structure (8 sensing channels, 13 drive channels)

Note: They are connected to the MCU via 4 wires: SDA, SCL, RST and INT.

GT9147:

The IIC address of GT9147 can be 0X14 or 0X5D. Within 5ms after the reset, if INT is high, 0X14 is used as the address, otherwise 0X5D is used as the address. For the specific setting process, please refer to the document: GT9147 Data Manual.pdf. In this chapter, we use 0X14 as the device address (excluding the lowest bit, converted into read and write commands: read: 0X29, write: 0X28)

Control command register (0X8040)

This register can be written with different values ​​to achieve different controls. We generally use the two values ​​0 and 2. Writing 2 can soft reset GT9147. After hard reset, generally write 2 to this register to implement soft reset. Then, write 0 to read the coordinate data normally (and end the soft reset).

Configuration register group (0X8047~0X8100)

There are 186 registers here, which are used to configure the various parameters of GT9147. These configurations are generally provided to us by the manufacturer (an array), so we only need to write the configuration given by the manufacturer into these registers to complete the configuration of GT9147.

Product ID register (0X8140~0X8143)

There are 4 registers here to store product ID. For GT9147, these 4 registers are read as 9, 1, 4, 7 (ASCII code format). Therefore, we can use the values ​​of these 4 registers to determine the model of the driver IC, and thus determine whether it is OTT2001A or GT9147, so as to perform different initializations.

Status Register (0x814E)

We only care about the highest bit and the lowest 4 bits. The highest bit is used to indicate the buffer status. If there is data (coordinates/buttons), the buffer will be 1. The lowest 4 bits are used to indicate the number of valid touch points, ranging from 0 to 5, 0 means no touch, and 5 means 5 touch points.

Coordinate data registers (30 in total)

There are 5 groups (5 points) here, and each group of 6 registers stores data. Take the coordinate data register group of contact 1 as an example. 
 
We generally only use the x and y coordinates of the contact, so we only need to read the data from 0X8150 to 0X8153, and combine them to get the coordinates of the contact. The other 4 groups are composed of 16 registers starting with 0X8158, 0X8160, 0X8168 and 0X8170, respectively for the coordinates of contacts 2 to 4. Similarly, GT9147 also supports register address self-increment. We only need to send the first address of the register group and then read it continuously. GT9147 will automatically increase the address, thereby increasing the reading speed.