Development of touch screen driver on embedded Linux platform

Due to the convenience, flexibility, space saving, and intuitive features of the touch screen, the input device of the embedded system is increasingly favored by various terminal product manufacturers. The Linux operating system is a popular choice for embedded systems because of its open source code and easy to cut. This article will discuss how to write a touch screen driver in the Linux operating system based on the construction hardware.

Introduction to the SPI interface

Serial Peripheral Interface The SPI bus technology is a full-duplex, synchronous serial interface from Motorola that provides a powerful four-wire interface (receiver, transmit, clock, and slave).

The slave device of the SPI shares a clock line with the master device, and the clock is always sent from the master device. When 823e is in master mode, the chip select signal line is disabled. If it is slave mode, its slave select line is enabled low. In this example, the 823e is the master device, so we chose a 823e GPIO (General Purpose I/O) as the chip select signal for the slave. Most synchronous serial data converters are easy to interface with, and their hardware is very powerful, so the SPI-related software is quite simple, giving the CPU more time to process other transactions.

Touch screen hardware

The touch screen input system consists of a touch screen, a touch screen control chip and a data processor. Touch screens can be divided into five categories according to their technical principles: vector pressure sensing, resistive, capacitive, infrared, and surface acoustic wave. Resistive touch screens are used in embedded systems.

The touch screen we chose is AMD's resistive touch screen AMT9502. The touch screen control chip is TI's analog-to-digital conversion chip ADS7846. The chip supports the SPI communication protocol, so we use the 823e SPI interface to communicate with the ADS7846 chip. The analog signal obtained from the touch screen is input to the 823e as a data processor after passing through the analog-to-digital converter.

software program

The 823e communicates with the touch screen controller via the SPI interface, so control of the touch screen is the operation of the SPI interface. Once the SPI interface driver has been written, communication with the touch screen controller can be established. After the linux kernel has finished running, the SPI interface is open and a portion of the memory has been allocated for it to use. At the same time, the SPI interrupt routine has been added to the wait queue. Once the SPI interface has an interrupt, the SPI interrupt service routine is woken up and starts running. This part of the work is done in the initialization function that runs during system startup. The following will be discussed in conjunction with the source code to discuss the initialization function, which focuses on two points.

Use of mICrocode

Because the network parameter space of SCCx conflicts with the parameter space of SPI, if you want to use SCCx as the network port and also use SPI driver, you need to load microcode and then reposition the parameter space of SPI. Micropatch is a file that loads microcode. The microcode in this file can be downloaded from the official Motorola website.

The CPM includes a portion of the bidirectional RAM port, called the parametric RAM, which includes USB, SCC, SMC, SPI, I2C, and IDMA channel operations. Among them, the SPI and I2C parameter areas can be relocated to another 32-bit parameter area. After reading the following code carefully, you can understand how this process works:

Spi=(spi_t*)&cp->cp_dparam[PROFF_SPI];

Printk("thespiaddris%pn",spi);

If((reLOC=spi->spi_rpbase))

{

Spi=(spi_t*)&cp->cp_dpmem[spi->spi_rpbase];

Printk("MICROCODERELOCATIONPATCHn");

}

The purpose of the above code is to first query whether the microcode has been used, and then obtain the relocated pointer (loading microcode and repositioning is done in microcode.c).

SPI descriptor in RAM

The descriptor for the SPI interface is stored in the buffer, and the address of the buffer is specified by the SPI buffer descriptor in the bidirectional RAM. The data to be sent is in the send buffer and the received data is stored in the send buffer. The buffer descriptor loops form a loop that helps to gradually transmit (receive) the data that you want to send (receive). It is because of these buffer descriptors that the communication processing module is able to complete the communication and to account for and handle the error.

A piece of code can be used to see how the above schematic diagram is implemented in the initialization function:

Spi->spi_rbase=r_rbase=dp_addr;

Spi->spi_tbase=r_tbase=dp_addr+sizeof(cbd_t);

/*Write the address of RXBDRING to POINTERTOSPIRXRING

Write the address of TXBDRINT to POINTERTOSPITXRING*/

Spi->spi_rbptr=spi->spi_rbase;

Spi->spi_tbptr=spi->spi_tbase;

The two codes above /* must be written, otherwise they will be read and written.

Tbdf=(cbd_t*)&cp->cp_dpmem[r_tbase];

Rbdf=(cbd_t*)&cp->cp_dpmem[r_rbase];

/* As you can see from this code, the address of RXBDRING is in bidirectional RAM*/

Tbdf->cbd_sc&=~BD_SC_READY;

Rbdf->cbd_sc&=~BD_SC_EMPTY;

/*Set the status of the RING, and the sent RING is set to be unprepared.

Accepted RING is set to not ready to accept status*/

Rxbuffer=m8xx_cpm_hostalloc(2);

Txbuffer=m8xx_cpm_hostalloc(2);/*Get two spaces*/

Tbdf->cbd_bufaddr=__pa(txbuffer);

Rbdf->cbd_bufaddr=__pa(rxbuffer);

/* memory map; and set DATAPOINTER to the address of RXDATABUFFER */

The above code is done in the initialization function. Once the initialization function works correctly, you can take the correct steps to communicate with the SPI port. After the above initialization, the cpm_install_handler function is called. The function of the function is to register the interrupt function into the kernel. Once the SPI port generates a hardware interrupt, the interrupt function is called. The interrupt function can be written according to the different needs of different systems. In this example, We make the data received by the SPI read once the interrupt function is called.

Next, how to send data is taken as an example to analyze how to operate SPI port communication.

Steps to send data

In this example, the SPI interface is set to master mode. In order to start the data transfer process, the kernel writes the data to be transferred to a data buffer and then configures the buffer descriptor for the purpose of the transfer. The following is a piece of code that sends data, explaining the process of transmission through the code.

MEMSet((void*)txbuffer,0,2);/*clear buffer*/

Tbdf->cbd_sc=BD_SC_READY|BD_SC_LAST|BD_SC_WRAP;

Tbdf->cbd_datlen=2;

/*Set the value of the status control register of the send buffer and the number of transmitted data*/

Rbdf->cbd_sc=BD_SC_EMPTY|BD_SC_WRAP;

Rbdf->cbd_datlen=0;

/* Since it is not intended to accept data, the number is 0*/

Cp->cp_spmode=0x777f;

Cp->cp_spie=0xff;

Cp->cp_spim=0x37;

/*Set the value of the SPI interface register to send data, set the SPI interface

The master or slave mode must be set in the send function. Otherwise, the data cannot be sent*/

Cp->cp_spcom|=0x80;/*Start sending data*/

Udelay (1000); / * must wait, otherwise it can not correctly read the value of the buffer status control register * /

If((tbdf->cbd_sc&0x8000))

Printk("spiwriteerror!");

mEMSet((void*)rxbuffer,0,2);

In the process of data communication, the most important is the timing. The correct timing can only be obtained through repeated experiments. Figure 3 is a logic diagram obtained during the experiment (Agilent's 1672G logic analyzer test results). Among them, CS is the chip select signal, CK is the clock signal, and DO is the data sent by 823e. You can use a logic analyzer to read whether the resulting data is consistent with the data sent by the device. The correct communication must be obtained after a long period of debugging.

Operation of the ADS7846

According to the ADS7846 manual, the driver must establish communication with the ADS7846 at initialization time. Therefore, the 823e first sends a command to the ADS7846 to get the communication after the ADS7846 reply. The driver calls the SPI read and write function to implement the operation of the ADS7846.

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