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This article mainly introduces the raspberry pie 4 rt-thread how to achieve SPI screen man-machine interface, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let the editor take you to understand.

Raspberry pie 4 rt-thread to achieve SPI screen human-computer interface 1. Preface

Raspberry pie 4's rt-thread has been constantly updated to fully tap the underlying hardware features of raspberry pie. At the same time, with the help of a variety of peripherals, raspberry pie 4 has become a more suitable platform for learning embedded development, verifying various peripheral functions and learning the operating system.

The development of all kinds of peripherals on raspberry pie 4 requires some embedded debugging experience and driver debugging methods, because although the raspberry pie has a lot of data, the chip data of the key core is very little, which is the development work at the application level. In order to learn embedded more deeply, it is not enough to understand the method, but to understand the principle. This paper mainly introduces the use of raspberry pie 4 images and touch-related images, so as to realize the transplantation and interface interaction of GUI.

two。 Introduction of raspberry pie 4 display interface

Raspberry pie 4 is displayed without any screen interface by default and can be connected to the HDMI interface.

In addition, the raspberry pie 4 has a display DSI cable interface with MIPI, which can be connected through cable arrangement.

The driver implementation of these two display interfaces has been implemented in the videocore of raspberry pie. For specific implementation details, you need to see the use of the relevant GPU. As a part of learning embedded images, it is no longer possible to do any underlying related development from the raspberry pie. To call the read and write operation of an image, you can use the communication pipeline between CPU and GPU as the data transmission channel. For more information, please see the driver implementation details of mbox:

Https://github.com/RT-Thread/rt-thread/blob/master/bsp/raspberry-pi/raspi4-32/driver/mbox.c

Put the image into a memory space, then send a message to GPU, ask it to go to that address to get the data, and finally you can see the display on the screen.

This kind of only display, if you want to achieve touch, but also need to carry out another interface implementation. One is that DSI can transfer the coordinates of the touch to GPU and get the coordinate data through mbox. Another common way to connect the HDMI screen is to connect the touch to the USB and get the coordinate points through USB. This is a common means of implementation.

In addition, Yuexue has a 3.5inch SPI screen from MHS-3.5inch RPi Display.

Http://www.lcdwiki.com/MHS-3.5inch_RPi_Display

The following figure is a basic demonstration:

The screen is driven by SPI bus, and the maximum spi rate supports 125Mhz. Support resistive screen touch. And the price is also very cheap. It is the focus of this paper.

3. Raspberry pie 4 SPI interface

If you want to drive the screen, you must first understand the SPI protocol and the use of the spi controller of the bcm2711 chip. All in-depth study of the rpi_DATA_2711_1p0.pdf document.

Https://gitee.com/bigmagic/raspi_sd_fw/tree/master/doc/raspi4

The point is that the SPI of Raspberry Pie 4 supports two modes, one of which is the standard SPI bus.

This is very basic, that is, the CE chip selection determines which device, the host transmits the SCLK clock signal, and then transmits the MOSI, the slave response data, MISO. This is the standard SPI protocol, you can review it.

The other is LOSSI mode.

That is, low-speed serial bus. After selecting the peripherals, the host can send the clock through SCL, and then send data through SDA. After receiving the data from the slave, it will also be transmitted to the host through the SDA cable. There is only one line to transmit data.

The way to distinguish is that when sending data bits, such as sending 8-bit data bits, a bit will be added to the front to indicate read and write. This is suitable for the use of low-speed peripheral bus. This model is not used for the time being and will be analyzed when it is used later.

4. Implementation of SPI screen on Raspberry Pie 4

I will not expand too many details, anyway, debugging took two days, the following mainly talk about the experience and steps of debugging.

When the standard SPI screen is connected, both touch and LCD use the same SPI cable.

The selection is made through the CS pin, which corresponds to two selections of the raspberry pie SPI0. I just started debugging LCD, as the main control of LCD is ILI9486, it seems that the chip manual, ready to follow the previous debugging experience, first read out the factory ID to try. I tried for a long time without reading the data. When I looked at the above picture carefully, I found that the pin on the SO was marked with Touch panel SPI data output. It turns out that it is not possible to output data. So change direction, first debug the touch, because the touch is more standard.

At the beginning, the touch could not read the data, because the SPI driver was not well written, the testing was not strict, and the understanding of SPI protocol was not deep. Later, it simply used a logic analyzer to track the data. Found that the rate is too high, the original touch support spi frequency is very low, only a few hundred K. I finally saw the data read out and changed.

Because it is a resistive screen, it reads 12-bit data, reads 2 bytes, and then calculates the offset to get 12-bit data, with an overall range of 0 to 2048. According to this law, constantly test the coordinates at the top and bottom of the screen, and finally get the following conclusions.

XPT2046:Width:320 High:480

No pressed: (0x800.0xfff)

-ETH----USB

| | (0x800re0x800) (0xfffpj0x800) | |

| | |

| | (0x800re0xFFF) (0xfffre0xfff) | |

-

Do not touch the data (0x800ji0xfff), the value of each angle can see the above information.

To calculate the coordinates, because of the resistance screen, the coordinates of each point are uniformly distributed, which has the linear relationship of quadratic equation. It is easy to calculate the coordinate points from the data obtained. It took a day to debug this. You can finally start debugging LCD.

Because the selections of the two pieces are different, and they can only be written but not read, it needs to be very accurate to debug. First of all, the LCD reset, that is, the No. 22 pin first high level, then low level and then high level. The reset action is completed at this time.

Next, note that the commands and data are controlled at a high and low level of GPIO. Pay attention to this detail. Then it is time to transfer the configured parameters, which are mainly referred to the similar implementation of lcd. When the parameter is passed, the panel of the screen is white and turns black, which is the key to prove that the parameter is written in. For the realization of this detail, debugging for a long time. Then write the data to lcd.

At the beginning, the speed of brushing the screen is visible to the naked eye, and each coordinate point is changing, which must be unacceptable, and then calculate that the frequency of SPI is less than 1MHZ. This certainly won't work, so configure the raspberry pie SPI to raise the clock to 125Mhz. Although the speed has been improved, it is still not enough. It takes almost 1 second to refresh the full screen. Finally, we directly skip the spi framework of rt-thread and directly operate the underlying SPI to write data. I found it was really much faster.

Use data to speak, use Image2LCD to generate an image, and do screen brushing test.

The final result is as follows:

Because the tick of the operating system is 10ms, so the test results are about 40 to 50ms, so the frame rate is about 20 to 25 frames. This is the measured data at present. Although the rate is not very high, it is acceptable.

Thank you for reading this article carefully. I hope the article "how to implement SPI screen human-computer interface with raspberry pie 4 rt-thread" shared by the editor will be helpful to you. At the same time, I also hope you will support us and pay attention to the industry information channel. More related knowledge is waiting for you to learn!

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