Embedded System Design

# Lab1 * Please explain what is `arm-linux-gnueabihf-gcc`? Why don’t we just compile with `gcc`? * eabi: 嵌入式ABI，嵌入式應用二進制街口指定了文件格式、數據類型、暫存器使用、堆積組織優化和在一個嵌入式軟件中的參數的標準約定。 * hf: armhf架構，用fpu計算，傳參數也用fpu中的浮點暫存器傳，省去了轉換，性能最好，但是中斷負荷高 * 因為 embedded 和 host 的計算機架構不同，所使用的library也不同，所以不能直接用 `gcc` * Can executable `hello_world` run on the host computer? Why or Why not? * 不行 # Lab2 * What are the cmake and make for? What is the relationship between them? * cmake & make 都是編譯工具 * CMake是一种跨平台编译工具，比make更为高级，使用起来要方便得多。CMake主要是编写CMakeLists.txt文件，然后用cmake命令将CMakeLists.txt文件转化为make所需要的makefile文件，最后用make命令编译源码生成可执行程序或共享库（so(shared object)）.它的作用和qt的qmake是相似的。 * Why there are so many arguments in the compilation command(step 4.4.2)? What are they for? * -I：這用於指定編譯器應該查找header files的其他包含目錄 * -L：這用於指定連接器應該查找library的其他庫目錄 * -Wl，-rpath-link：這用於為連接器指定運行時dependent shared library搜索路徑 RUNPATH。這些路徑用於在執行期間查找動態庫（.so文件） * provides a hint to the linker about where to find dependent shared libraries during the linking process. It doesn't affect the runtime behavior of the resulting binary or library. * -lpthread：這鏈接了libpthread庫 * -lopencv_world：這鏈接了OpenCV庫，具體是libopencv_world庫 * What is `libopencv_world.so.3.4.7` for? Why do we need to use `LD_LIBRARY_PATH=. ./demo` to run the executable? What would happen if we just run with `./demo` ? Why? * .so: dynamically linked shared object libraries，在執行時動態載入 * The "world" module in OpenCV is a unified binary/library that contains all the individual OpenCV modules combined. Instead of linking against each module's library separately * LD_LIBRARY_PATH是Linux環境變數名，該環境變數主要用於指定查找共享庫（動態連結庫）時除了默認路徑之外的其他路徑(/lib & /usr/lib)。 * 程式執行時，當執行函數動態連接.so時，只會從找/lib & /usr/lib，若找不到library則會發生錯誤 * It is so complex and difficult to show a picture by using the framebuffer. Why don’t we just use `cv::imshow()` to do this work? * cv::imshow(): 在指定視窗中顯示影像(受到螢幕解析度的限制) * No Windowing System (such as X11 (on Linux)) * Direct Control & Performance * What is a framebuffer? * framebuffer 是把螢幕上的每個點對映成一段線性記憶體空間，程式可以簡單的改變這段記憶體的值來改變螢幕上某一點的顏色 * What is the result of the command below? Why? * screen redirect to file * file redirect to frame buffer * So, 螢幕不會改變 ``` sudo cat /dev/fb0 > fb0 sudo cat fb0 > /dev/fb0 ``` * You can find there is a file named `fb1` under `/dev` directory. What is the difference between `/dev/fb0` and `/dev/fb1` ? Why we use `/dev/fb0` rather than `/dev/fb1` ? * `/dev/fb0`: primary, 是embedded framebuffer(LCD) * `/dev/fb1`: secondary ___ ### LABs Details * 透過 `open` + `ioctl` 來取得 framebuffer 資訊 (/dev/fb0) * 藉由 `ofstream` 寫入到 LCD framebuffer * DOS: 顯卡 bios 直接對螢幕寫屏 * Linux: 顯卡硬體抽象化，提供 framebuffer 讓用戶直接寫屏 * RS-232(UART): 按照1個bit 接著1個bit傳輸 (串列通訊) * 只用一條線來傳資料，所以它沒有額外的clock * 非同步收發傳輸器 * 要設定 baud rate ，盡量跟對方傳輸速度一樣 * 1. `cv::imread` 或 `cv::VideoCapture` 取得圖片資訊 * 2. BGR to BGR565 (16-bit image) --> LCD requirement * 3. ofstream 操作螢幕寫入 * `cv::VideoWriter`: 錄影 * `cv::imwrite`: 截圖 * multi-thread: 偵測按鍵輸入 https://zhuanlan.zhihu.com/p/558495267 https://ecomputernotes.com/computer-graphics/basic-of-computer-graphics/what-is-frame-buffer https://www.cnblogs.com/createyuan/p/4437912.html https://www.cnblogs.com/liangliangge/p/14403773.html https://makerpro.cc/2016/07/learning-interfaces-about-uart-i2c-spi/ https://ithelp.ithome.com.tw/articles/10325085?sc=rss.iron https://www.strongpilab.com/rs232-uart-difference/