外部设备——例如，通用串行总线(usb)端口或系统时钟。
External devices — For instance, Universal Serial Bus (USB) ports or the system clock.
一旦检验总线速度，可以信赖的写定时器、串行I/O和其他时钟有关的程序。
Once you verify the bus speed, you can confidently write the timer, serial-I/O, and other clock-dependent routines.
一旦检验总线速度，可以信赖的写定时器、串行I/O和其他时钟有关的程序。
Once you verify the bus speed, you can confidently write the timer, serial-I/O, and other clock-dependent routines.
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