Real-Time Windows Target

Real-Time Kernel

The Real-Time Windows Target uses a small real-time kernel to ensure the real-time application runs in real time. The real-time kernel runs at CPU ring zero (privileged or kernel mode) and uses the built-in PC clock as its primary source of time:

• Timer interrupt -- The kernel intercepts the interrupt from the PC clock before the Windows operating system receives it. This blocks any calls to the Windows operating system. Because of this, you cannot use Win32 calls in your C-code S-function.

• The kernel then uses the interrupt to trigger the execution of the compiled model. As a result, the kernel is able to give the real-time application the highest priority available.

  To achieve precise sampling, the kernel reprograms the PC clock to a higher frequency. Because the PC clock is also the primary source of time for the Windows operating system, the kernel sends a timer interrupt to the operating system at the original interrupt rate.

  Technically, the kernel is provided as a VxD on Windows 98, and Windows Millennium Edition and as a kernel-mode driver on Windows 2000, Windows NT, and Windows XP.

• Scheduler -- The timer interrupt clocks a simple scheduler that runs the executable. The number of tasks is equal to the number of sampling periods in the model with multitasking mode. With single-tasking mode, there is only one task. The maximum number of tasks is 32 and faster tasks have higher priorities than slower tasks. For example, a faster task can interrupt a slower task.

• During execution, the executable stores data in buffers. Later, the data in these buffers is retrieved by the Scope block. The scheduling, data storing, data transferring, and running the executable all run at CPU ring zero.

• Communication with hardware -- The kernel interfaces and communicates with I/O hardware using I/O driver blocks, and it checks for proper installation of the I/O board. If the board has been properly installed, the drivers allow your real-time application to run.

• The Analog Input, Analog Output, Digital Input, Digital Output, Counter Input, and Encoder Input blocks call the drivers for input and output. You can choose to have a driver block use values equal to voltage, normalize values from 0 to +1, normalize values from -1 to +1, or use the raw integer values from the A/D or D/A conversion press. Drivers also run at CPU ring zero.

• Simulink external mode -- Communication between Simulink and the real-time application is through the Simulink external mode interface module. This module talks directly to the real-time kernel, and is used to start the real-time application, change parameters, and retrieve scope data.

Features

Real-Time Application