QDK PIC24/dsPIC-XC16 - Quantum Leaps

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QDK PIC24/dsPIC-XC16 www.state-machine.com/pic 5 Implementing “Zero Interrupt Latency” with the NMI As noted in Section 3.2, the interrupt locking policy in this QP port locks only interrupts with IPL from 1 to 6. The highest-level interrupts with IPL=7 are not locked at all. In other words, IPL=7-interrupts are nonmaskable (they are NMIs). This interrupt locking policy has two important consequences. First, the IPL=7-interrupts cannot call any QP services (e.g., NMIs cannot post events to QP tasks), because any system call from an NMI would run the risk of corrupting the internal QP data since the protection by critical sections does not apply for NMIs. On the flip side, however, the IPL=7-interrupts run completely “free” with the interrupt latency determined only by the underlying hardware and independent on the critical sections implemented in the QP. This is what is meant here by the term “Zero Interrupt Latency”. Such NMIs (IPL=7-interrupts, in this case) have many potential uses. They are very useful when the software must perform some simple operations very fast, but most of the time the software does not need to communicate with the task-level of QP active objects. Consider for example a fast interrupt-driven UART interface. The ISR that receives bytes must remove the bytes from the FIFO very fast or else the FIFO will quickly overflow. However, the ISR might buffer the bytes and only need to notify the task level when the buffer fills up, which is relatively infrequently. NOTE: NMIs (IPL=7-interrupts, in this case) can be equally well used in the non-preemptive QP port as well as in the preemptive port with the QK kernel. 5.1 Communication between NMIs and QP From the previous discussion it is obvious that the NMIs must be able to communicate from time to time with the task level (active objects in QP). The question is how to achieve such communication without calling QP services directly The solution is to call QP services indirectly, by triggering a regular (maskable) interrupt from the NMI. In the PIC24/dsPIC architecture you can very easily trigger an ISR by explicitly setting the corresponding interrupt flag (the same that you explicitly clear in the ISR). The triggered interrupt can call QP services, such as posting or publishing events to QP active objects. The subtle, but important consideration is the communication between the NMI and the triggered maskable interrupt. Typically, such communication must occur by means of atomically-accessed shared variables. In the PIC24/dsPIC CISC architecture most 8- and 16-bit variables are read and written atomically (in one machine instruction), so implementing this way of communication is quite easy. In other CPUs, such as RISC load-store architectures, the implementation of atomic access is trickier and requires using special swap (SWP) instructions. 5.2 Implementation Considerations for the NMIs Because the NMIs run completely “free” and outside the QP framework, they don’t need to follow any conventions associated with regular maskable ISRs that can call QP services. Generally, the NMIs should run as fast as possible, because the interrupt latency for NMIs is determined typically by the execution time of the NMI itself. (In this QP port, only one IPL level 7 is available for NMIs, so NMIs cannot preempt each other). NMIs can be coded in C, and in fact the XC16 compiler offers options to make these interrupts very efficient. On of these options is __shadow__, which uses fast context save and restore to the shadow registers. Obviously, you can use only one such fast ISR in your system to avoid corruption of the shadow registers. And finally, you should generally avoid calling any functions from the NMI, as this would force the compiler to save and restore w0-w7. Copyright © Quantum Leaps, LLC. All Rights Reserved. 26 of 35
QDK PIC24/dsPIC-XC16 www.state-machine.com/pic 6 BSP for the Microstick II Board The Board Support Package (BSP) for PIC24/dsPIC and Explorer 16 board (see Figure 1) with the nonpreemptive “vanilla” scheduler is located in the directory: \examples\pic24_dspic\vanilla\ xc16\dpp-qk-microstick2_pic24\ for PIC24 and \examples\pic24_dspic\vanilla\ xc16\dpp-qk-microstick2_dspic\ for dsPIC. The BSP consists of the following files: 1. bsp.h contains the Board Support Package interface (BSP) 2. bsp.c contains the implementation of the BSP, which includes all ISRs and all platform-specific QP callbacks. 6.1 Setting the Sizes of Stack and Heap You set the sizes of stack and heap through the MPLABX IDE, as shown in Figure 11. Figure 11 Setting the size of heap and stack Copyright © Quantum Leaps, LLC. All Rights Reserved. 27 of 35
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