QDK PIC24/dsPIC-C30 - Quantum Leaps

More documents

Recommendations

Info

QDK PIC24/dsPIC-C30 www.state-machine.com/pic 3.4 QP Idle Loop Customization in QF_onIdle() The cooperative “vanilla” kernel can very easily detect the situation when no events are available, in which case QF_run() calls the QF_onIdle() callback. You can use QF_onIdle() to suspended the CPU to save power, if your CPU supports such a power-saving mode. Please note that QF_onIdle() is called repetitively from the event loop whenever the event loop has no more events to process, in which case only an interrupt can provide new events. The QF_onIdle() callback is called with interrupts locked, because the determination of the idle condition might change by any interrupt posting an event. The PIC24-dsPIC CPU supports several power-saving levels (consult the data sheets of particular PIC24/dsPIC devices for details). The following piece of code shows the QF_onIdle() callback that puts Pic24-dspic DSP into the IDLE power-saving mode. Please note that Pic24-dspic architecture allows for an atomic setting the low-power mode and enabling interrupts at the same time. Listing 6: QF_onIdle() for the non-preemptive (“vanilla”) QP port to PIC24/dsPIC (1) void QF_onIdle(void) { /* entered with int disabled, NOTE01 */ (2) LED_ON (IDLE_LED); /* blink the IDLE LED, see NOTE02 */ (3) LED_OFF(IDLE_LED); (4) #ifdef Q_SPY . . . /* output Q-SPY trace data to the UART (see Section 7) */ (5) #elif defined NDEBUG (6) __asm__ volatile("disi #0x0001"); (7) Idle(); /* transition to Idle mode, see NOTE03 */ #else (8) QF_INT_ENABLE(); /* enable interrupts, see NOTE01 */ #endif } (1) The QF_onIdle() callback is always called with interrupts disabled to prevent any race condition between posting events from ISRs and transitioning to the sleep mode. (2-3) The LED D10 on the Explorer 16 board is used to visualize the idle loop activity. The LED is toggled on and off, which causes the LED to “glow” at the intensity proportional to the rate of invocations of the idle callback. The idle LED is toggled with interrupts still disabled, which means that the intensity of the LED “glow” is not skewed by preemptions from interrupts or tasks. (4) If QSPY software tracing is enabled, the QF_onIdle() callback tries to output one byte from the trace buffer to the SCI. See Section 7 for more information about QSPY software tracing. (5) The low-power mode stops the CPU clock, so it can interfere with the debugger. Here, the lowpower mode is activated only in the Release build configuration when the macro NDEBUG is defined. (6) Interrupts are locked for just one following machine instruction, which ensures that interrupts will unlock after the IDLE mode. (7) The IDLE mode is activated by executing the IDLE instruction. NOTE: The PIC24-dsPIC allows for atomic transition to the Idle or Sleep modes with interrupts locked, as it should be done for a deterministic transition to the low-power mode (see [Samek 07a]). (8) When the idle/sleep mode is not used, interrupts are simply enabled. Copyright © Quantum Leaps, LLC. All Rights Reserved. 16 of 35
QDK PIC24/dsPIC-C30 www.state-machine.com/pic 4 Preemptive Configuration with QK The QP port to PIC24/dsPIC with the preemptive kernel (QK) is similar to the “vanilla” port. In particular, the interrupt locking/unlocking policy is the same, and the BSP is almost identical, except for some extra considerations for the ISRs. 4.1 QK-specific ISR Entry and Exit Macros (file qk_port.h) The preemptive QP port to PIC24/dsPIC also allows writing ISRs in C, so no explicit assembly programming is necessary. However, the preemptive QK kernel requires (as all preemptive kernels do) notifying the kernel about entering and exiting the ISR, so that the kernel can avoid context switching inside ISRs, but knows when to perform asynchronous preemptions when the last nested ISR returns to the task level. While working with the compiler-generated ISRs, the biggest problem for a preemptive kernel is to reliably detect that a given ISR is not nesting on top of another ISR, so that only the last interrupt returning to the task level performs context switch, if necessary. NOTE: Unlike most other processors, PIC24/dsPIC does not disable interrupts upon the entry to the interrupt service routine. PIC24/dsPIC merely raises the current IPL to the level of currently serviced interrupt. This means that the currently serviced interrupt can be preempted by a higher-level interrupt even at the very first instruction. Consequently, incrementing the interrupt nesting counter (QK_intNest_) is not a reliable way of accounting for interrupt nesting, because it can miss an interrupt nesting level no matter how early in the ISR it is performed. For PIC24/dsPIC, a reliable way of detecting nesting of interrupts is to check the preempted status register SR, which is saved on the interrupt stack as shown in Figure 6. Only if the preempted IPL (bits SR) is non-zero, the current interrupt preempts the task level and performing a context switch is allowed. Figure 6: Interrupt stack frame for PIC24/dsPIC Low memory High memory 15 PC SR PC Register saved by the C30 compiler Register saved by the C30 compiler … Register saved by the C30 compiler 0 IPL3 status bit (CORCON). Stack pointer (w15) While checking the stacked SR is quite easy in assembly, it is much trickier for the compiler-generated ISRs. The compiler generates different stack frames depending on the actually clobbered registers in the body of the ISR as well as the attributes of the ISR and even the optimization level used to compile the code. Consequently, it is difficult to know the offset of the stacked SR from the current stack pointer w15 that is accessible inside the ISR, (again see Figure 6). Copyright © Quantum Leaps, LLC. All Rights Reserved. 17 of 35
Page 1 and 2: QP state machine frameworks for PIC
Page 3 and 4: 1 Introduction This QP Development
Page 5 and 6: QDK PIC24/dsPIC-C30 www.state-machi
Page 17: QDK PIC24/dsPIC-C30 www.state-machi
Page 37: QDK PIC24/dsPIC-C30 www.state-machi

QDK PIC24/dsPIC-C30 - Quantum Leaps

Create successful ePaper yourself

Delete template?

Save as template?