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ST7LITE1 Datasheet(PDF) 34 Page - STMicroelectronics |
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ST7LITE1 Datasheet(HTML) 34 Page - STMicroelectronics |
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34 / 131 page  ST7LITE1 34/131 8 INTERRUPTS The ST7 core may be interrupted by one of two dif- ferent methods: maskable hardware interrupts as listed in the Interrupt Mapping Table and a non- maskable software interrupt (TRAP). The Interrupt processing flowchart is shown in Figure 19. The maskable interrupts must be enabled by clearing the I bit in order to be serviced. However, disabled interrupts may be latched and processed when they are enabled (see external interrupts subsection). Note: After reset, all interrupts are disabled. When an interrupt has to be serviced: – Normal processing is suspended at the end of the current instruction execution. – The PC, X, A and CC registers are saved onto the stack. – The I bit of the CC register is set to prevent addi- tional interrupts. – The PC is then loaded with the interrupt vector of the interrupt to service and the first instruction of the interrupt service routine is fetched (refer to the Interrupt Mapping Table for vector address- es). The interrupt service routine should finish with the IRET instruction which causes the contents of the saved registers to be recovered from the stack. Note: As a consequence of the IRET instruction, the I bit will be cleared and the main program will resume. Priority Management By default, a servicing interrupt cannot be inter- rupted because the I bit is set by hardware enter- ing in interrupt routine. In the case when several interrupts are simultane- ously pending, an hardware priority defines which one will be serviced first (see the Interrupt Map- ping Table). Interrupts and Low Power Mode All interrupts allow the processor to leave the WAIT low power mode. Only external and specifi- cally mentioned interrupts allow the processor to leave the HALT low power mode (refer to the “Exit from HALT“ column in the Interrupt Mapping Ta- ble). 8.1 NON MASKABLE SOFTWARE INTERRUPT This interrupt is entered when the TRAP instruc- tion is executed regardless of the state of the I bit. It will be serviced according to the flowchart on Figure 19. 8.2 EXTERNAL INTERRUPTS External interrupt vectors can be loaded into the PC register if the corresponding external interrupt occurred and if the I bit is cleared. These interrupts allow the processor to leave the Halt low power mode. The external interrupt polarity is selected through the miscellaneous register or interrupt register (if available). An external interrupt triggered on edge will be latched and the interrupt request automatically cleared upon entering the interrupt service routine. Caution: The type of sensitivity defined in the Mis- cellaneous or Interrupt register (if available) ap- plies to the ei source. 8.3 PERIPHERAL INTERRUPTS Different peripheral interrupt flags in the status register are able to cause an interrupt when they are active if both: – The I bit of the CC register is cleared. – The corresponding enable bit is set in the control register. If any of these two conditions is false, the interrupt is latched and thus remains pending. Clearing an interrupt request is done by: – Writing “0” to the corresponding bit in the status register or – Access to the status register while the flag is set followed by a read or write of an associated reg- ister. Note: the clearing sequence resets the internal latch. A pending interrupt (i.e. waiting for being en- abled) will therefore be lost if the clear sequence is executed. 1 |
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