Question

Assume that for a PIC24FJ64GA002 the pin for CN13/RB13 has a pushbutton switch connected to it and the change notification interrupt for RB13 is enabled. For the following code snippet, how many times does the -CNInterrupt ISR execute if the pushbutton is presses and released one time? Assume no switch bounce and explain your answer. The ISR is void _ISR _CNInterrupt (void) f DISABLE_RB13_CNINTERRUPTO; II CN13IE 0; .cN1F = 0;

Answer 1

Internet Servise Routine for Change Notification:

Lets first explain the above program ,

void ISR_CNInterrupt( void ) is a function which is defined to clear the _CNIF and _CN13IE interrupt flag and bit.

ie. _CNIF = 0 is used to clear the interrupt flag before exiting the function

also _CN13IE=0 will clear the pin 13 which is connected to push button

now the main function,

CONFIG_RB13_AS_DIG_INPUT() will set the pin RB13 as the input from the switch ( ie.push button)

ENABLE_RB13_CN_INTERRUPT() is a macro used to set _CNxIE bit associated with RB13 port ( ie. _CN13IE=1)

then, _CNIF=0 is to clear the interrupt flag

_CNIP=2 is used to choose a priority and

_CNIE=1 is used to enable the change notification general interrupt.

while(1) a forever executing loop for main routine.

Now , we know that the change notification can be used to wake a system processor from the sleep or idle mode.

The above given code assumes the pushbutton switch SW1 is attached to the port RB13 , also it is the change notification input CN13 . CONFIG_RB13_AS_dig_input is the macro which defines the port RB13 is the digital input to the system. and individual change notification is enabled using the macro ENABLE_RB13_CN_Interrupt. also before the while(1) loop is entered the interrupt flag is cleared, the priority is assigned and global change notification in enabled. while enabling the interrupts it is important to clear the associated interrupt flag before the interrupt is enabled to avoid an immediate jump to the ISR while interrupt flag was set prior to interrupt being enabled.

note : interrupt enable will not prevent the interrupt flag from being set. it only prevent the jump to ISR.

here the priority level for this interrupt is needed to be above 0 , so priority level =2 is just arbitrary.

Pressing SW1 generates a change notification interrupt , causing a jump to _CNInterrupt() ISR , which clears the change notification interrupt flag(_CNIF=0)

Upon ISR return, execution resumes in the while (1) { } body. Two change notification interrupts are generated for each press and release of the switch.( ie. one for the press and release ).