Lecture 9 - Interrupts (1)

Lecture 9 - Interrupts (1)

1. Functions

   • A small self contained program
   • When Called;
     • The Main Program is Suspended
       • The Function Runs
         • The Main Program Continues, once the function is completed

2. The Stack

   • A Temporary Store
   • First In Last Out store
     • Most recently stored info is the first to be retrieved
     • The first info stored is last to be retrieved
   • When stack limits are exceeded, data is lost
   • Stack Operation
     • Program Counter points to the current position in the program
       • When a function, subroutine or procedure is called the current value of the Program Counter is pushed on to (stored in) the Stack
         • At the end of the subroutine call, the Program Counter value is 'popped' from the stack back into the Program Counter and program resumes previous program from correct location
   • Use of Stack Pointer in Operating a Stack in Memory
     • Mbed implements the Stack in Data Memory (RAM)
       • An area of Data Memory is assigned to the Stack and cannot be used for any other application
         • In the mbed, the stack starts at the top of memory and 'grows' downward
   • Operation of the Stack Pointer (SP)
     • During main() SP points at top of RAM
       • During function call, return address to main() stored at next available location
         • SP Decremented (Goes down stack)
         • During another function call, return address to previous function stored at current location of SP
           • SP Decremented (Goes down stack)
           • On completion, SP is incremented and data at SP location is loaded into the Program Counter to recommence function

3. Interrupts

   • Unusual event that requires the CPU to stop normal program execution and perform service related to the unusual event
   • Event types
     • External
     • Internal
     • A serial port receiving data
     • The A to D converter completing a conversion
   • Hardware Induced Function Calls

4. Interrupt Service Routine (ISR)

   • Section of Code written to respond to the interrupt which has occurred
   • When an interrupt occurs:
     • Process completes the current instruction then:
       • Saves Program Counter on Stack
         • Jumps to the start address of the ISR
           • Executes ISR
   • On Completion of the ISR, the Processor:
     • Restores Program Counter from Stack
       • Restarts Interrupted Program

5. Interrupt Vector

   • Allows the processor to know exactly where to find a particular ISR
   • Is the starting address of the ISR
   • For each possible source of interrupt in the mbed there is an interrupt vector
   • Program flow jumps directly to the ISR for that particular interrupt source
   • Mostly. e.g. only one Vector for all INT pins

6. Interrupt Input