Exceptions and Interrupts

1
Exceptions and Interrupts
2
Exceptional Circumstances

• At home - how do you know when someone wants to
  talk to you on the telephone?
• A. Periodically pick up the phone and see if
  someones there
• This is known as polling

• B. Wait for the phone to ring and then answer it
• This is exception- or interrupt-driven

3
When Interrupting is Nice

• Coordinating I/O operations
• Notifies the CPU that an input is ready or an
  output can be changed

• Timing
• Periodic (clock-driven) interrupts remind the CPU
  of the passage of time
• Multi-tasking
• Updating counters
• Timing during interactive programs

• Errors
• Interrupts can notify of error situations

4
Exception Types

• Hardware (called Interrupts or Resets)
• Reset
• User-defined interrupt
• Timer operations
• CPU operations monitor failure

• Software (not on PSoC)
• Illegal instruction
• Divide by zero
• Overflow

5
Im Not listening...

• Interrupts can sometimes be ignored (or masked)
• Good for when the CPU is doing something more
  important
• When the interrupt mask is set, interrupts are
  hidden (masked) and ignored

• Interrupts may be prioritized
• The fire alarm is more important than the oven
  timer which is more important than the telephone

• Non-maskable interrupts cannot be ignored
• NMIs take precedence and interrupt any task

6
Servicing Interrupts and Exceptions

• When an exception occurs
• CPU decides whether to do anything

• CPU saves current state
• PC, Flag Register
• Global interrupts disabled

• Jump to interrupt service routine (ISR)
• Execute routine
• RETI - return from interrupt

Works like a Call, except Flag register is saved,
too.

• Restore state
• Flag Register (re-enables global interrupts)

• Return to user program (restore PC)

7
Interrupt Vector Table

• When an interrupt occurs, control of the program
  moves to the interrupt service routine (ISR)
• Similar to a subroutine
• But how do we know where the ISR is?

• The address of the ISR is provided by the
  interrupt vector table
• IVT has one entry for each type of interrupt
• Each entry is indexed by interrupt type
• PSoC vector table has four bytes for each entry
• Usually holds a single LJMP instruction that
  passes control to the ISR

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Interrupt Vector Table
0004h LJMP to code for handling low voltage
indicator
0010h LJMP to code for handling analog col 2
interrupt
001Ch LJMP to code for handling GPIO interrupt
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Full PSoC Interrupt Vector Table
Name Identifies source of interrupt
Device Availability
Priority If interrupts happen at the same time,
highest priority interrupt wins
Address Address of interrupt vectorNote Only 4
bytes, usually just an LJMP
10
Programming With Interrupts

• Write the interrupt service routine
• What action is to be performed when a particular
  interrupt happens?
• End with RETI

• Link the interrupt vector to the ISR
• At interrupt vector location, write LJMP myISR
• Usually taken care of by PSoC Designer

• Write the main routine
• Enable the interrupt to be serviced
• Int_MSKx registers control various interrupts
• Write a 1 into the appropriate bit to enable
  the interrupt
• See next slide for details

• Globally enable interrupts
• Set GIE bit in flag register use
  M8C_EnableGInt

• Wait. That is, do nothing

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Interrupt Mask Registers

• The registers INT_MSKx turn individual interrupts
  on or off
• Write a 1 to the appropriate bit to turn
  interrupts on

• The 29466 chip were using has 4 analog columns
  and 4 digital rows (highlighted)
• See useful equates in M8C.inc under System and
  Global Resource Registers

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Example - GPIO ISR
Whenever GPIO pin goes high, update and display a
count
1. Select interrupt on pin in Design view(Dont
forget pull-down mode!)
2. Write the GPIO ISR
// ISR for the GPIO interruptPSoC_GPIO_ISR
inc count // increment count mov A,0 //
position LCD at 0,14 mov X,14 call
LCD_1_Position mov A, count call
LCD_1_PrHexByte // print out count reti // all
done - return
Interrupts cant have parameters Must use globals
3. Link ISR to GPIO vector at 001C (Usually done
already in boot.asm)
org 001Ch ljmp PSoC_GPIO_ISR
export count export _main area
bss count blk 1 // define counter area
text _main M8C_EnableGint call
LCD_1_Start call LCD_1_Init or REGINT_MSK0,
00100000 //1 in GPIO position loop jmp loop
4. Write the initialization and main routine
Do-nothing program!