Interrupts and
reset operations
Overview
Introduction to interrupts
– What are they
– How are they used
 68HC11 interrupt mechanisms
– Types of interrupts
– Interrupt priorities
 Resets
and their operation/use
 Readings: Text, sections 3.6 -- 3.10
Interrupts: an overview
 Mechanism for responding to external
events
– CPU suspends execution of current routine
– Jumps to a special interrupt service routine
(ISR)
– After executing the ISR, it returns to what it
was executing before
Interrupts: an overview
Why use interrupts? ... Efficiency!
– Interrupts increase processor system efficiency
by letting an I/O device request CPU time only
when that device needs immediate attention.
– “Main” programs can perform routine tasks
without continually polling for I/O device status.
Interrupts: an overview
Interrupts allow the processor to interact
with slow devices in an efficient manner
– Consider Lab 2:
» Required that a counter be updated every
second while monitoring a digital input
Pseudocode:
Interrupts: an overview
Loop:
update counter
delay
check for digital input
goto Loop
» What about inputs that occur during delay?
Interrupts: an overview
– Use a timer that can generate an interrupt
» Pseudocode
Loop:
check for input
goto Loop
Timer_ISR:
update counter
reset timer
return
Interrupts: an overview
Interrupt Vectors
– Interrupt Vector Table located at $FFC0-$FFFF
in memory (ROM area)
– Each type of interrupt has an entry in the table
» Entry contains address of interrupt service
routine (16-bit value)
» See p.3 of the Programming Reference
Guide or Table B-2 in text
Interrupts: an overview
 What happens when an interrupt occurs?
– CPU finishes processing current instruction
– CPU automatically stacks registers to save the
current state of the processor
» Pushed onto stack in following order:
PC, IY, IX, ACCA, ACCB, CCR
– Fetches address of ISR from vector table
– Jumps to ISR address
Interrupts: an overview
– ISR should end with RTI instruction
(Return from Interrupt)
» Automatically pulls registers off stack
» Returns to original program (using value of
PC that was pulled
Interrupts: an overview
 Sources of interrupts
– I/O
» SCI, SPI
» Parallel I/O (STRBA)
– Timer, pulse accumulator, real-time interrupt
– External pins
» XIRQ*, IRQ*
Interrupts: an overview
– Software interrupt
» SWI instruction
– Illegal opcode trap
– COP failure
– Clock monitor
– RESET* pin
Interrupts: an overview
 Masks and enables
– Some interrupts are maskable
» If interrupt is masked (or disabled), it will
be ignored by the CPU
» Can be enabled/disabled by setting bits in
control registers
– Non-maskable interrupts
» Can be used for interrupts that should never
be ignored
Interrupts: an overview
– I bit in CCR is the interrupt request mask
» If set, all maskable interrupts are disabled
Use CLI and SEI instructions to clear/set
I bit is initially set after system reset
» To only mask some interrupts, clear I and set
the individual masks in the control registers
– X bit in CCR is the XIRQ mask
» Masks the XIRQ* pin (non-maskable interrupt)
» Initially set after system reset
» Software can clear it, but cannot set it
68HC11 interrupts -- 18 in all
Non-maskable -- 3 types
– XIRQ*
» On reset, this interrupt is masked
» During initialization, XIRQ can be enabled using
a TAP instruction (clear the X bit in the CCR)
» Once enabled during initiation, its state will not
change
» Used for high priority interrupt sources -- safety
68HC11 interrupts -- 18 in all
– Illegal opcode fetch
» When an opcode is decoded, if it is invalid, this
interrupt will occur
» User should write trap routine to deal with the
error
– Software generated interrupt (SWI)
» Instruction SWI behaves like an interrupt
» Enables development tools such as breakpoints
in Buffalo monitor
68HC11 interrupts -- 18 in all
Maskable -- 15 types
– These interrupts are masked as a group by the I
bit in the CCR
– IRQ*
» External pin
» Primary off-chip maskable interrupt
» Can be set to either low-level or falling-edge
sensitive
68HC11 interrupts -- 18 in all
Default is level sensitive (IRQE=0 in
OPTION register)
Edge sensitive can be set within first 64
cycles after reset (IRQE=1 in OPTION
register)
– Other interrupts based on operation of internal
support hardware -- timers, serial I/O, parallel I/O, etc.
68HC11 interrupts -- 18 in all
 Interrupt Priority
– What if two interrupts occur at the same time?
» Interrupts have assigned priority levels
PSEL3-0 in HPRIO register can be used to
designate highest priority interrupt
– Default is IRQ*
68HC11 interrupts -- 18 in all
» Higher priority interrupt is serviced first
When CPU recognizes an interrupt, it sets the
I bit
– Prevents additional interrupts
 I bit is restored by RTI
» It is possible to clear the I bit in the ISR (CLI
instruction) to allow the ISR to be interrupted
(nested interrupts)
 Usually a bad idea
68HC11 resets
 Resetting the processor brings the
system up into a known point from which
normal operations can be initiated
– Primarily concerned with the initialization of
operating conditions and key register values
 Similar to interrupt except that registers
are not stacked
68HC11 resets
 68HC11 supports 3 types of resets
– Power on or RESET*
» Occurs when a rising edge is detected on input
power Vdd (i.e., at power up) or when user asserts
the input RESET* line
» Power up reset delays reset actions for 4096
clock cycles to allow clock to stabilize
» RESET* must be held low for 6 clock cycles in
order to be recognized (vs. it being used as an
output signal)
68HC11 resets
– Computer Operating Properly (COP) watchdog
timer failure reset
» When activated, causes the processor to reset if
no activity is detected for a long period of time
» Processor must periodically execute code
segment to reset the watchdog timer to avoid the
reset
 Write $55 to COPRST ($103A) followed by
writing $AA
68HC11 resets
» Examples of use:
System is waiting for sensor input but
sensor has failed and will never provide
input
EMI may cause interference in fetching
instructions/data
» Enable the watchdog during initialization
operations (NOCOP bit in CONFIG register)
68HC11 resets
– Clock monitor reset
» Causes a reset if clock frequency drops
below 10 kHz
» Clock frequencies from 10 kHz to 200
kHz can cause unpredictable reset actions
» Once the low frequency clock is detected,
system should have ~1000 clock cycles to
reset before clock dies completely (based
on time constant of crystal)
68HC11 resets
68HC11 Instructions
– RTI -- Return from Interrupt
– CLI -- Clear I bit in CCR
– SEI -- Set I bit in CCR
– WAI -- Wait for Interrupt
» Stacks registers and waits for an unmasked
interrupt
» Reduces power consumption
68HC11 resets
– STOP
» If S bit in CCR is set, this acts like a NOP
» Else
 Causes all system clocks to halt
 Minimum power standby mode
 Registers and I/O pins are unaffected
 Can recover with RESET*, XIRQ*, or
unmasked IRQ* signal
68HC11 resets
Specifying Interrupt Service Routine
Addresses
– The starting addresses of all interrupt service
routines are specified in the jump table at addresses
FFC0 -- FFFF -- ROM area in the HC11
– Table contents and thus the ISR addresses must
be specified at the time of manufacture
68HC11 resets
68HC11 resets
– In evaluation units such as the EVBU, we can not
change the contents of the ROM jump table (the table
had to be completed during manufacture)
» In the table, the ISR starting addresses are
specified to be in the RAM area of the memory
system
» This RAM area is, in effect, a second
“pseudojump table” for user to specify the real ISR
starting addresses -- sort of like indirect addressing
 Each entry is 3 bytes -- allows for
unconditional jump statement to the ISR
address
68HC11 resets
» Thus, to use interrupts in your programs, you
must
Determine the ISR starting address specified
in the jump table at FFC0
 At that specified address in RAM, store the
opcode for a JMP instruction followed by the
address of the ISR
68HC11 resets
» Table 3-2 in the EVBU manual lists the jump
table addresses for the ISRs
 WARNING: the addresses in the tables
should ALL start with 00 instead of E0. For
example, the clock monitor ISR is at addresses
$00FD-00FF in RAM
68HC11 resets
 Reentrant subroutines
– Any subroutines called by an ISR should be reentrant,
especially if used elsewhere in your program
» A “reentrant” subroutine can be called again before it
returns from a previous call
– Example: Subroutine to convert 8-bit hex value into 2 ASCII
digits
;******************
; Hex_To_ASCII: Calls routine Convert_Nibble to convert 4-bit
; value to ASCII
; Input: Hex value in ACCA
; Output: ASCII digits in ACCA and ACCB
68HC11 resets
Temp_Storage DS 1 ; temporary result
Hex_To_ASCII:
TAB
ANDB
#$0F
JSR
Convert_Nibble ;
STAB
Temp_Storage
TAB
LSRB
LSRB
LSRB
LSRB
JSR
Convert_Nibble
LDAA
Temp_Storage
result in B
68HC11 resets
 Reentrant Subroutines
– To make subroutines reentrant, don’t use
allocated memory for passing parameters or
temporary storage
– Use the stack or registers instead
;******************
; Hex_To_ASCII: Assume Convert_Nibble is also
reentrant.
; Input: Hex value in ACCA
; Output: ASCII digits in ACCA and ACCB
;******************
68HC11 resets
Hex_To_ASCII:
TAB
ANDB #$0F
JSR Convert_Nibble ; result in B
PSHB ; save on stack
TAB
LSRB
LSRB
LSRB
LSRB
JSR Convert_Nibble
PULA ; get first result
RTS
68HC11 resets