A Play
Core Timer
Interrupts
Acted by the
Human Microcontroller
Ensemble from ENCM511
Unanswered questions
1. How do you tell C++ that this
function is an ISR and not a
standard function?
2. Why do you need to tell C++
that this function is an ISR
and not a standard function?
3. What is the difference (in
coding) between an ISR and a
standard function?
4. How does an interupt get
latched, why and where?
5. Why do I have to tell the timer
that the interrupt has been
serviced, and how do I do it?
6. What does “volatile” mean?
7. Why will “optimized code”
probably not work for
interrupt service routines if
the keyword volatile is not
used?
4/8/2015
Example Task 2 – file 2
(C++ or ASM)
extern volatile int foo_flag;
Tell “C++” that I am not a
function but I am an ISR –
interrupt service routine
???How declare?? ISR_count( ) {
foo_flag--;
}
Tell the timer that the
interrupt has been serviced
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
2 / 21
Timers available on Blackfin
• Watchdog timer – Hardware Reference 15-49
• Core timer – Hardware Reference 15-45
• General purpose timers 15-1
– Pulse Width Modulation
– Pulse Width Count and Capture
– External Event
• Application of timers to provide code safety and
improved version of UseFixedTimeASM( )
• Introduction to timer interrupts
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
3 / 21
Act 1 – The MAIN task
number_of_interrupts should have been a VOLATILE
Key for exam – working by luck does not count
Works – by luck – because code compiled with “DEFAULT
MODE” where all variables are treated as “volatile” by default
Would not work in “RELEASE MODE” where “unneeded” or
“unused” code isCORE
removed
by the compiler
4/8/2015
Timer Interrupts
-- a play, automatically
Copyright M. Smith, ECE, University of
4 / 21
There are two ways to
compile your C++ code
• Debug mode – simplest possible code generated
– not optimized often “slow”
• Release mode – repeated and not used code
removed by the compiler – faster
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
5 / 21
The assembly
code generated
from main.cpp
(Debug option)
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
Number of interrupts
checked each time
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around the
loop
The assembly
code generated
from main.cpp
(Release
option)
Number of
interrupts
NOT
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
checked
7 / 21
In loop
Main.cpp as
assembly code
Release option
“Volatile” used
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
8 / 21
How NOT to add a C++
interrupt service routine
This instruction ends
in the standard
return from
subroutine generated
by the C++ compiler
Code means
(Acts like an RTS)
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
9 / 21
Differences between C++ compiler
generated code and ours
Compiler uses the instructions
to build a “stack frame”.
Actually these instructions
“work faster”
We could use this approach
which achieves the same
thing
LINK 0 (save RETS and FP to
stack)
LINK 0 (save RETS and FP to
stack)
Function code here
Function code here
P0 = [FP + 4] (copy RETS into
P0
UNLINK (recover RETS and
FP)
JUMP(P0) (use copy of RETS
inside P0
4/8/2015
UNLINK (recover RETS and FP)
(Now wait for RETS value to be
fetched from stack so that it
can be used)
RTS
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
10 / 21
How NOT to add a C++
interrupt service routine
Since ISR’s can
be caused to
happen at
“ANY” time by
external
signals you
can’t code
parameter
passing or
parameter
returning
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
11 / 21
A proper
C++ ISR
After recovering all
registers we need to
RTI -- return from interrupt
NOT – repeat NOT
return from subroutine
Using P0 =[FP + 4];
UNLINK etc.
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
Save and
recover of
all registers
(volatile and
nonvolatile)
FP? ASTAT?
Is saving all
registers needed?
Would saving
12 / 21R7
and P1 be enough?
Players need to audition for the
part of the Core Timer Registers
• Core Timer Scale Register TSCALE
– Requires ability to communicate with TCOUNT register
• Core Timer Count Register TCOUNT
– Requires ability to count backwards in steps of TSCALE
+ 1 (If TSCALE = 1 then timer works twice as slowly)
• Core Timer Period Register TPERIOD
– Requires ability to communicate with TCOUNT register
• Core Time Control Register TCNTL
– Leadership role – has ability to put timer into low power
mode, disable timer, enable auto reload feature which
place TPERIOD into TCOUNT whenever TCOUNT reaches
zero (causing an interrupts). Has a “sticky bit”
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
13 / 21
Core Timer Action
You set Core timer register TSCALE to 0 (decrement by 0 + 1)
You set register TPERIOD to 0x2000
You set register TCOUNT to 0x4000
You enable timer using control register TCNTL
TCOUNT is decreased by 1 until it reaches 0 (0x4000 system clock ticks)
When TCOUNT reaches 1, interrupt is caused and
TCOUNT is reloaded with TPERIOD (0x2000) – counts down again
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
14 / 21
• The play is about to start
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CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
15 / 21
Now we need
to add the actors
Main( ) -- 2 actors
• Doing something
• Number_Interrupts
4/8/2015
ISR routine – 1 actor
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
16 / 21
The play starts
• The main program and ISR operation will
now be demonstrated
• The narrator will now “Build ALL”, and
then DEBUG | RUN
• WHAT ELSE IS NEEDED MAKE THE ISR
WORK?
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
17 / 21
Now we need
to add the actors
Timer – 4 parts
• Core Timer Scale
Register TSCALE
• Core Timer Count
Register TCOUNT
• Core Timer Period
Register TPERIOD
• Core Time Control
Register TCNTL
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
18 / 21
Starting the play
Attempt 2
Main( ) -- 2 actors
• Doing something
• Number_Interrupts
ISR routine – 1 actor
Me_ISR);
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
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Important system registers
• Core interrupt mask register IMASK
– Must be super person with ability to stop /
start all interrupts in the world
– Controls IVTMR interrupt bit
• Core Interrupt Latch register ILAT
– Has ability to remember if interrupt has
occurred (been latched) but is being ignored –
bits set from one to zero when interrupt has
been accepted “recognized” by the processor
• Core Interrupt Pending Register IPEND
– Read but not written – indicates that interrupt
is active (recognized) or nested
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
20 / 21
Now we need
to add the actors
Timer – 4 parts
• Core Timer Scale
Register TSCALE
• Core Timer Count
Register TCOUNT
• Core Timer Period
Register TPERIOD
• Core Time Control
Register TCNTL
4/8/2015
CORE SYSTEM
REGISTERS – 1
person with many
hands
• Core interrupt mask
register IMASK
• Core Interrupt Latch
register ILAT
• Core Interrupt
Pending Register
IPEND
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
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Real demonstration
Main( ) -- 2 actors
• Doing something
• Number_Interrupts
• ISR routine – 1 actor
• Timer registers – 4
actors
• Core registers – 1
actor
Me_ISR);
4/8/2015
CORE Timer Interrupts -- a play,
Copyright M. Smith, ECE, University of
22 / 21