A Play Core Timer Interrupts Acted by the Human Microcontroller Ensemble from ENCM511

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 2 / 21 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? 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 }

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 3 / 21 Timers available on Blackfin Watchdog timer – Hardware Reference Core timer – Hardware Reference 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

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 4 / 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 is removed by the compiler automatically

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 10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 5 / 21

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 6 / 21 The assembly code generated from main.cpp (Debug option) Number of interrupts checked each time around the loop

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 7 / 21 The assembly code generated from main.cpp (Release option) Number of interrupts NOT checked In loop

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 8 / 21 Main.cpp as assembly code Release option “Volatile” used

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 9 / 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)

Differences between C++ compiler generated code and ours Compiler uses the instructions to build a “stack frame”. Actually these instructions “work faster” LINK 0 (save RETS and FP to stack) Function code here P0 = [FP + 4] (copy RETS into P0 UNLINK (recover RETS and FP) JUMP(P0) (use copy of RETS inside P0 10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 10 / 21 We could use this approach which achieves the same thing LINK 0 (save RETS and FP to stack) Function code here UNLINK (recover RETS and FP) (Now wait for RETS value to be fetched from stack so that it can be used) RTS

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 11 / 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

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 12 / 21 A proper C++ ISR Save and recover of all registers (volatile and nonvolatile) After recovering all registers we need to RTI -- return from interrupt NOT – repeat NOT return from subroutine Using P0 =[FP + 4]; UNLINK etc. FP? ASTAT? Is saving all registers needed? Would saving R7 and P1 be enough?

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 13 / 21 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”

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 14 / 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

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 15 / 21 The play is about to start

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 16 / 21 Now we need to add the actors Main( ) -- 2 actors Doing something Number_Interrupts ISR routine – 1 actor

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 17 / 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?

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 18 / 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

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 19 / 21 Starting the play Attempt 2 Main( ) -- 2 actors Doing something Number_Interrupts ISR routine – 1 actor Me_ISR);

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 20 / 21 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

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 21 / 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 CORE SYSTEM REGISTERS – 1 person with many hands Core interrupt mask register IMASK Core Interrupt Latch register ILAT Core Interrupt Pending Register IPEND

10/31/2015 CORE Timer Interrupts -- a play, Copyright M. Smith, ECE, University of Calgary, Canada 22 / 21 Real demonstration Main( ) -- 2 actors Doing something Number_Interrupts ISR routine – 1 actor Timer registers – 4 actors Core registers – 1 actor Me_ISR);