Programming with interrupt

We will show how interrupt works.  We will program (in a high level language) the interrupt.  We use the simulated IoT (Internet of Things) board to show how to program a small device.    The software to run all the demo is here  iot-rz-19.zip

platform: IoT board simulator
explain IoT board

First, let us show a simple example that use interrupt to do the work. We show how to run a clock using interrupt.  This is the program (in Rz). "clock" is a global variable. int0 is a special subroutine (called Interrupt Service Routine).  In this example, main() does not do anything.  It spends all the time running "while(1) doze()".  doze() puts processor to sleep and waits for the interrutp to occur.

clock

int0()
  clock = clock + 1 
  print(clock)

main()
  clock = 0
  while(1)
    doze()

In the IoT board, the default setting of the IoT simulator is to run for 10,000 clocks. There are two interrupts already wired to two timers. The interrupt0 is enable and the timer0 is 150 (clocks).

How interrupt work

To understand the above program we need to know how interrupt work. A CPU is interrupted when an external signal triggerred it. Once CPU recognised the interrupt, it completes the current instruction then jump to "interrupt service routine" (ISR for short).  ISR performs some task. When it finishes, it "reti" (return from interrupt).

See what is the assembly code of the above program

:int0     ; Interrupt Service Routine
...
  ld r1 clock
  add r2 r1 #1
  st r2 clock
...
reti       ;  return from interrupt

:main
...
  mov r1 #0
  st r1 clock
  jmp L103
:L104
  swi        ;   <<<<<<<  sleep and wait for int
:L103
  mov r1 #1
  jt r1 L104
...
  ret rads

You probably notice that call ISR is similar to call to a subroutine with "jal r2 ads" and return from subroutine by "ret r2".

However, "jal" occurs by software but  "call ISR" is done by hardware (external signal)

If we put a periodic clock to the interrupt pin, then the interrupt can occurs periodically (for example every 20 ms) without software intervention. This is how the IoT board is wired.

How ISR knows where to jump to? 

We must set the "interrupt vector" which is the address of the ISR.  In IoT platform it is at M[1000].

Observe the first line in main

:main
...
  mov r1 #int0     ; ***** store &int0 to M[1000]
  st r1 1000
  mov r1 #0
  st r1 clock
  jmp L103
...

on IoT platform we have two interrupts, int0 and int1. Let us demo them

Use two interrupts to run two programs concurrently

num
letter

int0()
  print(num)
  num = num + 1

int1()
  printc(letter)
  letter = letter + 1



Use interrupt to run multitask

The next example shows a more "realistic" multitask program. We can do "foreground/background"  multitask using "main" to run a continuous task and "int0" to run a periodic (or other intermittent) task.  This is usually called "foreground/backgroud". 

main() will run "background"
int0() will run "foreground"

We do not use "doze" in main.  Here is the program

num
char

int0()
   print(num)
   num = num + 1

delay()
  i = 0
  while(i < 10)
     i = i + 1

main()
  settimer0(400)
  num = 1
  char = 65
  while(1)
    printc(char)
    char = char + 1
    if(char > 90) char = 65
    delay()

The main program prints a continuous stream to characters ABC...ZA...Z.... and from time to time int0() prints 1, 2, 3 .... .  The routine delay() is run to slow down the main program so that you can see the output on the screen clearly.  Assume this program file named "multi.txt".  Here is what you see on your screen when compile, assembly, and run this program.

Graphics anyone?

 The final program will do some graphics!   IoT board contains an input port 10 that is connected to a wave generator (sine wave).  We can read the port using int0().  This is called "sampling".   The value is the amplitude of a sine wave.  This is the program, read sine wave.

int0()
  x = readport(10)
  print(x)

main()
  while(1)
    doze()

Here is the output from the screen

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