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AN #22 - Two examples about Multi Tasking Print
Multitasking with BASCOM-8051

The following two application notes are provided by David H Lawrence one of our US resellers. As you will find out at http://www.rhombusinc.com David is specialized in Industrial Embedded Processing.

The application note code and another example can be downloaded to

Download file #1
Download file #2

 

'-------------------------------------------------------------------------------
' (c) Copyright 2000 Rhombus Greenville SC
'Code can be used by others providing this header is included in the source.
'===============================================================================
'Getting started in Real-Time Control using BASCOM Part 1
'===============================================================================

'INTRO

'For those who have not yet applied themselves to real-time control using 8-bit
' MCU/MPUs, then these notes and segments of code may help you get started.
'Or if you normally avoid Interrupts, then this first part may change that by
' showing how Timer0 can be expanded into as many timers as you wish, and with
' very little code.

'I am experienced with the 51 at assembler level but new to the Bascom compiler.
' What surprises me the most is that despite the convenience of using a
' compiler, it does not appear to be restricting in any way - hence I hope to
' avoid the use of any assembler code.
'
'Whilst most of what will be described was written a good 15-25 years ago, I
' have checked and it does appear to fit in neatly with present day theory for
' simple real-time control and is classified as 'Co-operative Multi-tasking'.
' There is no forced switching of tasks as with a RTOS, and hence all CPU time
' is devoted to 'getting the job done' and no storage is needed to hold all the
' intermediate states of pre-empted tasks.

'OVERVIEW OF MACHINE CONTROL

'All but the most simple machines require multiple tasks operating in parallel.
' Each task consist of a series of states(steps) with clearly defined logic that
' determines the transition from one state to the next.For example a tank drain
' valve is open, the low level float switch indicates empty but it is known that
' an additional 40 secs needs to be timed for a complete drain and to finally
' close the valve. In parallel a safety locking sequence must be performed on
' another part of the machine before a steam valve is opened.
'
'It is that sort of parallel control that is easily handled by PLCs where there
' is a continuous loop scanning all Inputs, then based on pre-defined logic so
' each task progresses from one stage(step) to the next and resulting in a new
' set of Outputs. With fast loops of Inputing, Processing and Outputing the
' tasks are effectively being controlled in parallel.
'
'When I/O is added that cannot be included in that main I/O loop, such as
' operator interfaces and host serial communication, then the software must use
' interrupts to take whatever time is needed from the main I/O loop to service
' the needs of those asynchronous events. It was at this stage of the PLC's
' history that the limitations of relay ladder logic became more than apparent.
' The PLC solutions to handle these new demands were far from elegant,whereas
' for MPU/MPCs the solutions are a natural and can be achieved very simply
' using the Bascom compiler.

'The methods to be described here have handled slow process control as mentioned
' above, and together with both serial & operator interfaces, They have also
' allowed an 11Mhz 8052 to precisely control a knitting machine's 100+ pneumatic
' outputs in perfect sync with needles flying by at 800 per second, and as the
' operator keyed in a new batch and the host collected production data.
'
'The sharing of CPU time is based on a priority ordered list of tasks and with
' all asyncronous or time critical events being interrupt driven. The main I/O
' tasks are prompted by timer set flags (only one in example) which can also
' serve to distribute CPU demands. Analog inputs can also benefit from using a
' timer rate related to the supply frequency. When each task is completed it
' returns execution to the top of the list.

'Interrupt routines are kept at an absolute minimum and any excessive processing
' needs are off-loaded to the main loop. This holds interrupt service latencies
' to a minimum and avoids the need for either hardware or software priority
' handling to satisfy critical timing issues.

'An example of off-loading interrupt servicing time would be to merely flag the
' timer event shown below, avoid all needs for stacking (it would not change the
' value of any registers, nor even the status flags), and carry out the updates
' in the main loop. For simplicity, the timer ISR below does its processing
' during the interrupt and that will normally be OK but for the most demanding
' applications.
'
'Timer0 ISR code is very small and should be self explicit. In order to show it
' working there is a main loop using those new timers as inputs, a little logic
' for reloading them, and their run status is output to a set of pins to allow
' scoping. For a steady trace the timers are running very fast at a resolution
' of 2mS. If using the Timer0 expansion code in your own applications then it is
' only a matter of defining a new constant for loading the timer.

'Using P3.5 as a scope trigger will show P3.2.3.4 effectively operating in
' parallel and from the same Timer0.

'Part 2 will add transparent keypad input to change the timer values whilst they
' are running & without affecting their operation until the Enter key finalises
' their new value and they snap to the new timing.

'----- INITIALISATION

Dim Timers(4) As Byte , Tic_cnt0 As Byte , Isr_temp As Byte , Io_flag As Bit

Const 2ms = -1793  ' (2/1000)*(11059200/12)-50reload

Config Timer0 = Timer , Gate = Internal , Mode = 1 '16 bit,own code reloads
On Timer0 Timer_0_int
Enable Interrupts 'enable the use of interrupts
Enable Timer0

Priority Set Timer0 'highest priority
Counter0 = 2ms
Start Timer0

'----- MAIN LOOP

'Dumb code to show a set of timers (Timer0 expanded) each running with their own
' individual values, and controlling their own output(a port pin) and effectively
' in parallel.

Main_loop
:
Do
'--Test for I/O prompt
 
If Io_flag = 1 Then Goto Io_control 'Dummy machine control

'--Test for waiting Host messages 'Nothing currently implemented
 
'If Msg_flag Then Goto Msg_rtn

'--Test for keypad activity
 
'If Key_flag Then GoTo Key_rtn

Loop

'----- MACHINE CONTROL

Io_control
:
 
'The 3 stages of Read_Inputs/Process/Write_Outputs
 
'All inputs here are Timer values and hence readily available internally.
 
'The processing here is merely to watch for Timers(1) to count down to zero
 
' and then reset its associated P3.5. On the next pass (when Io_flag is found
 
' set in the Main_loop) we reload all 4 timers with their individual values &
 
' set their individual flags.
 
'All subsequent passes with Timer(1)<>0 has pins P3.2.3.4 updated to reflect
 
' when their counts have reached zero.
 
'That is not a separate Output stage as such, but if we had remote I/O then
 
' the equivalents of P3.2.. would be internal bits and once all processing
 
' was complete, we would output them as a distinct & separate operation as in
 
' a PLC.
 Io_flag
= 0
 
If Timers(1) = 0 Then
 
If P3.5 = 1 Then 'Dont restart till 1 pass later
  
Reset P3.5
 
Else
 Timers
(1) = 20 : Set P3.5
 Timers
(2) = 15 : Set P3.2
 Timers
(3) = 10 : Set P3.3
 Timers
(4) = 5 : Set P3.4
 
End If
 
Else
 
If Timers(2) = 0 Then Reset P3.2 'Could avoid multiple resets
 
If Timers(3) = 0 Then Reset P3.3 ' but no real savings
 
If Timers(4) = 0 Then Reset P3.4
 
End If
 
Goto Main_loop

 
'We return at the highest priority level ignoring the tasks below this one
 
' but know that they will be reached on the next pass, now that the Io_flag
 
' is reset. *Except* if the interval set between scans is not realistic: eg
 
' if the Control routines take 50mS and the Io_flag is set every 40mS then

 
' all other functions will be blocked out. Timing the different functions in
 
' the Main_loop and allocating realistic intervals is not a difficult task.

'----- INTERRUPT SERVICE ROUTINE

'On-chip Timer0 over-flow interrupts steal insignificant slices of CPU time in
' order to update any number of independent timers. The timer values halt at zero
' thereby doubling as 'Done' flags. Where timing ranges cannot be covered by
' single byte values it may be more economical to group them with multiple
' Tic_cnts in preference to expanding all timers to use multiple bytes.

Timer_0_int
:
 
Counter0 = 2ms
 
Start Timer0
 
Set Io_flag
 
Inc Tic_cnt0
 
If Tic_cnt0 => 1 Then '1=2mS for easy scoping
 Tic_cnt0
= 0 '5/50 =10mS/100mS more typical
 
For Isr_temp = 1 To 4
 
If Timers(isr_temp) <> 0 Then Decr Timers(isr_temp)
 
Next
 
'If Timers(0) <> 0 Then Decr Timers(0) 'Alternatively trade code size
 
'If Timers(1) <> 0 Then Decr Timers(1) ' for speed
 
'If Timers(2) <> 0 Then Decr Timers(2)
 
'If Timers(3) <> 0 Then Decr Timers(3)
 
End If
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