> As has been pointed out by a few others, do some research on
state-machines.

A state-machine should not be feared as something complex - it is simply a
structured way of looking at a set of conditional jumps or case statements.
Despite this it is a very powerful and useful model of many types of
reality.

Another major aspect of state machines is that they lend themselves
absolutely ideally to interrupt driven operation. Trying to perform multiple
tasks ("multitasking") without using state machines (or a Real Time
Operating System (RTOS) which effectively does the same thing) is very very
hard. With state machines, on each interrupt you generally increment (or
decrement) a timer and then inspect a list of activities to see if each
requires "service" at the resultant timer value. (Rather than sharing a
single timer, you may have one "timer" or software counter per task and
process the related task when the timer reaches zero.) Any task requiring
service  directs you to its current state. Appropriate activities (if any)
are performed. A decisions is made as to what the next state should be and
the appropriate timer is reset to wake you up again in this new state in due
course. It may sound a little complex if you've never done it but it makes
life much much easier than alternatives.



As a typical example I have a multi state-machine application (versions
happen to be on AVR & SX but that's irrelevant) that

- reads a 9600 baud hardware UART and carries out basic actions
- * implements a 4800 baud software UART to talk to a GPS unit
- * reads a magnetic card swipe reader (2 channel clock and level data
inputs),
- switches on and off data and power to a printer and GPS,
- * reads the speed of a serial square wave (vehicle speed), ,
- Operates a serial input shift register to add extra input lines.
- Writes to a parallel data port.
, (* Optionally implements another software UART to talk to a terminal, )
- * Implements a circular buffer to buffer GPS and magnetic card reader data
to hw UART.
- Sends a subset or all or no GPS data to hw UART.

Items marked with a * are interrupt driven state machines that operate
entirely autonomously as "virtual peripherals" (to coin a phrase :-) ). They
need no program intervention to operate and may as well, from the
programmer's point of view) be hardware peripherals (once you've got their
software working :-) ).


Example of an interrupt driven software UART Receiver
This requires ONLY 2 states!
Assume
    An IRQ occurs at 4 x the bit rate
    At each IRQ process the current state.
    A timer named Timer (just a memory location) is decremented once per
interrupt

This is indicative of state machine method - not of good programming style
:-)
(ie this is to show method, not how it would necessarily be done in detail).

NO bells and whistles here (no start bit recheck, stop bit check etc)
Data "appears" in DatatOut when a byte is received and flag ReadyFlag is set
to 1.
User must retrieve byte before another byte is received and reset ReadyFlag
to 0.
(Could as easily be interrupt on output)
Data is N81 (8 data bits no parity, 1 start, 1 stop).
E&OE (written on the fly).

____________

Initialisation ; call me once at start up
    State = 0
    ReadyFlag=0
    Timer = 1

; __________________

; INTERRUPT CODE:
    ; once per interrupt
     Decrement Timer
    If Timer > 0 then goto otherstuff
    If State = 0 then goto State0
    If State = 1 then goto State1
Otherstuff:
    ..................
; ______

/ State0:
If input = high then     ; Loop forever while line idle
    Timer = 1    ; do state 0 again in 1 tick
If input = low then     ; start bit
    Timer = 6                ; no debounce check
    State = 1                 ; next time do bit inputting
    Counter = 8           ; inputting 8 bits
    Data = 0
goto otherstuff     ; or some other common end point


/ State1:    ; input a bit
Place input bit into carry
Rotate Data right with carry    ; input bit moves into MSB
Decrement Counter
If Counter = 0 then
    State = 0
    ReadyFlag = 1
    DataOut = Data
    Timer = 1               ; half bit time
If Counter >0 then
    timer = 4               ; wait one bit time then come back here
goto otherstuff     ;  Djkstra would be sad :-)

; That's it.

;Try it, you'll like it.

            ; Russell McMahon

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu