Rabbit 2000, Getting Started

Getting Started Manual

15

The statement at (3) waits for a time delay, in this case 200 ms.  The costatement is being 
executed on each pass through the big loop.  When a 

waitfor

 condition is encountered 

the first time, the current value of 

MS_TIMER

 is saved and then on each subsequent pass 

the saved value is compared to the current value.  If a 

waitfor

 condition is not encoun-

tered, then a jump is made to the end of the costatement (4), and on the next pass of the 
loop, when the execution thread reaches the beginning of the costatement, execution 
passes directly to the 

waitfor

 statement.  Once 200 ms has passed, the statement after 

the waitfor is executed.  The costatement has the property that it can wait for long periods 
of time, but not use a lot of execution time.  Each costatement is a little program with its 
own statement pointer that advances in response to conditions.  On each pass through the 
big loop, as little as one statement in the costatement is executed, starting at the current 
position of the costatement’s statement pointer.  Consult the Dynamic C Reference Man-
ual for more details.

The second costatement in the program debounces the switch and maintains the variable 

vswitch

.  Debouncing is performed by making sure that the switch is either on or off for 

a long enough period of time to ensure that high-frequency electrical hash generated when 
the switch contacts open or close does not affect the state of the switch.  The 

abort

 state-

ment is illustrated at (5).  If executed, the internal statement pointer is set back to the first 
statement within the costatement, and a jump to the closing brace of the costatement is 
made.

At (6) a use for a shadow register is illustrated. A shadow register is used to keep track of 
the contents of an I/O port that is write only - it can’t be read back. If every time a write is 
made to the port the same bits are set in the shadow register, then the shadow register has 
the same data as the port register. In this case a test is made to see the state of the LED and 
make it agree with the state of vswitch. This test is not strictly necessary, the output regis-
ter could be set every time to agree with vswitch, but it is placed here to illustrate the con-
cept of a shadow register.

To illustrate the use of snooping, use the watch window to observe 

vswitch

 while the 

program is running.  Add the variable 

vswitch

 to the list of watch expressions.  Then 

toggle 

vswitch

 and the LED.  Then type 

<ctrl-U>

 to observe 

vswitch

 again.

3.4  Advantages of Cooperative Multitasking

Cooperative multitasking, as implemented with language extensions, has the advantage of 
being intuitive.  Unlike preemptive multitasking, variables can be shared between differ-
ent tasks without having to take elaborate precautions.  Sharing variables between tasks is 
the greatest cause of bugs in programs that use preemptive multitasking.  It might seem 
that the biggest problem would be response time because of the big loop time becoming 
long as the program grows.  Our solution for that is a device caused slicing that is further 
described in the Dynamic C Reference Manual.