ZBasic System Library
343
ZBasic Microcontrollers
Type
Subroutine
Invocation
WaitForInterval(flags)
Parameter
Method
Type
Description
flags
ByVal
Byte
A set of flag bits that control the operation. See the
discussion below.
Discussion
This routine allows a task to suspend itself and wait for an interval timer to expire. The length of the
interval is set by the routine SetInterval. Note that there is only one interval timer that is shared by all
tasks. This means that at most one task may be awaiting the expiration of an interval at any one time. If
another task is already awaiting an interval, calls to WaitForInterval will return immediately.
The bit values for the flags parameter are described in the table below.
Interval Timer Flag Values
Value
Description
&H01
Wait until the next interval expires.
&H02
Reset the interval counter to its original value.
The remaining bits are currently undefined but may be employed in the future.
After a call to SetInterval the interval counter is decremented on every RTC tick. When it reaches
zero, if a task is awaiting the expiration of the interval, that task will be scheduled to run immediately. If
no task is awaiting the expiration of the interval, the fact that the interval expired is recorded and the
interval counter is reset to the original value.
If the flags value is zero when a task calls WaitForInterval and an interval expiration has previously
been recorded (with no waiting task), the call will return immediately. Otherwise, the task will be
suspended until the interval expiration. If the flags value is &H01, the task will be suspended until the
next expiration of the interval. If the flags value is &H03, interval counter will be reloaded and then the
task will be suspended until the interval expires. The last mode of operation is similar to a task calling
Sleep. The difference is that when the interval expires, the task is immediately reactivated. With a
Sleep call, the task will execute again when its sequential turn comes up.
A task awaiting the expiration of an interval has lower priority than one awaiting an interrupt. Note that a
task awaiting the expiration of an interval will exhibit some latency between the expiration of the interval
and when the waiting task begins execution. The latency depends on a number of factors including the
specific instruction being executed at the time and the number and frequency of system interrupts that
need to be handled. Instructions that may take a long time to execute such as OutputCapture, ShiftIn,
ShiftOut, X10Cmd, etc. will introduce more latency than simple instructions like assigning a value to a
variable.
Example
Call SetInterval(1.0)
Do
Call WaitForInterval(0)
<other code here>
Loop
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