### Periodic Timing using the Timer() Command

The BasicX provides a real time clock which is useful in providing for such periodic tasks as turning on a motor for the first five minutes of each hour or similar.

However the Timer() command permits one to set the time for performing a task at any value up to 86400 seconds.

For example, consider an application where a task which takes a variable period of time is performed and it is desired to perform this task periodically. In the following routine, the task is one of flashing an LED a number of times (1 to 10) which may take from 1.0 to nominally 10.5 seconds and it is desired to repeat this every 20 seconds.

The current time in elasped seconds since midnight is fetched using the Timer command. The alarm time is calculated by adding the period, in this case 20 seconds.

The task is then performed and the program then loops until the current time is greater than or equal to the alarm time. Of course, the program could be performing other chores during this looping.

A problem occurs when the alarm time exceeds 86,400 seconds. The clock will simply reach 86,399 and then roll over to zero and thus current time will never be greater than or equal to alarm time. This is handled by subtracting 86400 from the alarm time and first looping until current time is less than or equal to alarm time (when the timer rolls over) and then continuing to loop until current time is greater than or equal to alarm time.

```' Periodic_2.Bas
'
'
' Copyright, Peter H. Anderson, Baltimore, MD, Oct, '99

Sub Main()

Dim RollOverFlag as Byte
Dim NumFlashes as Integer
Dim CurrentTime as Single
Dim AlarmTime as Single

Call OpenSerialPort(2, 9600)	' for debugging
Call PutTime(23, 59, 0.0)	' for testing roll over of timer

NumFlashes = 0

CurrentTime = Timer()

Do
AlarmTime = CurrentTime + 20.0	' every 20 seconds
If (AlarmTime > 86400.0) then	' there was a rollover
AlarmTime = AlarmTime - 86400.0
RollOverFlag = 1
Else
RollOverFlag = 0
End If

NumFlashes = NumFlashes + 1
If (NumFlashes > 10) Then
NumFlashes = 1
End If

Call Flash(NumFlashes)

' now time for the duration of the 20 seconds
If (RollOverFlag = 0) then
Do
CurrentTime = Timer()
Loop Until (CurrentTime >= AlarmTime)

Else
Do
CurrentTime = Timer()
Loop Until (CurrentTime <= AlarmTime)	' wait for the timer to
' roll over

Do
CurrentTime = Timer()
Loop Until (CurrentTime >= AlarmTime)

End if

CurrentTime = AlarmTime

Loop

End Sub

Sub Flash(ByVal NumFlashes as Integer)	' Flash LED a number of times

Dim N as Integer
Dim str as String

str = "..."
Call PutStr(str)

For N = 1 to NumFlashes
Call PutPin(22, 0)
Call Sleep(0.5)
Call PutPin(22, 1)
Call Sleep(0.5)
Next

End Sub
```