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ZBasic System Library
16
ZX Microcontroller Family
specific details). If the flag is False, the routine sets it to True and then goes about using the timer.
When it has finished its function, it sets the flag back to False.
Your code can use the timer busy flag as the parameter to the Semaphore() function in order to get
exclusive access to the timer. Of course, you must set timer busy flag to False when your code is
finished with the timer to indicate that the timer is no longer in use. Likewise, you may want to acquire a
semaphore on a timer busy flag for the I/O Timer before calling a System Library routine that uses I/O
Timer. If you succeed in setting the semaphore you’ll know that the timer is not already in use. An
example of code for this purpose (for ZX devices that use Timer1 for the I/O Timer) is shown below.
' wait until the timer is available
Do While (Not Semaphore(Register.Timer1Busy))
Call Sleep(0.5)
Loop
' use the timer
Call LockTask()
Register.Timer1Busy = False
Call ShiftOut(12, 13, 8, &H55)
Call UnlockTask()
Note, particularly, the line immediately before the call to ShiftOut(). After the semaphore is acquired
Regsister.Timer1Busy will be True. Unless it is set to False, the call to ShiftOut() will fail
because that subroutine will think that the timer is in use.
Caution: setting the busy flag for a timer to True and never setting it back to False will prevent System
Library routines that require that timer from functioning.
Processor Speed Issues
Although the standard clock speed for ATmega-based ZX devices is generally 14.7456MHz, special
versions are available that run slower and faster. ZX devices based on the ATxmega run at 29.4912MHz.
The table below summarizes the differences that arise due to the difference in operating speeds.
Processor Speed Variations
Issue
7.3728MHz
14.7456MHz
18.432MHz
29.4912MHz
RTC Tick Frequency
512 Hz
512 Hz
500 Hz
512 Hz
RTC Fast Tick Frequency
512 Hz
1024 Hz
1 KHz
1024 Hz
RTC Timer Frequency
115.2 KHz
230.4 KHz
72 KHz
230.4 KHz
Multi-tasking Time Slice
1.95 mS
1.95 mS
2.0 mS
1.95 mS
Default TimerSpeed1 Units
135.6 nS
67.8 nS
54.4 nS
67.8 nS
Default TimerSpeed2 Units
1.085 µS
1.085 µS*
434 nS
271 nS
CountTransitions() Sample Rate
204.8 KHz
409.6 KHz
512 KHz
737.3 KHz
Note, particularly, that the value for TimerSpeed2 at 14.7456MHz is scaled to match the value
corresponding to operating at 7.3728MHz. This is done for compatibility with BasicX devices that operate
at the latter speed. Consult the preceding sections for information on which routines use the
TimerSpeed1 and TimerSpeed2 values.
It is highly recommended to use the built-in values Register.CPUFrequency,
Register.RTCTickFrequency, and Register.RTCTimerFrequency instead of using hard-coded
values. Doing so also simplifies code that must run on multiple devices that operate at different speeds.
See the descriptions in the ZBasic Language Reference Manual for more details.
Detailed Descriptions
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