ZBasic System Library
83
ZBasic Microcontrollers
Type
Function returning Long
Invocation
CountTransitions(pin, interval)
Parameter
Method
Type
Description
pin
ByVal
Byte
The pin on which logic transitions will be counted.
interval
ByVal
Single or
Long
The time interval specified in seconds or I/O Timer ticks,
respectively, during which transitions will be counted. See the
discussion below for information on range and resolution.
Discussion
When called, this routine will begin counting logic transitions on the specified pin and will continue until
the specified interval has elapsed. During the counting process processor interrupts are disabled. This
strategy allows high precision in measuring the interval but has the drawback that other processes that
utilize interrupts will not function correctly. Among such affected processes are all serial communication
and multi-tasking. For this reason, the counting interval should be kept as short as possible. RTC ticks
that occur during the counting process are accumulated and the RTC is updated when the counting is
finished.
The specified pin, which you must configure to be an input before calling, is sampled at a fixed rate of
approximately 1/36 (ATtiny, ATmega) or 1/40 (ATxmega) of the CPU frequency. The sample rate, default
resolution and maximum measurement interval are shown in the table below for various CPU
frequencies. You may modify the range and resolution of the measurement interval by modifying the
built-in variable Register.TimerSpeed1. See the special section I/O Timer Prescaler Values for more
details.
Important Values for ZX Devices
Processor
Family
Frequency
Sample Rate
Default
Resolution
Maximum
Interval
ATmega
7.3738 MHz
204.8 KHz
4.883 µS
10,485 sec.
ATmega
14.7456 MHz
409.6 KHz
2.441 µS
5,242 sec.
ATmega
18.432 MHz
512.0 KHz
1.953 µS
4,194 sec.
ATxmega
29.4912 MHz
737.3 KHz
1.356 µS
2,912 sec.
For generic target devices, which can operate at an arbitrary main frequency and RTC frequency, the
important values related to CountTransitions are shown in the table below where F_CPU is the main
processor frequency.
Important Values for Generic Target Devices
Processor
Family
Sample Rate
F_SAMP
Default
Resolution
Maximum
Interval
ATtiny, ATmega
F_CPU/36
1/F_SAMP
2147483647/F_SAMP
ATxmega
F_CPU/40
1/F_SAMP
2147483647/F_SAMP
Resource Usage
This function uses the I/O Timer and disables interrupts during the counting process. However, RTC ticks
are accumulated during the process and the RTC is updated upon completion. The maximum number of
missed RTC ticks that can be tracked is 65,535. A measurement interval longer than that number of RTC
fast ticks will result in incorrect RTC accumulator values. The maximum measurement interval for correct
adjustment the RTC is shown in the tables above.
Compatibility
In BasicX missed RTC ticks are not accounted for.
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