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--- en:avr:timers [2010/07/26 17:43] – corrected typos Wember
+++ en:avr:timers [2020/07/20 09:00] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== Counters/Timers ======
-Counters, which in some sense can be called timers, are one of the most important sub-functions of a microcontroller. These enable to precisely time processes, generate signals and count events. A counter converts the number of input cycles to a binary value using an array of triggers. The maximum number of counted cycles depends on the length of this array and this is marked by the length of the binary code. AVR has 8- and 16-bit counters. If a timer has reached its maximum value (255 in 8-bit and 65535 in 16-bit counters), the next cycle will generate an overflow and the counter resets back to 0. A counter's clock signal can come from the clock signal of the microcontroller and in this case it is possible to decrease its value using a prescaler. Some AVRs have an internal independent clock signal generator, which can be modified to run faster using a frequency multiplier. Counters also differ in application cases and work modes.
+Counters, which in some sense can be called timers, are one of the most important sub-functions of a microcontroller. These enable to precisely time processes, generate signals and count events. A counter converts the number of input cycles to a binary value using an array of triggers. The maximum number of counted cycles depends on the length of this array, and this is marked by the length of the binary code. AVR has 8- and 16-bit counters. If a timer has reached its maximum value (255 in 8-bit and 65535 in 16-bit counters), the next cycle will generate an overflow and the counter resets back to 0. A counter's clock signal can come from the clock signal of the microcontroller, and in this case it is possible to decrease its value using a prescaler. Some AVRs have an internal independent clock signal generator, which can be modified to run faster using a frequency multiplier. Counters also differ in application cases and work modes.
-===== Counter's default mode =====
+===== Counter's Default Mode =====
-In the default mode, a counter does nothing more than count sequential numbers all the time. Its value can, of course, be read and changed from the program at any time. The only additional function in the default mode is to cause an interrupt on counter overflow. The default mode is typically used to execute a section of the program at certain intervals.
+In the default mode, a counter does nothing more than continually count sequential numbers. Its value can, of course, be read and changed from the program at any time. The only additional function in the default mode is to cause an interrupt on counter overflow. The default mode is typically used to execute a section of the program at certain intervals.
 <box 100% round #EEEEEE|Example>
@@ Line 42: / Line 42: @@
 </code>
-The counter in this example will not generate the interrupt in exactly 10 ms though, in order    to do that it would require giving the counter a decimal value and this is not possible. To achieve a precise interval between the interrupts, both the prescaler value and the initial value of the counter have to be chosen so that their division results in an exact number. This is not always possible, especially with 8-bit counters as their value range is quite small. To achieve a more precise or larger interval, a 16-bit counter can be used.
+The counter in this example will not generate the interrupt in exactly 10 ms, though, in order to do that it would require giving the counter a decimal value, and this is not possible. To achieve a precise interval between the interrupts, both the prescaler value and the initial value of the counter have to be chosen so that their division results in an exact number. This is not always possible, especially with 8-bit counters as their value range is quite small. To achieve a more precise or larger interval, a 16-bit counter can be used.
 </box>
-==== External clock counter ====
+==== External Clock Counter ====
 It is also possible to use an external clock source as a counter's clock signal. AVR has a pin called Tn for this purpose, n marking the number of the counter. External clock signal and the polarity can be selected using the prescaler register.
-==== Timing events ====
+==== Timing Events ====
 Since the counters allow timing operations, more complex AVR microcontrollers have an option to time specific events on a hardware level. This part of the counter is called an input capture unit. There is a choice between two events: the logical change in the value of a special input pin or in the value of the analog comparator result. If the selected event occurs, the counter's value is written to a special register, from where it can be read at any time. If the event is longer than the overflow time of the counter, the program has to count the overflows as well and take them into account when calculating the final result.
@@ Line 70: / Line 70: @@
 	// The result is valid only if the counter
-	// has not overflown yet
+	// has not overflowed yet
 	if (!(TIFR & (1 << TOV1)))
 	{
@@ Line 103: / Line 103: @@
 </code>
-The program fires an interrupt each time a rising front occurs in the external signal. During the interrupt, the counter is checked for overflows - this can happen if the frequency of the signal is below 122 Hz (8 MHz / 2<sup>16</sup>) and in this case the value of the counter doesn't reflect a real period any more. The frequency is calculated using 32-bit numbers to get the inverse of the period. First thing is to set the counter to 0, because the timer works on the same clock signal as the processor and each instruction execution occurring after the external event shortens the measured period corrupting the result. The maximum measured frequency is limited by the time spent in the interrupt program.
+The program fires an interrupt each time a rising front occurs in the external signal. During the interrupt, the counter is checked for overflows - this can happen if the frequency of the signal is below 122 Hz (8 MHz / 2<sup>16</sup>) and in this case the value of the counter doesn't reflect a real period anymore. The frequency is calculated using 32-bit numbers to get the inverse of the period. The first thing is to set the counter to 0, because the timer works on the same clock signal as the processor and each instruction execution occurring after the external event shortens the measured period corrupting the result. The maximum measured frequency is limited by the time spent in the interrupt program.
 </box>
-Catching events and registering the time it took for them to occur can also be resolved at the software level. It is possible to use external or other interrupts and read the value of the counter during these events. The hardware-level event catching is meant to run independently form the main program and time relatively short (or frequent) events.
+Catching events and registering the time it took for them to occur can also be resolved at the software level. It is possible to use external or other interrupts and read the value of the counter during these events. The hardware-level event catching is meant to run independently from the main program and time relatively short (or frequent) events.
-===== Signal generating =====
+===== Signal Generating =====
-More complex counters can generate a signal, in addition to timing the length of one. For this purpose the counter has an output compare unit and a compare match output unit. The output compare unit has registers with the same bit-width as the counter and the values of these registers are compared to the value of the counter while it is running. An interrupt can be generated and special pins' values can be changed each time the counter's value is equal to the value in the compare unit register. At this moment a pin can either be set high or low or inversed. The signal is generated by changes in the value of the output pin.
+More complex counters can generate a signal, in addition to timing the length of one. For this purpose the counter has an output compare unit and a compare match output unit. The output compare unit has registers with the same bit-width as the counter and the values of these registers are compared to the value of the counter while it is running. An interrupt can be generated and special pins' values can be changed each time the counter's value is equal to the value in the compare unit register. At this moment a pin can either be set high, low or inversed. The signal is generated by changes in the value of the output pin.
-In some signal generating modes, the counter's maximum value can be altered. The counter's physical size will remain the same, but a comparison register is used to reset the counter at a specific count. The previous examples could also be solved by using this method, but the function is rather for changing the period of the signal. In addition to that, a counter can be configured to a mode where it works with both incrementing and decrementing.
+In some signal generating modes, the counter's maximum value can be altered. The counter's physical size will remain the same, but a comparison register is used to reset the counter at a specific count. The previous examples could also be solved by using this method, but the function is rather for changing the period of the signal. In addition to this, a counter can be configured to a mode where it works with both incrementing and decrementing.
-The counters and the signal generating modes using them are one of the most complex peripheral modules in an AVR. Writing about all of them here would be a huge waste of time and typically there is no need to know everything in order to use them. The following describes one of the most common PWM signals in robotics. The rest can be read from the AVR documentation.
+The counters and the signal generating modes using them are one of the most complex peripheral modules in an AVR. Writing about all of them here is beyond the scope of this text, and typically there is no need to know all aspects in order to use them. The following describes one of the most common PWM signals in robotics. The rest can be read from the AVR documentation.
 ==== Pulse Width Modulation ====
@@ Line 123: / Line 123: @@
 <box 100% round #EEEEEE|Example>
-Task: using an 8MHz ATmega128, generate two speed regulating servo motor signals. Use pin PB5 (OC1A) to generate a pulse width of 1 ms and pin PB6 (OC1B) to generate pulse width of 2 ms.
+Task: Using an 8MHz ATmega128, generate two speed regulating servo motor signals. Use pin PB5 (OC1A) to generate a pulse width of 1 ms and pin PB6 (OC1B) to generate pulse width of 2 ms.
 <code c>

en/avr/timers.1280166189.txt.gz · Last modified: 2020/07/20 09:00 (external edit)