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STM32 TIMERS #8. Cascading Timers || Make 48bit Counter || External clock mode
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Feb 4, 2023
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View Video Transcript
0:00
foreign
0:01
[Music]
0:09
hello and welcome to controllers Tech
0:13
this is the continuation of the stm32
0:16
timer series and we will continue with
0:19
the timer synchronization
0:21
today in the eighth video of the series
0:23
we will discuss how to Cascade the
0:25
timers together and make a bigger
0:27
counter than the default 16-bit
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the video title says making a 48-bit
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counter but actually you can make the
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counter however big you want depending
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on the number of timers you have
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you must watch the previous video to
0:42
understand how the cascading works for
0:44
these timers the video is in the top
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right corner
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we will first see what is the need for
0:50
such a big counter
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let's assume our APV timer clock is at
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90 megahertz and we use the prescaler of
0:58
9 to bring it down to 10 megahertz
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this 10 megahertz is counter frequency
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basically the counter will be counting
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at this rate
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this means each count in the counter
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will take 0.1 microseconds
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now most of the time is in stm32 are
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16-bit in size so the Maximum Auto
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reload value we can set is 65535
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this is the maximum number of counts the
1:27
counter can count
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this makes the period of the counter
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equal to 6.5 milliseconds
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and the lowest frequency we can measure
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with this counter will be 150 Hertz
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so ideally the counter can measure the
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highest frequency of 10 megahertz and
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lowest of 150 Hertz
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obviously it cannot measure 10 megahertz
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due to the limitations like interrupt
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processing time but let's assume for now
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that it is possible
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now the problem is how do we measure
1:59
even lower frequencies like 100 Hertz or
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50 hertz or even lower ones like one
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Hertz
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of course you can reduce timer clock by
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increasing prescaler but then it will
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also reduce the highest frequency you
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can measure
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the issue can be resolved by using a
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larger counter
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say for example if we had a 48-bit
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counter we could get the auto reload
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value of
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65535 cube
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the counter is still running at 10
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megahertz but the maximum number of
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counts have been increased now and so it
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can measure even the very low
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frequencies
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we will achieve this by cascading the
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timers together so depending on how many
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timers you can Cascade you can make the
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counter as big as you want
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let's start the cube IDE and create a
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new project
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I am using stm32f446re
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give some name to the project and click
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finish
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first of all I am selecting an external
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Crystal to provide the clock
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the board has 8 megahertz Crystal on it
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and I want the system to run at 180
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megahertz
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let's configure the timers now
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I am configuring the timer 1 in the
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input capture mode so that we can
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measure the input signal
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the pin pa8 will be used as the input
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pin for the signal
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actually since I gave you the example
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with a PB clock the 90 megahertz let's
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change the system clock to 90 megahertz
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the apb2 timer clock is at 90 megahertz
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now and the timer 1 is connected to it
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so the timer 1 is also running at 90
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megahertz
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let's use the prescaler of nine this
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will bring the clock down to 10
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megahertz
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also I am using the auto reload value of
4:00
10 000. I will show the calculation in a
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while
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the timer 1 is the master timer so we
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will enable the trigger event selection
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to update event
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here is the configuration for the input
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capture and I am leaving it as it is
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let's understand the calculation now
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the counter one frequency is 10
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megahertz right now
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this means that each count in the
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counter 1 takes 0.1 microseconds
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I have set the auto reload value to 10
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000 so the counter one will have a
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period of one millisecond
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this means the counter one can measure
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the signal whose period is between 0.1
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microseconds to 1 millisecond
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of course 0.1 microsecond is an ideal
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case and it's not possible for this
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microcontroller to measure it
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now for the signals with a period more
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than one millisecond we will Cascade
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another timer with this one
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I am configuring the timer 3 for this
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purpose
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timer 3 will be configured in the slave
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mode and that 2 in the external clock
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mode 1. the trigger Source will be from
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the timer one so we will set it to ITR
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0. watch the previous video if you don't
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understand this
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now the timer 1 output frequency will be
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equal to APB time a clock divided by the
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prescaler times the auto reload value
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this will be equal to one kilohertz
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timer 3 will be clocked by this
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frequency and if we keep the prescaler
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at 1 the counter 3 will be running at
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one kilohertz two
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this means each count in the counter 3
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will take one millisecond
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if we set the auto reload to maximum 65
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535 the counter 3 will have a period of
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65 seconds
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basically the count of three can measure
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frequencies between one kilohertz and
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0.015 Hertz
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now think of these two counters as the
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single counter and we have ourselves the
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frequency bandwidth of 10 megahertz to
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0.015 Hertz
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this is a huge bandwidth considering we
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were only able to achieve up to 150
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Hertz with the single counter
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we will use the input capture mode with
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the interrupt so enable the respective
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interrupt for the timer 1.
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that's it for the cube MX configuration
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click save to generate the project
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inside the main function we will start
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the timer 1 channel 1 in the input
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capture interrupt mode
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also we will simply start the timer 3 so
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that its counter can also count
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when the rising Edge will be captured by
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the timer 1 an interrupt will be
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triggered and the input capture complete
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callback will be called
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we will write the rest of the code
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inside this callback function
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here first of all I am defining some
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variables which we will need later
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counter 1 and 3 will hold the values of
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the respective counters
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his first captured will be used to reset
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these counters
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now first we will check if the Callback
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is called by the timer 1 channel 1.
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then we will check if the first Edge is
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captured or not
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if this is the first Edge then reset the
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counters and set the first capture to 1
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indicating the first Edge has been
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captured
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now when the interrupt is triggered by
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the second Edge we will first read the
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respective counters
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and now calculate the frequency
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to calculate the frequency we will
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convert the counter 3's value in terms
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of the counter one's value
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basically every tick encounter 3 is
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equivalent to having 10 000 ticks in
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counter one so to make them equal we
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will multiply counter 3 with 10 000.
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since the overall counter value is now
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in terms of the counter 1 to calculate
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the frequency we will simply divide the
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counter one's clock by the counter value
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after the frequency has been calculated
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we will reset the first captured so that
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the whole process can start from the
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beginning
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let me revise this one more time
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when the first Edge arrives we will
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reset the counters
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now when the second Edge arrives we will
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read the counter values
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then convert all the counter values in
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terms of the counter one and finally use
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the counter one clock to calculate the
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frequency
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if you are confused about this part you
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can think of this as the simple formula
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counter is equal to counter one plus
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counter two times the counter one's
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reload Value Plus counter three times
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the counter two's reload value times the
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counter one's reload value and so on
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of course this is considering the three
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timers are in Cascade mode with timer
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one clocking the timer 2 which is
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clocking the timer three
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this is it for the coding part let's
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build and debug the code now
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I am using the any triple five module
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which basically outputs square waves of
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different frequencies
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I have connected its output to the logic
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analyzer and also to the input capture
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pin of the stm32
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let's add the counters and frequency to
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the live expression
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for some reason it's not working
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all right I have found the issue
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here instead of timer Channel One it
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should have been how Tim active Channel
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One
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let's build and debug the code again
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now it's working all right
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you can see the frequency is two hundred
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thousand
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if we check the logic analyzer it also
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gives similar values
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now I am going to reduce the frequency
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values
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you can see the frequency is decreasing
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and now it's around 97 kilohertz
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let's check the analyzer
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here we have a similar frequency
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now the reading is around 13.9 kilohertz
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and you can see the similar reading in
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the logic analyzer
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let's reduce it further
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notice that so far the counter 3 is
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always zero this is as we discussed the
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counter one is capable of reading up to
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one kilohertz and if the signal period
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is less than one millisecond the counter
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3 cannot count up
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so as long as our frequency is higher
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than one kilohertz the counter three
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will remain zero
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but if we go lower than one kilohertz
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now the counter three also starts
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contributing to the frequency
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measurement
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right now the frequency is around 558
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Hertz
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and this is exactly what the analyzer is
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also showing
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let's keep going lower
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now the frequency is around 17.5 Hertz
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as we discussed in the beginning this is
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way lower than the limit we set for a
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single 16-bit counter
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let's keep reducing
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this 0.57 Hertz is the minimum frequency
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achievable using this module I have
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and if we measure it in the analyzer it
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is exactly the same
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I cannot show lower values than this but
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with my current setup I can go as low as
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0.015 Hertz
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so this is how cascading the timers can
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be used to increase the counter size to
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whatever resolution you want
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this wasn't exactly a 48-bit counter but
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if you do use that you can achieve an
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extremely high frequency bandwidth
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as I mentioned earlier to calculate the
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frequency while using the 48-bit counter
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you can use the counter one clock divide
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by the counter one plus the counter 2
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times the 65
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535 plus the counter three times the 65
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535 squared
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basically we have to convert every other
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counter's value in terms of the counter
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one value and then divide the counter
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one clock by the overall counter value
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so I hope you understood the concept the
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need and the process of implementing the
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48-bit counter
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we will continue with the timer series
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and more videos will be coming out soon
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which will cover some other timer modes
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this is it for today
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the link to download the code is in the
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description below
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leave comments in case of any doubt
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keep watching and have a nice day ahead
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[Music]
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