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FreeRTOS Tutorial 4 || Counting Semaphore || STM32 || NO CMSIS
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Feb 13, 2020
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View Video Transcript
0:08
hello everyone welcome to controllers
0:12
tech today in this video I am going to
0:16
walk you through the counting semaphores
0:18
in free rtos like I mentioned in the
0:22
post I am NOT going to use the CMC's API
0:25
and instead I will be using free RTOS
0:29
functions directly if you want to browse
0:32
more videos on our TOS you can check out
0:36
the playlist on the top right corner
0:38
let's start by creating the project in
0:41
stm32 cube ide i am using f 4 4 6 r e
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give the name to the project and click
0:57
finish
1:04
this is the cube MX and let's begin the
1:08
setup I am selecting the external
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crystal for the clock let's enable the
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free RT OS enable to use the counting
1:28
semaphore leave everything as it is a
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default task will be created and we
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can't remove it from here
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next select the time-based source
1:49
anything other than systick
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I am enabling you at 2:00 in interrupts
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mode to communicate with the PC let's
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set up the clock now I have 8 megahertz
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crystal on the board and I want the
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controller to run at maximum possible
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clock
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click Save to generate the project
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here is the main dot C file first let's
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open the CMC say ciao I am going to
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include all these directly to my project
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although I don't need all of them I am
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including just incase comment out the CM
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sis file now we are not going to use any
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of the CM sis function here
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[Music]
3:10
I need to include few more files that is
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STD lip and string dot H you will see
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their usage in a while I am removing the
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default task related functions that were
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created by default
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[Music]
3:40
you
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[Music]
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let's start the process by defining the
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task related functions first here HPT
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handler will be the handler for the high
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priority task and HPT task is the entry
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function where the task code will be
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written in the same way I am creating
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three more handlers for the medium
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priority task low priority task and very
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low priority task next we need to create
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a handler for the semaphore I am naming
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it counting sem now I am creating a
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resource which have three values and
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some other variables that are going to
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be used our X data is going to store the
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value received from UART
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[Music]
4:59
[Music]
5:04
first we need to create the semaphore X
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semaphore create counting takes two
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parameters first is the maximum number
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of count you want and second is the
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initial count value I am creating the
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semaphore with three tokens because the
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resource have three values and three
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tasks can access it simultaneously if
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the semaphore is created the handler
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that is counting saime will have any
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other value than null next we need to
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create the tasks X tasks create takes
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the following parameters first is the
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task code or entry function the name
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stack.size
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task parameter priority and the handlers
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address you can give any name to these
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tasks name is not used anywhere in the
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program similarly I have created three
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more tasks with different priorities
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based on their names and finally we will
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start the scheduler now we have to write
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the tasks I will start with high
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priority tasks first
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[Music]
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it's going to print this string when the
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control enters the task now we will take
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the semaphore X semaphore take have two
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arguments first is the semaphore Handler
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and waiting time port max delay means
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it's going to wait forever for the
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semaphore to become available if the
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semaphore is acquired successfully these
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statements will be executed i teo a is
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the function to convert integer to the
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string first parameters is the integer
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value then the character array and then
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the format type after this I am joining
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all these strings and finally send it to
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the UART
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increment the index variable and added a
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delay of three seconds
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in the beginning of the task let's
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release three semaphores
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so that three tasks can acquire them
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medium priority task is also going to be
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same other than releasing semaphore one
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of the very important property of
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counting semaphore is that you can
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release the semaphore at any point in
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the code
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unlike binary semaphore it's not
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necessary to release the semaphore by
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the tasks which have acquired it
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so I have created four tasks with
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different wait time let's build the code
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there are no errors so flash it to the
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board I am using Hercules for the serial
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monitor
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let's put breakpoint inside each task
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okay let's run the program as expected
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the HPT will run first and hit the
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breakpoint counting semaphore was
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created and the statement from HPT note
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that at this point there are three
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tokens available as we release them
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while entering the HBT semaphore was
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acquired data was accessed and hbt was
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suspended for 3 seconds semaphore wasn't
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released by the HPT so there are 2
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tokens available at this point
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now the NPT runs accesses data and goes
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into suspension without releasing the
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semaphore
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[Music]
10:22
touken available is one LPT runs
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accesses the data and goes into
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suspension without releasing the
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semaphore again tokens available are
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zero control have entered the VL PT
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let's see what happens now the l PT
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couldn't get the semaphore because there
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is no token available now l PT will come
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out of suspension in one second and
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preempt the VL PT and it will try to
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take the semaphore as there is no
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semaphore token available all these
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tasks will go in the waiting forever
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states
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[Music]
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[Music]
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[Music]
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next let's cup the you are to receive
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one byte of data in the interrupt mode
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once the interrupt is triggered a
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callback function will be called and we
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must write that too if the received data
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is R we will release the semaphore here
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releasing the semaphore from ISR is not
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that straightforward
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first we must create a variable called
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higher priority task woken and
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initialize it with PD false then we have
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to release the semaphore using X
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semaphore give from ISR whose parameter
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will the address of the variable that we
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created just now along with the handler
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the idea behind this is let's say the
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interrupt was triggered when the control
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was in LP t while the interrupt is being
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processed the higher priority task free
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emptied the lower priority
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now when the ISI exits the control
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shouldn't go back to the lower task and
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therefore high priority task woken
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parameter will be automatically set true
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port n switch will do the context switch
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if the variable is true if the variable
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is false the port ends which have no
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effect you must write this in every is
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our function
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let's not give these semaphores at the
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beginning also I am going to modify the
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tasks a bit instead of suspending they
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will be deleted at the end passing null
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means delete the current task
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the LPT won't delete but it will keep
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going in the suspension let's build the
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code and flash it to the board
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I forgot to start the reception in the
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callback function
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skip the breakpoints let's see semaphore
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created successfully HPT runs but
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couldn't get the semaphore because there
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is none available same for the MPT l PT
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and v l PT all tasks are waiting for the
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semaphore token to become available
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now let's and the character are from PC
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to the controller as you see on
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receiving are the semaphore was released
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and the hbt resumes and delete itself
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again MPT resumes and deletes itself l
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PT resumes and delete now only the l PT
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is left and it will keep resuming every
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500 milliseconds
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let's take a look at the another
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important function in the free RT OS
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you xmf or get count gives the number of
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available tokens of the counting
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semaphore I am again using I Toa to
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convert the number to the string and I
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am going to release the semaphores in
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the beginning I am using same setup for
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the other tasks also
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[Music]
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let's build and run the code now so hbt
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task runs and there are three token
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available right now the number of tokens
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decreases because the tasks are not
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releasing the semaphore when the VL PT
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runs there is not token available and
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therefore it will wait for it forever
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when we send the character our the
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semaphore get released and the VL PT
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runs and again go into the suspension
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till the next token become available
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[Music]
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let's not delete the tasks and keep them
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under suspended states
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[Music]
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[Music]
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I am going to release three semaphores
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from the ISR if the character R is
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received let's debug the code at the
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beginning three semaphores were released
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the number of tokens available decreases
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when there is no token available every
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task will be waiting for the semaphore
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[Music]
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when the character are is sent three
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tokens will be released and the three
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top priority tasks will run again these
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tasks will go in the waiting State for
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the semaphore and same thing will happen
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again if I send the character note that
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only the task h PT m PT and l PT runs
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this is because the VL PT tasks get
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preempted while waiting for the
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semaphore this is it about the counting
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semaphore you can implement it according
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to your requirement
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remember that counting semaphore can be
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released from anywhere in the code
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irrespective of the task holding it
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[Music]
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those who like to have themed like this
18:50
go to eclipse marketplace click popular
19:00
[Music]
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this is the theme I am using darkest
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dark theme this is it guys I hope you
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understood the concept leave comments in
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case of any doubt to download the code
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go to the link in the description have a
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nice day
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you