Programmable Interval Timer 8253/8254 – Peripheral Interfacing with 8085 Microprocessor

Programmable Interval Timer 8253/8254 – Peripheral Interfacing with 8085 Microprocessor


Do subscribe to Ekeeda channel and press bell icon to get updates about latest Engineering HSC and IIT-JEE Mains and Advanced videos. Hello Friends,In this video we will study about the 8253 or 8254 for programmable interval timer it’s block diagram and pen description so let’s start with the topic. The programmable interval timer its IC number is 8253 or 8254 it’s an especially designed timer which is used for the Microprocessor for the counting and the timing operations so we can say that it’s an especially designed chip for the Microprocessors to perform the timing and counting operations. And these timing and counting operations they are implemented through software that means we are writing the software instructions we are programming the chip and then the chip is performing the timing and the company operations that is by the name of this IC is programmable because it can be programmed we have the software for it and interval timer because it is providing the time intervals or it is providing the time delays at some specific time intervals okay so we can have fear that these are timing and counting it is provided by the or it is implemented through the software so now comes that what is the need of this programmable interval timer why the microprocessor needs the use of this chip so let’s take an example that if the microprocessor suppose it’s performing any task so microprocessor it is performing any task and it wants that after one after the completion of one task it takes some time delay okay suppose for T seconds it wants the time delay between two tasks so if we have like first task we have AI then the microprocessor is going to take a time delay of T seconds and then it will perform the second task suppose J now this time delay of T seconds if we want in through the microprocessor then the software a solution of this time delay is that we will have some we will have some timer here and we will write a deal a subroutine time delay subroutine is there and this delay subroutine it is providing the time delay of T seconds now to get the time delay of T seconds we have to initialize a register or register pair by some count so for this time delay we will need to initialize a register or register pair by account which is equivalent to the time delay of T seconds okay now each time the microprocessor it performs the task because it has to perform any task and after itis first circuit has completed then it will all that subroutine it will initialize the register and register pair by the count then it will decrement the count until it becomes zero and then the count is zero then it is going to perform the J toss and then J task will be completed after the completion of J task it will again have a delay of three seconds so again it is going to call the subroutine it will initialize the register or the register pair with the count it will decrement the count until it becomes zero so this has become a complex task okay every time the microprocessor it has to call the subroutine and because every time it wants the time delay of T seconds so in between the task I and J the microprocessor cannot perform any other does because it has to concentrate on the execution of the subroutine so that it can get a perfect time delay of T seconds so if we see that when the microprocessor is performing a lot of tasks and every time it needs to have a break or it have it requires a time delay then this task this complete procedure is becoming very complex and the microprocessor is bit busy in it it cannot perform any other task in between so to release the microprocessor from all these complexity we can use an external timer and what this external timer it is going to do that external timer will every time the microprocessor it completes the task that timer it will work it will initialize the counter and it will decrement the counter and it is going to provide the time delay of T seconds so the microprocessor it is not busy in executing the subroutine it is free from its job it can perform any other job and that task of providing third time delay of T seconds will be done by the external so that is the need of an external timer and that is why the eight to five three or into PI for programmable interval timer is developed for use by the micro processors to perform the timing and accounting operations so that is the need of an external timer or we can say that the programmable interval timer eight two five three and eight two five four these are a two five three it is the lower version and a two five four it is the higher version of the programmable interval timer so now we will start with the features of eight two five three or a two five four so now let’s come to the features of eight two five three and eight two five four you so as the programmable interval timer eight to five three or eight to five for it is performing the timing and accounting operation so it will need some timers so this I see it consists of three identical 16-bit doctors TM 0 T em1 and em2 so these counters they can be initialized by a 16 16-bit count and then these counters are going to perform the counting operations now so it has three 8-bit three identical 16-bit captors TM 0 TM 1 TM 2 now these counters these are basically the dumb factors means they are initialized with the highest value of the count and then the count will be decrement at each transition from high to low of the clock input so these are three identical 16-bit counters these are the dumb compass and they will count at each high-to-low transition of clock input suppose we have a clock here clock pulse is there so every high this is high and this is low so every high to low transition it is going to count the counters okay it is going to decrement suppose we have started with 100 so from high to low if it is going then it will decrement the counter by 1 it will become 99 then again from high to low it is going to decrement the counter it will become 98 so in this way the counter will work till it’s become 0 so this down counters means it is decrementing the count at each high to low transition of the clock input now as it is having the three counters so these three counters they can be operated in any of the 6 modes of 8 to 5 3 the 8 to 5 10 8 to 5 for Isis this timer it is having six modes 6 operation modes are there and these timers the 3 timers they can be operated in any of the six modes independent of the operation like if it is TM not it is working in mode 1 it will be independent from the other two timers which are working in some different modes okay so the functioning of each of the timer it is independent of each other so we can say that we can we can change the modes of operation of all the three timers or we can say that the timer’s are software programmable we can write the software instructions for it we are going to write the control board for each of the modes and those control boards they are going to decide that which timer is working in which more so we can say that the timers are software programmable we can change the operation okey-dokey so this was about the three timers of the eight two five three I see now then comes the maximum clock input the maximum clock inputs to the eight two five three it is 2.6 megahertz so we can say that the timer’s can work up till the frequency of two point six megahertz or we can say that it is the maximum operating frequency of eight two five three timers now next we have the feature that as this eight two five three timers they are both in association with the microprocessor so every microprocessor will be different we can have 8-bit 16-bit and 32-bit processors we can have different types of microprocessor so this 8 2 5 3 & 8 2 5 4 they have the feature that they are compatible with almost all types of microprocessor now as the programmable interval timer we have the two icees eight two five three and also the eight two five four eight two five four is the updated version of the higher version of eight two five three so we have written here that the maximum clock input to the eight two five three it is two point six megahertz so in the higher version this operating frequency has been increased till megahertz so this is the feature of eight two five for that it can have the operating frequency of ten megahertz also another feature of eight two five three and eight to five for programmable interval timer ICS or that in the eight to five food we have a command which is known as read back command so in eight two five three and eight two five four we were having the counters and the counters they are initialized with the count value and this count value when it becomes zero then the counter it has provided that specific time delay now if you want to read the count value at any time then this read back command is useful so in eight two five three this feature is not available that you cannot read the value of the count at any time only in eight two five four we have this read back command so this read back come on it allows the user to check the count value the programmed mode that the timer is working in which of the six modes and the current mode and current status of the counter so all these things can be checked by the user at any time using this read back command and this feature is only available in the eight to five four I see so these were the features of the programmable interval timer Isis eight two five three and eight two five four so now let us come to the pin description and the block diagram of the programmable interval timer eight two five three here we will study about eight two five three only so coming to the pin diagram we will have this is the pin diagram of eight two five three you can see that it’s a 24-pin I see we have 24 pins here out of these 24 pins the eight pins from one to eight they are for the data bus nativist is the 8-bit data buses here from D naught to d7 it is the bi-directional data bus then for each of the time but we have the three signals two input signals that is the clock and gate and one output signal is out so each time or it is having two input signals the clock and the gate and one output signal out so we have the three timers timer 0 1 and 2 so each timer will have the 3 signals clock which is denoted by CLK then gate and out so close and get other input signals and out is the output signal of the time so we have four timers 0 we have clock zero out zero gate zero four time four one we have clock one gate one and out one these are further timer one and these three are for timer – this is for timer 0 so the three timers they will have the three signals which are occupying the pins 9 10 11 13 14 15 and 16 17 and 18 then we have the ground and the power supply pins okay so power supply pin it is at the 24 that is positive and ground is connected at the twelfth number pin then we have a 1 and a node these are the timer selection bits through these address lines a not an event though a 2 5 3 it is going to decide that which of the register it has to select ok then we have tube select read and write this is write signal this is read and this is chip select so these are the 24 pins of the IC 8 2 5 3 and these pins are descriptions their names are written here so let’s study each of the pins one by one first we will see the block diagram because block diagram is also related to it so this comes the block diagram of the 8 to 5 3 or 8 to 5 for programmable interval signal here we have the data bus buffer which is connected to the data bus this is our data bus bi-directional it is an 8-bit bi-directional data bus so we have d not 2d seven eight bits represented here and this date of us is buffered it is connected to the internal bus okay so that the data if it is to be transferred to any of the counters it can be transmitted through this transfer through this internal bus then we have read writes logic through this readwrite logic we have the signals reach write a naught and a 1 they are connected then we have the chip select signal also connected towards it the function of this readwrite logic it is to provide the control signals to all the other components of the 8 to 5 3 Ising then we have the control world register ok so control got register it has the control board which decides that bitch mode in which mode the three factors are operating now in the counters they have the two input signals clock and gate and one output signal out for each of the timers we have the three signals associated with it the counters they are connected to the internal bus so that the data can be transferred easily between the various blocks so this is the block diagram and it is the pin diagram of the 8 2 5 3 I see now let us study all the pins and all the blocks of the if I see one by one first we will start with the block data bus buffer and the 8-bit bi-directional data bus so our first is data bus and data bus before so the data bus it is the bidirectional 8-bit data bus and the buffer is also a bi-directional 8-bit buffer okay now the use of this buffer is to interface the timer 8 to 5 3 with the system data bus because the programmable interval timer it is connected to the micro processor it will perform the timing and counting operations for the micro processor so it will be connected through those data bus of the microprocessor through this data bus buffer ok so the function is to interface the timer with the micro processor data bus ok now the operation of this the buffer data bus buffer it is controlled by the chip select line so the operation of this data bus buffer it is dis controlled by the chip select line and this chip selects line if it is low because it’s an active low signal so when it is no it means that it is telling that the microprocessor it wants to communicate with the 8 to 5 3 IC so if this is the microprocessor and it is the IC 8 2 5 3 so if the microprocessor wants to communicate with 8 2 5 3 then it’s going to send the chip selects signal it is going to activate and this chip selects signal we have in between the data bus buffer is also there so data bus buffer it is going to transfer the data between the micro processor and the 8 2 5 3 if this chip select line it is enabled so the operation of this data bus buffer it is controlled by the chip select line if it is low then only this buffer will act if it is high then this buffer it will go into the high impedance state so we can say that this data before it is controlling the transmitting and the receiving of the data between a 2 5 3 and microprocessor now when the data it is transmitted by the microprocessor by the in and the out instructions these instructions are used to transfer data byte or to receive a data byte from a peripheral device peripheral device are the input/output devices so when a 2 5 3 it is connected to micro processor it is acting as an input/output device so the micro processor is going to use the in and out instructions for transmitting the data or receiving the data from the peripheral device or from the timer so this data was buffer it is going to see or transmit the data when microprocessor is executing the instructions in and out so we can say that the data bus buffer it has three basic functions first function is programming the modes of eight to five three because the boats they are decided by the Control Board and Control Board is provided by the microprocessor so microprocessor is going to write the control board or through the data bus buffer into the control raj system so indirectly we can say that the data bus buffer it is controlling the programming of the modes of 8 to 5 3 now the second function of 8 this data bus buffer is that this database buffer it is used to load the count value in the tangles and this count value it is provided by the microprocessor so the time period for which it runs the time delay so that time delay according to that time delay it will calculate the count value and that value will be loaded by the microprocessor through the data bus buffer in the timer’s timer 0 number 1 and timer now third function is to reading the count value from the time owns the time was because we have loaded the count in the timers then the counter angles they are going to decrement the count angle 20 coming after the comp reaches zero value so any time if we want to read the count value from the timer’s from the three timers then we can read it through the database buffer so they thomas buffer we can say that it is used to read and write the count value into the timers or from the timers and also it is used to program the modes of 8 to 5 3 through the control board which is written in the control register now this database buffer it is connected to the database of the microprocessor so it is we can say that data bus buffer is connected to D not to d7 pins of the microprocessor okay and the transfer of data it is taking place through these pins D naught to d7 now these pins D naught to d7 they are usually in the high impedance state they are initially in high impedance state now when the chip selects signal it is enabled means its value is zero it means that microprocessor it wants to communicate with the chip and then these lines these D not to d7 pins they get activated and the data is transferred now if the read signal is activated means its value is 0 because it’s an active low signal so it will be active and its value is 0 or if the write signal it is active means x value is 0 then if read is 0 then the count value will be read by the microprocessor and if write signal it is activated then the count is loaded into the timers so we can say that the D not to d7 pins they are used according to the chip select signal and read and write signals and then the data bus buffer will book okay so data is transmitted through these pins D not to d7 from the microprocessor to the timer 8 2 5 3 okay now next block in the block diagram of 8 2 5 3 it is the read and write control logic you can see in the diagram here we have the read and write control logic and in this logic we have the signals reach write a not even and it is also connected to the data bus buffer and chip selects signal is also there also this signal is connected to the control board register so the function of this read write logic it is to accept the data from the control bus and also from the address bus from the address bus a naught a 1 it is getting that which of the register internal register it has to select and then through this it also generates the control signals and these control signals are given inside the IC ok so the function the read and write control logic is to accept input it accepts the input for the system control bus and in turn and generates the control signals for the overall device operation so you can see that here we have the read signal so when it is activated then the counters will be read F right is activated then the count will be loaded into the counters a not an even they will decide that which register it has to select and this logic it will work only when the strip select signal is enabled because if it is enabled it means that it 2 5 3 and microprocessor they want to communicate with each other if it isn’t disabled means if it’s value is 1 then the chip will not be selected and the data bus buffer will be in the high impedance state and go transfer of bakeable takes place so this read and read and write control logic it is a its function it is decided by the chip select line ok it is enabled if it is enabled then only this logic will work if it is disabled then no operation will occur because the chip is not selected okay so the most important signal here it is the chip selects signal so this trip Civic signal it is used to enable the communication between the eight two five three and the microprocessor through the data bus okay so the data bytes they are only be transmitted or received by the microprocessor only when the chip this a two five three chip it is selected through this chip select signal and this chip selects signal it is an active low signal so how this chip select signal it is enabling the communication when this chip selects signal is low it will enable the data bus buffer I’m the database buffer because it is connected to the database of the microprocessor so data will be transmitted or received through this so what this chip sonic signal is doing it is enabling the data bus buffer if it is zero then this data bus buffer will be enabled if it is 1 then this buffer will be in the high impedance state now this chip sonic signal it is decided by some circuitry because the address lines the 8 to 5 3 it is connected as a peripheral device to the microprocessor so peripheral device it will happen 8-bit address now out of 4 this 8-bit address they are going to use the 8 address lines of the microprocessor like a not to a 7 these are the address lines which will be used for addressing the 8 to 5 3 chip now out of these 8 address lines a naught and a 1 they are connected to the read and write control logic and rest of the lines that is from a2 to a7 they are used to form the chip select signal cs bar so these address lines how they are connected to form the chip select signal the every microprocessor they will have a different circuitry for it so the configuration of the system employs the some type of circuitry to define the chip selects signal now the next signals in the read and write control logic are the read and write signals the read and write signals they control the direction of data transfer between the a to 5/3 and the microprocessor so they are controlling the direction of data transfer on the 8-bit data bus if read signal because it’s an active low signal it is having a bar over it so it will be active when its value is zero so when this signal it is low that means that the count value which is there on the three counters counter bond counter 0 and counter two so that count value will be read by the microprocessor so the data it is going to transfer from the counters or from the a25 3ic to the microprocessor so when this is low CPU is reading the data from the a two five three in the form of count value now comes the right signal this signal is also an active low signal so it’s value it is active when its value is zero now when the signal is Rho it means that the data which is available on the data bus it will be written in the three counters or in the control register so when it does load the CPU is writing the data into the a two five three in the form of mode information that is in the form of control God because control word contains the information about which mode will be active so more information or loading the count values into the counters so count value on the control word it will be written by the CPU into the eight two five three when this write signal is enabled now if these two signals they are high okay means this is high and this is also high now when these two signals are high then the data bus buffer which is enabling the data transfer which is connected between the eight two five three and CPU it will go into the high impedance state if the chip sonic signal it is enabled it’s zero means the chip is selected but the read and write signals they are higher so the data bus buffer it will go into the high impedance state and no transfer of data will take place between the eight two five three and the microprocessor so this is the function of the read and write signals the two of the signals which are connected to the region write logic for a naught and a 1 these are the two input lines which will decide that the microprocessor it will write or read the data from which of the timers and which the control register so which of the internal register will be connected that will be decided by a naught and a 1 so these two input lines they allow the microprocessor to specify that which of the internal register means either the control register timer 0 timer 1 in timer 2 which of these register is going to be used for the data transfer so now let’s see the table for it that how the two lines they are deciding the transfer of data so we will see that the a1 and anon lines and the cs bar chip select read and write all how all these signals they are deciding that which operation is going to be performed by the 8 to 5 3 so when this chip selects signal it is zero then only 8 to 5 3 will be will communicate with the microprocessor now when this read signal is 1 and write signal is 0 means this is activated this is not activated because these two signals are active low signals so they will work only when their value is 0 so right operation will takes place now if the value of a 1 and a not it is 0 0 then counter 0 is selected and here write signal is activated so operation will be to load the count value into the counter 0 so this even it what they are 0 so counter 0 will be selected and here the write signal it is 0 exactly so it will be a write operation so the operation will be to write the count value into the counter 0 so this will be the operation when the value of a 1 and a naught is 0 0 and right signal is activated now we have again chip selects signal is 0 read is one write is 0 and even and a naught they are 0 1 then the counter 1 will be selected and again it will be a write operation so the function will be to load counter 1 then we have all the signals will be same here if we have 1 0 then lambda 2 will be selected so the operation will be to load the counter 2 then we have the chip selects signal is 0 for each signal is again 1 this right is 0 and here if we have 1 1 then the operation will be because the control register is selected so here the operation will be to write the mode word that is to write the control board into the control register so 0 0 it is basically selecting the counter 0 if this is the value of a 1 and a naught if it is 0 1 then 1 first counter is selected if it is 1 0 then second counter is selected and if it is 1 1 then control register is selected now these four operations these are the write operations because write signal is activated here now if read signal is activated then the selection will be based on a 1 and a knot and then grid operation will be performed so here we will have 0 Reid will be activated right will be 1 then 0 0 so the counter 0 is selected so counter 0 will be read so read the timer 0 then we have 0 0 1 if it is 0 1 then counter 1 is selected so it will read the timer 1 then again chip selects signal 0 read signal is 0 right is 1 it is 1 0 then it will read the timer 2 now if it is 1 1 then control register will not be read because for the control register no read operation is associated with it so bun-bun will have no function no operation will be there then if the value of this cs is 0 read signal is 0 1 is 1 and then it is 1 1 ok so we can say that no operation is performed and the data bus buffer will be in the impedance state ok now next if the chips on its signal is but it means that the chip 8 2 5 3 is not selected and whatever will be the value of this read write even a not the chip is not selected so we can say that the operation is that the chip is disabled and the buffer it is in the high impedance state because chip selects signal is 1 so chip is not selected here now if the chip select signal is 0 and the read and write signals they are at logic 1 but because their value has to be 0 for the read and write operation and if the value is 1 1 and whatever will be the value of a 1 and a not again no operation will be performed and again the buffer will be in the high impedance state ok because these signals has to be low any one of the signal has to be low to perform the read and write operation so these in though these four operations they are the write operations because write signal is activated and these three are the read operations because read signal is activated ok so this is the table which is showing that which internal register is selected according to the value of evil and the a node now these a 1 and a node these are basically the address lines of the microprocessor so the eight address lines are deciding the address of the might of the 8 to 5 3 IC so out of the eight address lines two are deciding that which internal resistor is connected and the rest of the lines they are deciding the chip select signals now next come the control board register so the control board register it is used to program the timers in different modes as we know that the eight two five three it has six modes so any of the timers can be programmed in any of the six modes so this modes are decided by the control board registers and the motor control the operations of the three timers now this control board resistor it is selected when the K not and even lines they are at logic one we have seen in the table that control register is selected when a naught and a 1 at a logic one now when the read signal is activated and this control register is selected then because control register it has no read operation associated with it so we cannot read the control register so when this read signal is 0 no no read operation is performed for control register we cannot read the control board so this will have no effect on it now if the value of the write signal it is zero means it’s low then the control board which is provided by the microprocessor it will be written into the control register now this control but it is an 8-bit word because the data was of the microprocessor it is 8-bit data bus so it be data is going to be written so control board it will be an 8-bit word and it will be written into the control register so control register will be an 8-bit register okay and this in bit but it is going to decide that the timers will work in which of the modes ok now when this write signal is activated and control that has been written into the control register so the counters will be loaded with their count value which is also provided by the data bus buffer so the next we have the counters the three counters which are present in the eight two five three and see now eight two five three it has three 16-bit counters and each couple has three signals withered and these three signals are first is clock second is gate and third is clock and gate other input signals and out is the output signal so we can say that the counter will have two input signals and one output signal first we will study the clock signal so the clock input pin it provides the 16 byte timer with the signal which decides that the time for it is going to decrement means it start the decrement process of the tack mode so when this clock signal will be given to the timer the timer it is already loaded with a count so it will start declaring the count until its value reaches to zero now these counters these 16-bit counters as they are the down counters so they are always going to decrement the count okay and also these counters they decrement the value of the count from every high to low transition of the clock input means that if this is the clock signal then high to low transition means this is high and this is low this is again high and this is again hello so from every high to low transition the counter is going to decrement suppose the counter it is initialized by a value of five so when the transition takes place from high to low of the clock input then the counter will be decrement so its value will become four again we have the high to low transition so again the counter is going to decrement so its value will become three and it will decrement in such way that the last value zero comes okay so for every high to low transition of the clock input the counter it is going to decrement the count value so this was the clock input now comes the gate so gate signal it is used to initiate or enable the counting process means the counting process or decrementing process it’s going to be start when the gate signal will be given okay and the effect of the gate signal it depends on which of the six modes of operation is being chosen for the timer’s so the time was they are going to start the decrementing process only when the gate signal is given to it and the clock input it is every time the clock pulse it is transiting from high to low then the count will become will be decremented then we have the output signal so this signal it provides an output from the timer that if you want to read the count value at any instant then we can read the come to back view from the output signal okay and at any time we can also read the counter and at the last when the counter has become zero then also it will provide us the output of the so these three signals the clock input gate and output they are associated with each of the timer’s timer 0 timer 1 and time 2 so the output of the timer this out function it provides an output from the timer and its function it is its function basically depends upon the mode of operation in which the timer is being programmed so we can say that its function so in 8 to 5 3 we have the three 16-bit counters which are the dumb counters that means they are the they are initialized with account and then they decrement the counter until its value becomes 0 now to understand the functioning that how the time was they were let us see the internal structure of the thymus so the internal structure of the dimer it consists of three elements input elements and counting elements and output element the function of the input element is to get the count which is loaded by the central processing unit of the microprocessor the microprocessor it is going to provide the count through the data bus buffer and that count will be loaded into the input element of the timer now once the input element has been initialized it will become the count will remain there until it is reloaded by the new count so if you have initialized to the count like suppose you have given the count value of 10 so 10 will remain in the input element until it is reloaded the timer it is reloaded by some other count okay so input element it is going to contain the count 10 in its if it I we have given the count value 10 then 10 will be loaded into the input element now this input element it will provide this count to the counting element and this counting element will start decrementing the count when it will be given this clock and the gate input as we know that the timer’s they are the down counter so it is going to decrement the count and it is going to decrement the counter for every high to no transition of the clock input because if see this is the clock pulse so for every high to low transition the counter is going to be decremented so suppose the counter we have loaded it with in so for this it will become nine for this it will become seven and for this high to low transition it will become seven so when the clock pulse is given the counting element will start decrementing the count now this counting of the decrementing of the account it is available at the output element because this output it is basically a transparent latch and the function of this latch is to whatever is the output whatever will be the input the output will be the same suppose like for first clock transition high to low transition the count has been decremented and its value has become 9 so the output element will give the output here as 9 okay so what’s this output element is giving it is providing the current value of the come at the its output okay now suppose that if you want to latch the output of the counter like if we want that this nine value is latched so this 9 will remain at the output and the counter it will continue its decrementing of the counter so 9 will be there then 8 then 7 so in this way the counting element it will continue decrementing the counter but at the output we are getting the last output which is 9 okay and the microprocessor can at any time it can read the value of the count from this output element now once the microprocessor has read the output from this output element then this output element it will again be updated and it is going to again show the value of the count which is currently this counting element is counting so once this line has been read by the microprocessor then again this output element will be updated and the suppose the current value of the count is 7 so it will again start showing the value of the counter so in this way the timer’s they work.they having these three input elements so let us write the function of each of the element input element so we can see that the function of the input element is that the count value in it is loaded into the input element and it remains in it this element until it is reloaded with another count value next we have the counting element so this some value which is loaded into the input element it is transferred to the counting element at an appropriate clock input you can see in the diagram that to the counting element the clock input and the gate input they are connected so at an appropriate clock input this low come which is loaded into the input element it is transferred to the counting element and once the count has been transferred into the counting element the down content that is decrementing the count it starts now this value of the count current value of the count it is available at the output element so in the diagram we can see that the output of the counting element it is connected to the input of the output element so the current value of the count okay here I have written that the current value of the count at any movement it is available at the input of the output element so whatever will be the current value of count in the counting element it is available at the input of this output element now output element I have said that it’s a large it’s a transparent large so whatever will be its input that input will be available at its output okay so it is acting as a latch here transparent latch so the current value because at the input of output element we have the current value of the count so that current value of count it is available at the output of this output element so the third element is the output element okay so the it’s a transparent latch and the output of the latch it follows the input input is what it is the current come value now the put of the the value of this account it can be latched by using appropriate mode because we know that there are different modes in which the eight two five three timers they can’t be programmed so depending upon the modes of operation the output of the count means the value of the sound can be latched so the value of the count it can be latched by issuing appropriate control word in the control register and once the output value of the counter it is latched the internal counting continuous means suppose that the current value of the counter pose it is nine so when we are latching this if we are latching the output of the counter then the output of the output element will become nine but the internal or decrementing of the account will continue okay and then this count this microprocessor you know it is always going to read the output of this output element okay so microprocessor always read the output of the output element now once it has read the this output element then the output element again becomes transparent and whatever will be the current value of the input that will be available at the output so once this nine has been read by the microprocessor the output element it will again become a transparent large and this seven six five four three two that means the current or decrementing of the counter will be available at the output element so this is how the time holds they work so in this video of a two five three and a two five for programmable interval timer we studied about these I sees their basic features and then we studied the block diagram and the pin diagram and then one by one we studied the description of all the blocks of a to 5 3 and the description of all the pins of a to 5 3 so I hope that this topic is clear to you thank you

6 thoughts to “Programmable Interval Timer 8253/8254 – Peripheral Interfacing with 8085 Microprocessor”

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