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THE 8253 IC(PIT)
Vatsal N Shah – IU1241090055Sharon Benedict – IU1241090056
Microprocessor & Interfacing
Electronics and Communication Engineering Dept.Indus University
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What is 8253?
Contents
PIN-DIAGRAM OF 8253
ARCHITECTURE OF 8253
MODES OF 8253
Interfacing with 8085
References
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What is 8253?
• The Intel 8253 is a programmable counter.• It includes 3 identical 16 bit counters.• It is packaged in a 24-pin DIP.• Six programmable modes.• Gate is used to enable or disable the counter.
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ARCHITECTURE OF 8253
D7-D0
RDWRA0A1
CS
DATABUSBUFFER
READ/WRITELOGIC
CONTROLWORD REGISTER
COUNTER 0
COUNTER 1
COUNTER 2
CLK 0
GATE 2
OUT 1
OUT 0
OUT 2
GATE 1
GATE 0
CLK 2
CLK 1
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D7
D1
D0
OUT 0
GATE 0
GND
D6
D5
D4
D3
D2
CLK 0
8253
VCC
OUT 2
GATE 2
CLK 1
OUT 1
GATE 1
RD
CS
A1
A0
CLK 2
WR
PIN DIAGRAM OF 8253
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Data Bus Buffer:• This block contains the logic to buffer the data bus to the
microprocessor and to the internal registers. It has 8 input pins, usually labelled as D7..D0, where D7 is the MSB.
Control Logic:• The control section has five signals: Read, Write, Chip select
and the address lines A0 and A1.• In peripheral I/O mode read and write signals are connected
to IOR and IOW while in memory mapped these are connected to MEMR and MEMW.
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• The control word register and counters are selected according to signals on lines A0 and A1 as shown below:
CS A1 A0 Select 0 0 0 Counter 0
0 0 1 Counter 1 0 1 0 Counter 2 0 1 1 Control Reg.
Control word Register:• This register is accessed when lines A0 and A1 are at logic
1.• It is used to write command word which specifies the
counter to be used, its mode and either read or write format.
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8253 Timer
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MODES OF 8253
MODE 0: Interrupt on terminal count.
MODE 1: Programmable 1 shot.
MODE 2: Rate generator.
MODE 3: Square wave generator.
MODE 4: Software triggered strobe.
MODE 5: Hardware triggered strobe.
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Mode 0: Interrupt on Terminal Count• The output will start off zero. The count is loaded and the
timer will start to count down.• When the count has reached zero the output will be set high,
and remain high until the next count has been reloaded.• This can be used as an interrupt.
Mode 1: Programmable One-Shot• The output will go low following the rising edge of the gate
input. • The counter will count and the output will go high once the
counter has reached zero.
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Mode 2: Rate Generator• This mode is used to generate a pulse equal to the clock
period at a given interval.• When count is loaded the OUT stays high until the count
reaches 1 and the the OUT goes low for one clock period.• The count is reloaded automatically and the pulse is
generated continuously.
Mode 3: Square Wave Generator• This mode is similar to mode 2. However, the duration of the
high and low clock pulses of the output will be different from mode 2.
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• Suppose n is the number loaded into the counter (the COUNT message), the output will be:
• high for n/2 counts, and low for n/2 counts, if n is even.• high for (n+1)/2 counts, and low (n-1)/2 for counts, if n is odd
Mode 4: Software Triggered Pulse. • The output will remain high until the timer has counted to
zero, at which point the output will pulse low and then go high again.
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Mode 5: Hardware Triggered Pulse • The counter will start counting once the gate input goes high,
when the counter reaches zero the output will pulse low and then go high again.
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Modes of Operation of 8253
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8253 Example
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References • http://www.circuitstoday.com/• http://
www.ti.com/lit/an/slyt145/slyt145.pdf• http://
8085projects.info/page/free-programs-for-8085-microprocessor.html
• http://www.learnabout-electronics.org/index.php
• https://www.youtube.com/watch?v=nxAQ1PFEd5U
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