Presentation on IC 8253.

1. Microprocessor &
Interfacing
THE 8253 IC
(PIT)
Vatsal N Shah – IU1241090055
Electronics and Communication Engineering Dept.
Indus University
1

2. What is
8253?
Contents
PIN-DIAGRAM
OF 8253
ARCHITECTUR
E OF 8253
MODES OF
8253
Interfacing
with 8085
References
2

3. 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.
3

4. ARCHITECTURE OF 8253
D7-D0
RD
WR
A0
A1
CS
DATA
BUS
BUFFER
READ/
WRITE
LOGIC
CONTROL
WORD
REGISTER
COUNTER
0
COUNTER
1
COUNTER
2
CLK 0
GATE 0
OUT 0
CLK 1
GATE 1
OUT 1
CLK 2
GATE 2
OUT 2
4

5. D7
D6
D5
D4
D3
D2
D1
D0
CLK 0
OUT 0
GATE 0
GND
8253
VCC
WR
RD
CS
A1
A0
CLK 2
OUT 2
GATE 2
CLK 1
OUT 1
GATE 1
PIN DIAGRAM OF 8253
5

6. 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.
6

7. • 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.
7

8. 8253 Timer
8

9. 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.
9

10. 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.
10

11. 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.
11

12. • 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.
12

13. 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.
13

14. Modes of Operation of 8253
14

15. 8253 Example

16. 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=nxAQ1PFE
d5U
16

17. 17

18. 18