PIT Programming Examples

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PIT Programming Examples

• Working with the modes of PIT

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Mode 0 interrupt on terminal count
The output in this mode is initially low, and
will remain low for the duration of the count if
GATE 1. Width of low pulse N?T Where N is
the the clock count loaded into counter, and T is
the clock period of the CLK input.
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Mode 0 interrupt on terminal count

• When the terminal count is reached, the output
  will go high and remain high until a new control
  word or new count number is loaded.
• In this mode, if GATE input becomes low at the
  middle of the count, the count will stop and the
  output will be low.
• The count resumes when the gate becomes high
  again.
• This in effect adds to the total time the output
  is low.

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Mode 1 programmable one-shot

• This mode is also called hardware triggerable
  one-shot.
• The triggering must be done through the GATE
  input by sending a 0-to-1 pulse to it.
• The following two steps must be performed
• Load the count registers.
• A 0-to-1 pulse must be sent to the GATE input to
  trigger the counter.
• Contrast this with mode 0, in which the counter
  produces the output immediately after the counter
  is loaded as long as GATE 1.
• In mode 1 after sending the 0-to-1 pulse to GATE,
  OUT becomes low and stays low for a duration of
  N?T, then becomes high and stays high until the
  gate is triggered again.

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Mode 2 rate generator

• Mode 2 is also called divide-by-N counter.
• In this mode, if GATE 1, OUT will be high for
  the N?T clock period, goes low for only one clock
  pulse, then the count is reloaded automatically,
  and the process continues indefinitely.

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Mode 3 square wave rate generator

• In this mode if GATE 1, OUT is a square wave
  where the high pulse is equal to the low pulse if
  N is an even number.
• In this case the high part and low part of the
  pulse have the same duration and are equal to
  (N/2)?T (50 duty cycle)
• If N is an odd number, the high pulse is one
  clock pulse longer.
• This mode is widely used as a frequency divider
  and audio-tone generator.

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Mode 4 software trigger strobe

• In this mode if GATE 1, the output will go high
  upon loading the count.
• It will stay high for the duration of N?T.
• After the count reaches zero (terminal count),
  it becomes low for one clock pulse, then goes
  high again and stays high until a new command
  word or new count is loaded.
• To repeat the strobe, the count must be reloaded
  again.
• Mode 4 is similar to mode 2, except that the
  counter is not reloaded automatically.
• In this mode, the count starts the moment the
  count is written into the counter.

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Mode 5 hardware trigger strobe

• This mode is similar to mode 4 except that the
  trigger must be done with the GATE input.
• In this mode after the count is loaded, we must
  send a low-to-high pulse to the gate to start the
  counter.

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Time delays in 80x86 IBM PC

• The following method of creating hardware time
  delays was first implemented in the IBM PC and
  compatible computers.
• To create a processor independent delay, IBM
  made PB4 of port 61H toggle every 15.085?s.
• CS holds 15.085?s in the following codes.

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Time delays in 80x86 IBM PC
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Time delays in 80x86 IBM PC

• Now a time delay of any duration can be created
  regardless of the CPU frequency as long as it is
  a 286 and higher PC.
• For example set CX33144 (33144?15.085?s0.5
  second)
• MOV CX, 33144
• CALL WAITF

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Time delays in 80x86 IBM PC
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Example
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Examples
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Generating Music on the IBM PC

• As mentioned earlier, counter 2 is connected to
  the speaker and it can be programmed to output
  any frequency that is desired.
• Look at the list of piano notes and their
  frequencies given in Fig. 5-5.
• Music frequency and time duration

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A delay of 250 ms
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Shape of 8253/54 Outputs
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Out0 pulse shape in IBM BIOS

• IBM BIOS programmed counter 0 to create mode 3,
  which is square-wave shape.
• Counter 0 is loaded with the number 65,536 and
  the clock period of input frequency 838 ns
  (1/1.193 MHz 838 ns), so the period of the OUT0
  pulse is equal to 65536?838 ns 54.9 ms (18.2
  Hz).
• OUT0 continuously sends out square wave pulses.

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Out1 pulse shape in the IBM BIOS

• IBM BIOS programmed counter 1 in mode 2, rate
  generator, with the value 18 loaded into the
  counter.
• OUT1 will be high for a total of 17 ?838 ns and
  go low for one pulse of 838 ns
• GATE1 is set to 1 permanently

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OUT2 pulse shape in the IBM BIOS

• IBM BIOS loads the value 1331 into counter 2.
• Since 1331 is an odd number, the OUT2 pulse is
  high for a total of (13311)/2 666 ? 838 ns and
  is low for a total of (1331-1)/2 665 ? 838 ns

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Sample Questions

• Preparation for Mid-Term Exam
• Microprocessors I

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Assembly Language

• Determine the Value of AX After the following
  Instructions

• MOV AX, 2345H
• MOV BX, 5432H
• ADD AX, BX 7777H
• MOV AL, BH 7754H

• MOV AX, 2345H
• ROR AL,1 23A2
• ADC AH,5 29A2

• MOV AX, 2345H
• MOV BX,1357H
• PUSH AX
• PUSH BX
• INC SP
• POP AX 4513

• MOV AX, 9245H
• ROR AL,1 92A2
• RCL AH,1 25A2

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Developing Simple Programs

• BCD to Hex Conversion
• SI BCD Source, DI Hex Destination, CX Size
• Hex to BCD Conversion
• SI Hex Source, DI BCD Destination, CX Size
• Common Divisor of two natural Numbers
• IsPrime
• AB
• Arithmetic Operations for Big Natural Numbers
• Start Address in SI, Size in CX
• Operations , Difference, , /,

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Using Special Interrupts

• Get a Number from user, find its divisors, show
  it to user
• Write the Term Hello in a color box in the
  center of the screen with borders. Use edge
  character codes for borders
• Navigate the cursor on screen

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System Design

• A Minimum System with 64k EPROM and 64K RAM
• Peripherals One 8259 as Master and 3 8259 Slave
  (Main Addresses 40H, 60H, 80H, C0H)
• Three PPI (Main Addresses 20H, A0H, E0H)
• ALL PPIs Can Cause Interrupt Through Master
• First PPI Controls 4 Stepper Motors
• Second PPI is for Display Controller

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System Programming

• Develop a Program that drives the stepper motors
  with equal rpm
• Digital Display Driver
• Interrupt of a timer (Every 10 ms with a 555)