chapter 04: peripherals interfacing with 8086 and applications€¦ · chapter 04: peripherals...

Compiled by Mr. Vishal Gaikwad, SIESGST

CHAPTER 04: Peripherals Interfacing with 8086 and Applications

8086-Interrupt structure.

Programmable peripheral Interface 8255.

Programmable interval Timer 8254.

Elementary features of 8259A and 8257 and interface.

Interfacing 8255, 8254 with 8086 and their applications.

University Questions

1. Write a program to blink bit 4 of port C using BSR mode of 8255 [5M_May-2015]

2. Draw and explain interrupt structure of 8086 with its IVT. [10M_May-2015, Dec_2015

3. Explain Mode 0 and Mode 1 of 8254 Timer/Counter IC with the help of timing diagram.

[10M_May-2015]

4. Explain different modes of operation of 8257 DMA controller. [10M_May-2015,May-

2017]

5. Explain the importance of DMA controller. Explain method of its interfacing with

8086. [10M_Dec-2015, Dec-2016,Dec-2017]

6. Explain in brief about programmable interval timer 8254. [5M_May-2016]

7. Explain 8086 interrupt structure and its method of interfacing with 8086

microprocessor with a suitable example.(any one interrupt) [10M_May-2016,May-

2018]

8. Explain in brief about programmable peripheral interface 8255. [5M_Dec-2016]

9. Write control word of 8255 to initialize port A as input port, port B and C as output port

group A and B in mode 0.[5M_May-2017]

10. State purpose of interfacing 8259 PIC to 8086. Explain interfacing of 8086-minimum

mode and 8259 single mode.[10M_May-2017]

11. What are the different types of interrupts supported by 8086 and explain IVT.

[10M_May-2017,Dec-2017]

12. Write control word of 8255 to initialize port A as input port, port B and C as output port

group A in mode 0 and Group B in mode 1.[5M_Dec-2017]

13. Explain Bit Set Reset mode of 8255 with application. [10M_Dec-2017]

14. Explain input output control word format of 8255.

Write control word of 8255 to initialize port A as input port, port B and C as output port

group A and B in mode 0. [10M_May-2018]


4.1 8086-Interrupt structure:

An interrupt is a condition that halts the microprocessor temporarily to work on a different task

and then return to its previous task. Interrupt is an event or signal that request to attention of

CPU. This halt allows peripheral devices to access the microprocessor.

Whenever an interrupt occurs the processor completes the execution of the current instruction

and starts the execution of an Interrupt Service Routine (ISR) or Interrupt Handler. ISR is a

program that tells the processor what to do when the interrupt occurs. After the execution of

ISR, control returns back to the main routine where it was interrupted.

The following image shows the types of interrupts we have in a 8086 microprocessor −

The different types of interrupts present in 8086 microprocessor are given by:

1. Hardware interrupt - 8086 has two pins to accept hardware interrupts, NMI and INTR.

2. Software Interrupts - These interrupts are caused by writing the software interrupt

instruction INT n where ‘n’ can be any value from 0 to 255 (00H to FFH). Hence all 256

interrupts can be invoked by software.

3. Error conditions (Exception or types) - 8086 is interrupted when some special

conditions occur while executing certain instructions in the program. Example: An error

in division automatically causes the INT 0 interrupt.

1. Hardware Interrupts –

Hardware interrupts are those interrupts which are caused by any peripheral device by sending

a signal through a specified pin to the microprocessor. There are two hardware interrupts in

8086 microprocessor. They are:

(A) NMI (Non Maskable Interrupt) – It is a single pin non-maskable hardware interrupt

which cannot be disabled. It is the highest priority interrupt in 8086 microprocessor. After its


execution, this interrupt generates a TYPE 2 interrupt. IP is loaded from word location 00008H

and CS is loaded from the word location 0000A H.

(B) INTR (Interrupt Request) – It provides a single interrupt request and is activated by I/O

port. This interrupt can be masked or delayed. It is a level triggered interrupt. It can receive

any interrupt type, so the value of IP and CS will change on the interrupt type received.

2. Software Interrupts –

These are instructions that are inserted within the program to generate interrupts. There are 256

software interrupts in 8086 microprocessor. The instructions are of the format INT type where

type ranges from 00 to FF. The starting address ranges from 00000 H to 003FF H. These are 2

byte instructions.

The total interrupt vector table is divided into three groups namely,

A. Dedicated interrupts (INT 0…..INT 4)

B. Reserved interrupts (INT 5…..INT 31)

C. Available interrupts (INT 32…..INT 225

Some important software interrupts are:

(A) TYPE 0 corresponds to division by zero(0).

(B) TYPE 1 is used for single step execution for debugging of program.

(C) TYPE 2 represents NMI and is used in power failure conditions.

(D) TYPE 3 represents a break-point interrupt.

(E) TYPE 4 is the overflow interrupt.

Interrupt Vector Table:

In an 8086 Interrupt system the first 1 Kbyte of memory from 00000H to 003FFH is reserved

for storing the starting addresses of interrupt service routines. This block of memory is often

called the interrupt vector table or the interrupt pointer table. Since 4 bytes are required to store

the CS and IP values for each interrupt service procedure, the table can hold the starting

addresses for 256 interrupt service routines. Following figure shows how the 256 interrupt

pointers are arranged in the memory table.


4.2 Programmable peripheral Interface 8255

The 8255 is a programmable peripheral interface i.e. PPI 8255.

PPI 8255 is a general purpose programmable I/O device designed to interface the CPU with its

outside world such as ADC, DAC, keyboard etc. We can program it according to the given

condition. It can be used with almost any microprocessor.

The Intel 8255A is a general purpose programmable I/O device designed for use with Intel

microprocessors. It consists of three 8-bit bidirectional I/O ports (24 I/O lines) that can be

configured to meet different system I/O needs. The three ports are designated as PORT A,

PORT B and PORT C. Port A contains one 8-bit output latch/buffer and one 8-bit input buffer.

Port B is same as PORT A or PORT B. However, PORT C is split into two parts- PORT C

lower (PC3-PC0) and PORT C upper (PC 7 -PC 4 ) by the control word. The four ports –two

8-bit PORTs and two 4-bit PORTs are divided in two groups

Group A (PORT A and upper PORT C)

Group B (PORT B and lower PORT C) for programming purpose.


These two groups can be programmed in three different modes:

1. Mode-0

2. Mode-1

3. Mode-2

Mode 0: In the first mode, mode-0, each group may be programmed in either input mode or

output mode (PORT A, PORT B, PORT C lower, PORT C upper).

Mode 1: In mode 1, each group may be programmed to have 8-lines of input or output (PORT

A or PORT B) and of the remaining 4-lines (PORT C lower or PORT C upper) 3-lines are used

for handshaking and interrupt control signals.

Mode 2: The third mode of operation, mode-2, is a bidirectional bus mode which uses 8- lines

(PORT A) only for a bidirectional bus and five lines (PORT C upper 4 lines and borrowing one

line from PORT C lower) for handshaking and control signals. PORT A and PORT B have

both input and output buffers and latches but PORT C has output latch and input buffer.

Pin Diagram of 8255:

The control word format of the 8255 is shown in Fig. below. The contents of the control register

are called the control word that specifies the input/ output functions of each port.


The control word register can be accessed to write the control word by selecting A1 and A0 in

high condition i.e. both ON. This register is not available for read operation.

As indicated in Fig, the most significant bit (D7) of the control word specifies either I/O

function or the bit set/ reset function. With D7= 1, the 8255 works in the I/O modes as shown

below.

With D7= 0, the 8255 works in BSR mode.

Port-C operates in BIT SET/RESET (BSR) mode with D7=0 as shown in above figure.

The BSR Mode in no way affects the functioning of port-A and port-B. Following steps are

essential to communicate with the peripherals through the ports of the 8255.

Determine address of ports-A, B, or C and of the control register according to the chip

select logic and the address lines A1 and A0.

Write the control word in the control word register.

Write input/output instructions in order to communicate with the peripherals through

ports-A, B, and C of the 8255.


4.3 Programmable interval Timer 8254:

The Intel 8254 are Programmable Interval Timers (PITs), which perform timing and

counting functions using three 16-bit counters.

The timer has three counters, numbered 0 to 2. Each channel can be programmed to operate

in one of six modes. Once programmed, the channels operate independently.

Each counter has two input pins – "CLK" (clock input) and "GATE" – and one pin, "OUT",

for data output. The three counters are 16-bit down counters independent of each other,

and can be easily read by the CPU.

Data bus buffer contains the logic to buffer the data bus between the microprocessor and

the internal registers. It has 8 input pins, usually labelled as D7-D0, where D7 is the MSB.

Read/write logic has 5 pins, which are listed below. The "X" denotes X is an active low

signal.

RD: read signal, WR: write signal, CS: chip select signal, A0, A1: address lines

Operation mode of the PIT is changed by setting the above hardware signals. For example,

to write to the Control Word Register, one needs to set CS=0, RD=1, WR=0, A1=A0=1.

The control word register contains the programmed information which will be sent (by the

microprocessor) to the device. It defines how each channel of the PIT logically works.

Each access to these ports takes about 1 µs.

Control Word Register and Operating Modes of 8254

The Control Word Register is selected by the Read/Write Logic when A1, A0 are 11.

To initialize the counters, the microprocessor must write a control word (CW) in this register.

Control Word of 8254 decide the operating mode of 8254.

Counter selection is also done using CWR of 8254.


Mode 0 (Interrupt on Terminal Count) –

Mode 0 is typically used for event counting. After the Control Word is written, OUT is initially

low, and will remain low until the counter reaches zero it is decremented by 1 after every clock

cycle. OUT then goes high and remains high until a new count or a new Mode 0 Control Word

is written into the counter. GATE = 1 enables counting, GATE = 0 disables counting.

Mode 1 (Hardware Retriggreable One Shot) –

OUT will be initially high. OUT will go low on the CLK pulse following a trigger to begin the

one-shot pulse, and will remain low until the counter reaches zero.

Mode 2 (Rate Generator) –

Initially value of OUT is low. When counting is enabled, it becomes high and this process

repeats periodically. Value of count = Input Frequency / Output Frequency. This mode works

as a frequency divider.


Mode 3 (Square Wave Generator) –

Counting is enabled when GATE = 1 and disabled when GATE = 0. This mode is used to

generate square waveform and time period (equal to count) is generated.

If N is count and is even then ontime of wave = N/2 and offtime = N/2

If N is odd the ontime = (N + 1) / 2 and offtime = (N – 1) / 2

Mode 4 (Software Triggered Strobe) –

In this mode counting is enabled by using GATE = 1 and disabled by GATE = 0. Initially value

of OUT is high and becomes low when value of count is at last stage. Count is reloaded again

for subsequent clock pulse.

Mode 5 (Hardware Triggered Strobe) –

OUT will initially be high. Counting is triggered by a rising edge of GATE. When the initial

count has expired, OUT will go low for one clock pulse and then go high again. After writing

the Control Word and initial count, the counter will not be loaded until the clock pulse after a

trigger.

For timing diagram refer attached pdf.

(Timing diagram is IMP in examination point of view)


4.4 Elementary features of 8259A and 8257 and interface:

8257 DMA Controller:

DMA stands for Direct Memory Access. It is designed by Intel to transfer data at the fastest

rate. It allows the device to transfer the data directly to/from memory without any interference

of the CPU. Using a DMA controller, the device requests the CPU to hold its data, address and

control bus, so the device is free to transfer data directly to/from the memory. The DMA data

transfer is initiated only after receiving HLDA signal from the CPU.

Following is the sequence of operations performed by a DMA –

- Initially, when any device has to send data between the device and the memory, the

device has to send DMA request (DRQ) to DMA controller.

- The DMA controller sends Hold request (HRQ) to the CPU and waits for the CPU to

assert the HLDA.

- Then the microprocessor tri-states all the data bus, address bus, and control bus. The

CPU leaves the control over bus and acknowledges the HOLD request through HLDA

signal.

- Now the CPU is in HOLD state and the DMA controller has to manage the operations

over buses between the CPU, memory, and I/O devices.

Features of 8257

- It has four channels which can be used over four I/O devices.

- Each channel has 16-bit address and 14-bit counter.

- Each channel can transfer data up to 64kb.

- Each channel can be programmed independently.

- Each channel can perform read transfer, write transfer and verify transfer operations.

- It generates MARK signal to the peripheral device that 128 bytes have been transferred.

- It requires a single phase clock.

- Its frequency ranges from 250Hz to 3MHz.

- It operates in 2 modes, i.e., Master mode and Slave mode.


Operating Modes of 8257:

The Operating Modes of 8257 can be programmed to operate in following modes:

1. Rotating Priority Mode 4. Fixed Priority Mode

2. Extended write mode 5. TC stop mode

3. Auto Load Mode

1. Rotating Priority Mode:

In rotating priority mode, the priority of the channels has a circular sequence. In this, channel

being serviced gets the lowest priority and the channel next to it gets the highest priority as

shown in Fig.

Thus, with rotating priority in a single chip DMA system, any device requesting service is

guaranteed to be recognized after no more than three higher priority services have occurred.

This prevents any one channel from monopolizing the system. The rotating priority mode can

be set by writing logic ‘1’ in the bit 4 of the mode set register.

2. Fixed Priority Mode:

In the fixed priority, channel 0 has the highest priority and channel 3 has the lowest priority.

Table shows the priority ratings.


In the fixed priority, after recognition of any one channel for service, the other channels are

prevented from interfering with that service until it is completed. If bit 4 of mode set register

is logic 0, Operating Modes of 8257 operates in fixed priority mode.

3. Extended Write Mode:

Microcomputer systems allow use of various types of memory and I/O devices with different

access time. If a device cannot be accessed within a specific amount of time it returns a “not

ready” indication to the 8257 that causes the 8257 to insert one or more wait states in its internal

sequencing. The extended write option provides alternative timing for the I/O and memory

write signals which allows the devices to return an early READY and prevents the unnecessary

occurrence of wait states in the Operating Modes of 8257. It does this by activating MEMW

and IOW signals earlier in the DMA cycle, giving more setup time.

4. TC STOP Mode:

If the TC stop bit is set, a channel is disabled (i.e. its enable bit is reset) after the terminal count

(TC) output goes high, thus automatically preventing further DMA operation on that channel.

To enable DMA operation on the channel it is necessary to set enable bit of the corresponding

channel in the mode set register. If the TC STOP bit is not set, the occurrence of the TC output

has no effect on the channel enable bits.

5. Auto Load Mode:

Auto load Mode when enabled, permits block chaining operations, without immediate software

intervention between blocks. In this mode, channel 2 parameters (DMA starting address,

terminal count and DMA transfer mode) are initialized as usual for the first data block. These

parameters are automatically duplicated in the channel 3 registers when channel 2 is initialized.

After the first block of DMA cycles is executed by channel 2 (i.e., after the TC output goes

high), the parameters stored in the channel 3 registers are transferred to channel 2 during an


‘update’ cycle and next block of DMA cycle is executed. This repeat block operations can be

used in applications such as CRT refreshing.

During the update cycle, it is necessary to prevent the CPU from inadvertently skipping a data

block by overwriting a starting address or terminal count in the channel 3 registers before those

parameters are properly auto-loaded into channel 2.

Interfacing of 8257 with 8086


4.4 INTEL 8259A Programmable Interrupt Controller

The 8259A is a programmable interrupt controller specially designed to work with Intel

microprocessor 8080, 8085A, 8086, 8088. The main features of 8259A programmable interrupt

controller are given below:

1. It can handle eight interrupt inputs. This is equivalent to providing eight interrupt pins

on the processor in place of one INTR (in 8085A)/INT(in 8086) pin.

2. The chip can vector an interrupt request anywhere in the memory map from 0000H to

FFFFH in 8085A microprocessor. However, all the eight interrupts are spaced at an

interval of either four or eight locations. This eliminates the major drawback of 8085A

interrupts in which all interrupts are vectored to memory location on page 00 H i.e.,

TRAP, RST7.5, RST6.5 and RST5.5 are vectored to memory locations 0024H, 003CH,

0034H and 002CH respectively.

3. It can resolve eight levels of interrupt priorities in a variety of modes. The priorities of

interrupts can be changed under running condition. Some of the desired lower priority

interrupts may be allowed to be acknowledged during the service of higher priority

interrupts.

4. Each of the interrupt requests can be masked individually similar to RST7.5, RST6.5

and RST5.5 interrupts of 8085A.

5. The status of pending interrupts, in service interrupts, and masked interrupts can be read

at any time similar to RST interrupts of 8085A.

6. The chip can be programmed to accept interrupt requests either as level triggered or

edge triggered interrupt request unlike your RST interrupts where some are edge

triggered and some are level triggered. However, all interrupts must be either level

triggered or edge triggered.

7. If required, nine 8259As can be cascaded in a master-slave configuration mode to

handle 64 interrupt inputs. In this case, the interrupting devices send their interrupt

requests either to slave 8259A or to master 8259A directly. The slave 8259As send their

interrupt to master interrupt request inputs and the master will send a single interrupt to

microprocessor interrupt pin INTR/INT.


Interfacing of 8259 with 8086

8259 can be interfaced with 8086 in following ways,

1. 8259 in single mode and 8086 in minimum mode

2. 8259 in single mode and 8086 in maximum mode

3. 8259 in cascaded mode and 8086 in minimum mode

4. 8259 in cascaded mode and 8086 in maximum mode

1. 8259 in single mode and 8086 in minimum mode

[ Figure : 8259 in single mode and 8086 in minimum mode]


- The A0 input of the 8259A is used to select one of the two internal addresses in the

device. A0 of the 8259A is connected to system line Al, so the system addresses for the

two internal addresses are FFF0H and FFF2H as shown below.

- The data lines of an 8259 are connected to the lower half of the system data bus; because

the 8086 expects to receive interrupt types on these lower eight data lines.

- RD and WR signals are connected to the system RD and WR lines.

- The interrupt request signal INT from the 8259 is connected to the INTR input of the

8086 and 𝐼𝑁𝑇𝐴̅̅ ̅̅ ̅̅ ̅ from the 8086 is connected to 𝐼𝑁𝑇𝐴̅̅ ̅̅ ̅̅ ̅ on the 8259A.

- As we are using single 8259 in the system SP/EN pin is tied high and CAS0-CAS2 lines

are left open.

- The eight IR inputs are available for interrupt signals.

2. 8259 in single mode and 8086 in maximum mode

[Figure: 8259 in single mode and 8086 in maximum mode]


In maximum mode as shown in the above figure the signals are provided to the 8259 is via

8288 bus controller.

3. 8259 in cascaded mode and 8086 in maximum mode

[Figure: 8259 in cascaded mode and 8086 in maximum mode]

In cascaded connection multiple 8259 are connected in master slave configuration.

The INT pin of the slaves are connected to the IR0-IR7 pins of the master and 𝐼𝑁𝑇𝐴̅̅ ̅̅ ̅̅ ̅ to the

𝐼𝑁𝑇𝐴̅̅ ̅̅ ̅̅ ̅ of the master 8259.

The CAS0-CAS2 signals work as an output for the master 8259 and acts as an input for the

slave 8259.

4. 8259 in cascaded mode and 8086 in minimum mode

Draw interfacing diagram with combinations of case1 and 3.

chapter 04: peripherals interfacing with 8086 and applications€¦ · chapter 04: peripherals...

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