8086 Pin Configuration and Signals

8086 Pin Configuration Overview

The 8086 microprocessor comes in a 40-pin DIP (Dual In-line Package). The pins are organized into different functional groups for address, data, control, and power signals.

Pin Categories:

• Multiplexed Address/Data: AD0-AD15 (16 pins)
• Address Only: A16-A19 (4 pins)
• Control Signals: Various control pins
• Status Signals: S0-S2, QS0-QS1
• Power and Clock: VCC, GND, CLK
• Mode Selection: MN/MX pin

Detailed Pin Description

1. Multiplexed Address/Data Pins (AD0-AD15)

These 16 pins serve dual purpose - they carry address during T1 state and data during T2, T3, T4 states.

Pin Numbers: 2-16, 39
Function: Bidirectional, multiplexed
T1: Lower 16 bits of address
T2-T4: 16-bit data bus

2. Address Pins (A16-A19/S3-S6)

These pins output the higher 4 bits of the 20-bit address during T1 state and status information during other states.

Pin Numbers: 35-38
A16/S3: Address bit 16 / Status bit 3
A17/S4: Address bit 17 / Status bit 4  
A18/S5: Address bit 18 / Status bit 5
A19/S6: Address bit 19 / Status bit 6

3. Status Signals (S0, S1, S2)

These pins indicate the type of machine cycle currently being executed.

Pin Numbers: 26, 27, 28
S2 S1 S0 | Operation
 0  0  0 | Interrupt Acknowledge
 0  0  1 | Read I/O Port
 0  1  0 | Write I/O Port
 0  1  1 | Halt
 1  0  0 | Code Access
 1  0  1 | Read Memory
 1  1  0 | Write Memory
 1  1  1 | Passive (no operation)

Control Signals

1. Clock Input (CLK)

Pin 19: Clock input pin that receives the system clock signal. Frequency typically ranges from 5-10 MHz.

2. Reset (RESET)

Pin 21: When high, resets the processor. Must be held high for at least 4 clock cycles.

3. Ready (READY)

Pin 22: Used to insert wait states for slow memory or I/O devices. When low, CPU waits.

4. Test (TEST)

Pin 23: Input pin used with WAIT instruction. CPU waits until TEST pin goes low.

5. Interrupt Request (INTR)

Pin 18: Maskable interrupt request input. Can be disabled by CLI instruction.

6. Non-Maskable Interrupt (NMI)

Pin 17: Non-maskable interrupt input. Cannot be disabled by software.

Minimum vs Maximum Mode Configuration

Mode Selection Pin (MN/MX)

Pin 33: Determines the operating mode of 8086.

• MN/MX = 1: Minimum Mode (single processor)
• MN/MX = 0: Maximum Mode (multiprocessor system)

Minimum Mode Pins (MN/MX = 1)

Pin 24: ALE (Address Latch Enable)
Pin 25: DEN (Data Enable)
Pin 26: DT/R (Data Transmit/Receive)
Pin 27: IO/M (Input-Output/Memory)
Pin 28: WR (Write)
Pin 29: HLDA (Hold Acknowledge)
Pin 30: HOLD (Hold Request)
Pin 31: RD (Read)

Maximum Mode Pins (MN/MX = 0)

Pin 24: QS0 (Queue Status 0)
Pin 25: QS1 (Queue Status 1)
Pin 26: S0 (Status 0)
Pin 27: S1 (Status 1)
Pin 28: S2 (Status 2)
Pin 29: LOCK (Bus Lock)
Pin 30: RQ/GT1 (Request/Grant 1)
Pin 31: RQ/GT0 (Request/Grant 0)

Queue Status Signals (Maximum Mode)

In maximum mode, QS0 and QS1 provide information about the instruction queue status.

QS1 QS0 | Queue Status
 0   0  | No queue operation
 0   1  | First byte of instruction from queue
 1   0  | Empty queue
 1   1  | Subsequent byte from queue

Practical Application:

These signals help external devices like 8087 coprocessor to monitor instruction execution.

Power Supply Pins

VCC (Pin 20)

Positive power supply: +5V ±5%

VSS/GND (Pin 11, Pin 40)

Ground pins: 0V reference

Power Consumption:

• Typical: 2.5W at 5V, 5MHz
• Maximum: 3.5W

Pin Configuration Diagram

Numerical Problems

Problem 1: Address Calculation

Question: If pins A19-A16 output 1010 and pins AD15-AD0 output 3456H during T1 state, what is the complete 20-bit address?

Solution:

A19-A16: 1010 (binary) = A (hex)
AD15-AD0: 3456H
Complete Address: A3456H

Verification:
20-bit address = A19A18A17A16 A15A14...A1A0
               = 1010 0011 0100 0101 0110
               = A3456H

Problem 2: Status Signal Decoding

Question: What operation is being performed when S2=1, S1=0, S0=1?

Solution:

S2 S1 S0 = 1 0 1
From status table: Read Memory operation
This indicates CPU is reading data from memory

Problem 3: Clock Frequency Calculation

Question: If 8086 operates at 8MHz, what is the time period of one clock cycle?

Solution:

Frequency = 8 MHz = 8 × 10⁶ Hz
Time Period = 1/Frequency
            = 1/(8 × 10⁶)
            = 0.125 × 10⁻⁶ seconds
            = 125 nanoseconds

Problem 4: Power Consumption

Question: Calculate current consumption if 8086 consumes 2.5W at 5V supply.

Solution:

Power = Voltage × Current
2.5W = 5V × Current
Current = 2.5W / 5V = 0.5A = 500mA

Practical Applications

1. Address Latching Circuit

External latches (like 74LS373) are used to separate address and data on AD0-AD15 pins.

2. Bus Interface

Pin signals are used to interface with memory and I/O devices in computer systems.

3. System Design

Understanding pin functions is crucial for designing microprocessor-based systems and troubleshooting hardware issues.

Summary

The 8086 pin configuration provides all necessary signals for building complete microprocessor systems. Understanding each pin's function is essential for hardware design, system integration, and troubleshooting. The multiplexed nature of address/data pins and the dual-mode operation make 8086 versatile for different system configurations.

Key Points to Remember:

• 40-pin DIP package with multiplexed address/data pins
• AD0-AD15 carry both address and data (time-multiplexed)
• Status signals S0-S2 indicate current operation type
• MN/MX pin selects between minimum and maximum mode
• Control signals manage bus operations and interrupts
• Proper power supply and timing are critical for operation