⏱️ Timing Diagrams – Understanding 8085 Microprocessor Operations

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A visual journey into how the 8085 microprocessor executes instructions step by step

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🧩 Big Picture Context

The 8085 Microprocessor works step-by-step using electrical signals and clock pulses.

A Timing Diagram shows how signals change over time during these steps — for example, when the CPU reads data, writes data, or fetches an instruction.

It's like a time schedule for all actions happening inside the processor.

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Time-based

Shows when signals change

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Visual

Graphical representation

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Sequential

Step-by-step process

🔧 1. What is a Timing Diagram?

A Timing Diagram is a graphical representation that shows the relationship between signals and time in the microprocessor.

It tells us when:

  • 📖 the CPU reads data from memory,
  • ✍️ writes data to memory,
  • 🔍 fetches an instruction,
  • 🔌 or communicates with I/O devices.

Each line or wave in the diagram represents a signal (like clock, read, write, IO/M', etc.) changing from High (1) to Low (0) according to time.

Clock
T1
T2
T3
T4
RD'
WR'

⚙️ 2. Important Signals in 8085

Signal Full Form Purpose
IO/M' Input/Output or Memory Tells whether the operation is with memory (0) or I/O device (1)
S1, S0 Status Signals Define the type of operation (Fetch, Read, Write, etc.)
RD' (Read) Read Signal Activated (Low) when the processor reads data
WR' (Write) Write Signal Activated (Low) when the processor writes data
ALE Address Latch Enable Helps separate address and data on the same lines (AD0–AD7)
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IO/M'

Differentiates between I/O and Memory operations

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RD'/WR'

Controls read and write operations

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ALE

Separates address and data on shared lines

⏳ 3. Timing Units

Term Meaning Example
T-State (Clock Cycle) The smallest unit of time controlled by the clock T1, T2, T3...
Machine Cycle A set of T-states that completes one basic operation Opcode Fetch = 4T
Instruction Cycle Combination of all machine cycles required for one instruction MOV A, B = 1 Machine Cycle (4T)

🧮 Example:

Instruction: MOV A, B

→ Fetch the instruction (Opcode Fetch – 4T)

→ Execute (move data between registers)

Total time = 1 Machine Cycle = 4 T-states

🧠 4. The CPU's Working Process

Every instruction runs in two main steps:

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(1) Fetch Cycle (FC)

The processor fetches (reads) the instruction from memory.

Example: Reading the opcode of MOV A, B from memory address 2050H.

⚙️

(2) Execute Cycle (EC)

The processor executes the fetched instruction.

Example: Moving data from register B to register A.

✅ Instruction Cycle = Fetch Cycle + Execute Cycle

🔁 5. Types of Machine Cycles in 8085

Type Description T-States Example
Opcode Fetch Cycle Reads the instruction code from memory 4T Fetching "MOV A, B"
Memory Read Cycle Reads data from memory 3T Reading data from address 2050H
Memory Write Cycle Writes data into memory 3T Saving a result into memory
I/O Read Cycle Reads data from an input device 3T Getting key input from keyboard
I/O Write Cycle Sends data to an output device 3T Displaying result on screen

🧩 6. Opcode Fetch Cycle (Detailed)

When a new instruction starts, the first thing 8085 does is fetch its opcode from memory.

🧾 Example:

Fetch the byte 41H from memory address 2105H.

🔄 Steps:

T1: CPU sends the address (2105H) on the address bus. ALE = 1 → lower 8 bits (A0–A7) are latched.
T2: IO/M' = 0 → indicates memory operation. RD' becomes LOW → tells memory to place data on data bus.
T3: Memory places the byte 41H (opcode) on the data bus. CPU reads this byte and stores it.
T4: Opcode is decoded. CPU understands it means MOV A, B.

After this, the CPU knows what to do next (copy data from B to A).

🕐 Total Time: 4T (4 clock cycles)

💾 7. Memory and I/O Read/Write Cycles

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Memory Read Cycle

Used when the CPU needs to read data from memory.

Example: Read data from address 2050H.

Takes 3T-states (T1–T3).

✍️

Memory Write Cycle

Used when the CPU wants to write (store) data into memory.

Example: Store result into address 3050H.

Takes 3T-states.

📥

I/O Read Cycle

Used to read data from an input device (keyboard, sensor).

Example: Get a key press.

Takes 3T-states.

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I/O Write Cycle

Used to send data to an output device (screen, LED, printer).

Example: Display output result.

Takes 3T-states.

🌍 8. Real-World Analogy

Imagine a teacher and a student:

The teacher (Memory/I-O) gives questions or receives answers.

The student (CPU) reads the question (Fetch), understands it (Decode), and writes the answer (Execute).

Each question-answer process takes fixed time steps — like T1, T2, T3 — similar to how the CPU completes its cycles in a fixed pattern.

👨‍🏫
Teacher

Memory/I-O devices

👨‍🎓
Student

CPU

📚
Question

Instruction

✍️
Answer

Execution

🧭 10. Summary Table

Concept Meaning Example
T-State Smallest unit of time One clock tick
Machine Cycle Group of T-states for one operation Opcode Fetch (4T)
Instruction Cycle Fetch + Execute MOV A, B
Opcode Fetch CPU reads instruction from memory 41H from 2105H
Memory Read/Write CPU reads/writes data to memory Reading/saving file
I/O Read/Write CPU interacts with devices Keyboard/display
Fetch & Execute Two parts of every instruction Read & perform action

Thank you for exploring 8085 Microprocessor Timing Diagrams!

Remember: Understanding timing is key to mastering microprocessor operations.