Design Address Decoding Logic and Interfacing Schematic
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Introduction to Memory Interfacing and Memory Map
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Today, we are going to discuss memory interfacing with the 8085 microprocessor. Can anyone tell me why memory is so crucial in microprocessor systems?
Memory stores data and programs that the microprocessor needs to operate.
Exactly! Memory plays a key role in storing both data and instructions. Now, what is a memory map?
It's a representation of how different memory sections are allocated.
Right! A memory map helps us visualize the allocation of the address space among different memory devices. Let's move on to designing a memory map for our configuration.
Calculating Address Lines and Assigning Memory Addresses
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Now letβs discuss how we calculate the number of address lines needed for a memory chip. For example, how would we calculate for a 2KB ROM?
We use the formula log2(N), where N is the number of bytes.
Exactly! So, log2(2048) gives us 11 address lines. What about for a 4KB RAM?
That would require log2(4096), which is 12 address lines.
Perfect! Now, letβs assign the starting and ending addresses for each memory device based on this information.
Address Decoding Logic
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Now we have our memory map. Next, we need to understand address decoding. Can anyone explain what it entails?
It's about ensuring each memory chip can be uniquely addressed without conflicts.
Correct! Higher-order address lines are decoded to generate Chip Select signals. How do we achieve this?
We can use logic gates or dedicated decoder ICs!
Great job! We will see examples of both in the next part.
Execution of Assembly Programs for Memory Operations
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Finally, we need to see how to perform operations with our memory setup! What is the basic structure of an assembly language program for writing data?
It typically starts with loading the address, moving data to the accumulator, and then moving that data to the memory location.
Exactly! Letβs write a simple program to store the value 55H in address 2050H. Can anyone share the key instructions weβd use?
We would use LXI, MVI, and MOV instructions.
Well done! Thus, we can write, read, and transfer data using assembly language programs.
Verification of Memory Operations
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Now that we've learned how to write and read data, how do we verify these operations with the 8085 trainer kit?
We can enter the machine code and check the memory locations to see if the values have been correctly written or read.
Exactly! We can also single-step through the program to observe the changes. Why is this important?
It helps us understand how the program operates and troubleshoot any issues.
Great insight! Let's summarize today's lesson.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The section explains how to design a memory map for RAM and ROM interfaces, describe the role of address and data buses in memory access, and demonstrate how to implement address decoding for microprocessor memory interfacing.
Detailed
In this section, we explore the key aspects of memory interfacing with the 8085 microprocessor, including the fundamental principles of memory mapping, address decoding logic, and memory operation procedures. The 8085 has a 16-bit address bus that allows for addressing up to 64 KB of memory locations, and this section provides a step-by-step process to design a memory map for different configurations of RAM and ROM. Key components such as the roles of address lines (A0-A15), data bus (D0-D7), and control signals (overlineRD, overlineWR) are discussed in detail.
A memory map is created by calculating the necessary address lines for the specified memory size and then assigning start and end addresses for each memory module. The discussion includes the importance of address decoding, where higher-order address lines are used to generate unique Chip Select signals for each memory chip, thereby preventing multiple chips from responding to the same address. Examples of address decoding logic using logic gates and decoder ICs are included to help illustrate these concepts. Lastly, specific assembly language programs are provided to demonstrate read/write operations in RAM and ROM, culminating in the verification of memory operations using an 8085 trainer kit.
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Common Connections
Chapter 1 of 5
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Chapter Content
β Common Connections:
β 8085 A0-A10 to ROM A0-A10.
β 8085 A0-A11 to RAM A0-A11.
β 8085 D0-D7 to ROM D0-D7 and RAM D0-D7.
β 8085 overlineRD to ROM overlineOE and RAM overlineOE.
β 8085 overlineWR to RAM overlineWE (ROM does not have overlineWE).
Detailed Explanation
In this part, we define how the 8085 microprocessor connects to the ROM and RAM chips. The address lines A0 to A10 of the 8085 are connected to the corresponding address lines of the ROM, allowing the microprocessor to access locations within the ROM. Similarly, for RAM, A0 to A11 are connected to ensure the microprocessor can access the entirety of its memory space. The data lines (D0 to D7) facilitate the transfer of actual data between these components. Control signals overlineRD for read operations and overlineWR for write operations are also connected accordingly, with ROM not needing a write line since it is read-only.
Examples & Analogies
Think of the 8085 microprocessor as a postal service that needs to send and receive letters (data) from different mailboxes (ROM and RAM). The address lines are like the street addresses leading to the mailboxes, ensuring that letters end up in the right place. Data lines are the letters themselves, and control signals are the postman's instructionsβwhether to deliver (read) or retrieve (write) letters.
Chip Select Generation for ROM
Chapter 2 of 5
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β Chip Select (overlineCS) Generation:
β For ROM (0000Hβ07FFH): Requires A15=0, A14=0, A13=0, A12=0, A11=0.
β Decoding logic: Connect A11, A12, A13, A14, A15 to a 5-input NOR gate. The output of the NOR gate connects to the ROM's overlineCS.
β Alternative (more common for simple blocks): If A11 is the primary differentiator for the lowest block, overlineA_11 can directly enable it, assuming higher bits are implicitly zero for this range. For absolute decoding, use inverters on A11-A15 feeding an AND gate, and that output to CS.
Detailed Explanation
In this chunk, we focus on generating a Chip Select (overlineCS) signal for the ROM. This signal is crucial for ensuring that when the microprocessor wants to read data from the ROM, it activates only the ROM and not any other connected devices. For the address range of the ROM (0000H to 07FFH), specific address bits (A15 to A11) must be set to zero. A 5-input NOR gate can be used to simplify this logic: when all A15, A14, A13, A12, and A11 are zero, the output enables the ROM. Alternatively, for cases where A11 serves as the key differentiator, it can directly control access to the ROM, simplifying the design.
Examples & Analogies
Imagine a library where only one librarian is allowed to serve a specific section of books (ROM) at any given time. The address bits A15 to A11 are like criteria that tell the librarian when it's their turn to help readers. If everyone else is off-duty (zeroes), the librarian for that section is activated. It's efficient because it ensures that readers asking for books from that section are only served by one librarian at a time, just as the microprocessor interacts specifically with one memory at a time.
Chip Select Generation for RAM
Chapter 3 of 5
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β For RAM (2000Hβ2FFFH): Requires A15=0, A14=0, A13=0, A12=1.
β Decoding logic: Connect A15, A14, A13 (inverted) and A12 (direct) to a 4-input AND gate. The output of the AND gate connects to the RAM's overlineCS.
Detailed Explanation
Next, we focus on generating the Chip Select signal for RAM. Similar to ROM, the purpose is to enable the RAM when the microprocessor addresses it. The address range for the RAM is from 2000H to 2FFFH, which means that for A15, A14, A13, A12; specific patterns of high and low signals create unique access. For RAM, the signals inform the AND gate: A15, A14, A13 should be inverted (since they are 0), and A12 should be high (1). When this condition is met, the RAM is activated, allowing data to be written or read from this memory.
Examples & Analogies
Think of activating a specific elevator (RAM) in a tall building with several elevators (memory locations). Just as only one elevator open when the right combination of buttons is pressed (the proper address lines), the RAM is activated to serve data requests when the right conditions (address signals) are met. The AND gate represents the decision-maker for when the elevator is authorized to operateβthe right button combination ensures the correct elevator is called.
Master Memory Enable
Chapter 4 of 5
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β Master Memory Enable: The IO/overlineM signal (low for memory operations) should be incorporated into the overall chip select logic for each device (e.g., ANDed with the address decode output before connecting to overlineCS).
Detailed Explanation
In this section, we introduce the Master Memory Enable signal, which is crucial for determining whether the memory interacts with the microprocessor. The IO/overlineM signal indicates when to treat operations as memory-related. By integrating this signal into the Chip Select logic, we ensure that even if the address lines indicate a memory device, it only activates if IO/overlineM is low. This prevents inadvertent attempts to read/write to memory when the microprocessor is addressing I/O devices instead.
Examples & Analogies
Consider a phone system where calling someone depends not just on dialing the right number (addressing) but also needing the phone line to be active (IO signal). If the line is busy (high), dialing won't connect (memory won't operate). Only when the line is open can the right call go through, ensuring you reach the correct person (memory chip) without interruption.
Conceptual Interfacing Diagram
Chapter 5 of 5
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β Conceptual Interfacing Diagram (similar to Theory section, to be drawn clearly in the practical file):
8085 Microprocessor
+-----------------+
| |
A0-A15 ------| Address Bus |
| |
D0-D7 -------| Data Bus |
| |
IO/M --------| Control Signals |
RD ----------| |
WR ----------| |
+-----------------+
|
| Address Bus, Data Bus, Control Signals
V
+---------------------------------+
| |
| Address Decoding Logic |
| (Gates/Decoder IC) |
| |
β
β
A11-A15 & IO/M--| Input Address Lines & Control | --- CS_ROM, CS_RAM
| |
+---------------------------------+
|
+------------------+------------------+
| |
V V
+-------------+ +-------------+
| ROM | | RAM |
| (e.g., 2KB) | | (e.g., 4KB) |
+-------------+ +-------------+
A0-A10 --- A0-A10 A0-A11 --- A0-A11
D0-D7 --- D0-D7 D0-D7 --- D0-D7
CS_ROM --- CS CS_RAM --- CS
RD --- OE RD --- OE
WR --- WE
Detailed Explanation
Here, we outline how to create a conceptual diagram illustrating the connections and logic circuit required for the interfacing of the 8085 microprocessor with ROM and RAM. The diagram provides a visual overview of how the address bus, data bus, and control signals interconnect with memory. It captures key features like the address decoding logic utilized to generate Chip Select signals and clearly delineates how signals flow between the microprocessor, the address decoding logic, and the actual memory components (ROM and RAM). This visual representation helps to solidify understanding of the previously described logic and connections.
Examples & Analogies
Creating a conceptual diagram is like designing a floor plan for a new house. The address lines, data lines, and control signals correspond to the structure's layout and connectionsβensuring that rooms (specific memory locations) are correctly placed and connected through doors (bus lines). Just as an architect uses these diagrams to visualize and plan construction, engineers use interfacing diagrams to effectively design microprocessor systems.
Key Concepts
-
Memory Map: A representation of memory allocation in an address space.
-
Address Decoding: A technique ensuring unique responses from memory chips based on address lines.
-
Chip Select (CS): A control signal to enable specific memory devices.
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Control Signals: Used to manage data transaction operations effectively.
Examples & Applications
Calculating address lines required for a 2KB ROM involves using log2(2048), resulting in 11.
For 4KB RAM, log2(4096) indicates a need for 12 address lines.
Memory Aids
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Rhymes
In memory address space, chips have their place, / Decoding their lines, for data we chase.
Stories
Once upon a time, a microprocessor lived in a world of memory. Each memory chip had its address, and through clever decoding, the micro could find the data it needed without confusion.
Memory Tools
ABC for Address Bus Control: A for Address lines, B for Bus data, C for Control signals.
Acronyms
RAM
Random Access Memory; ROM
Flash Cards
Glossary
- Memory Map
A graphical or tabular representation of how memory addresses are allocated in a system.
- Address Decoding
The process of determining which memory chip is selected based on the higher-order address lines.
- Chip Select (CS)
Signal used to enable a specific memory chip in a system, preventing conflicts.
- Assembly Language
A low-level programming language that provides a symbolic representation of the machine code.
- Control Signals
Signals that manage the operations of data transfer in memory and I/O devices.
Reference links
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