Entry 2: Pull-up and pull-down resistors

In entry 1, we briefly discussed the fundamental logic gates:

• NOT gate
• AND gate
• OR gate
• NAND gate
• NOR gate

I asked Grok how to demonstrate the workings of an logic gate. The answer was very clear, with the exception of "pull-up" and "pull-down" resistors, so let's begin there.

Pull-up and pull-down resistors.

I asked Grok "Explain "pull-up" and "pull-down" resistors and how I can demonstrate these using buttons, resistors and led."

Explanation of Pull-Up and Pull-Down Resistors

Pull-up and pull-down resistors are used in digital circuits to ensure that an input pin (e.g., on a microcontroller, logic gate, or IC) has a defined voltage level (HIGH or LOW) when it’s not actively driven by a switch, button, or another circuit. Without them, inputs can "float," picking up noise or random voltages, leading to unpredictable behavior.

Pull-Up Resistor

• Definition: A resistor connected between a pin and the positive supply voltage (e.g., +5V or VCC).
• Purpose: Pulls the pin’s voltage to HIGH (logical 1) when no other signal overrides it.
• Behavior:
  • When a switch connects the pin to ground (0V), the pin goes LOW (0).
  • When the switch is open, the resistor "pulls up" the pin to VCC (HIGH).
• Typical Value: 1kΩ to 10kΩ (depends on current and speed requirements).
• Use Case: Common in active-low switches (pressed = 0, released = 1).

Pull-Down Resistor

• Definition: A resistor connected between a pin and ground (0V).
• Purpose: Pulls the pin’s voltage to LOW (logical 0) when no other signal overrides it.
• Behavior:
  • When a switch connects the pin to VCC (e.g., +5V), the pin goes HIGH (1).
  • When the switch is open, the resistor "pulls down" the pin to ground (LOW).
• Typical Value: 1kΩ to 10kΩ.
• Use Case: Common in active-high switches (pressed = 1, released = 0)

Demonstration Using Buttons, Resistors, and an LED

To demonstrate pull-up and pull-down resistors, we’ll build two simple circuits using a button, resistors, and an LED. We won’t use a logic gate IC here (to focus purely on the resistor effect), instead directly observing the voltage state with the LED.

Materials Needed

• Breadboard
• Power supply: 5V (e.g., USB or battery).
• Push button (momentary switch).
• LED (e.g., red).
• Resistors:
  • 220Ω or 330Ω (for the LED).
  • 10kΩ (for pull-up or pull-down).
• Jumper wires.

1. Pull-Down Resistor Demo

Goal: Show that the input defaults to LOW (LED off) and goes HIGH (LED on) when the button is pressed.

Circuit Setup:

1. Connect +5V to one side of the button.
2. Connect the other side of the button to a breadboard row (call this the "input node").
3. Connect a 10kΩ resistor from the input node to ground (pull-down).
4. Connect the input node to the LED’s anode.
5. Connect the LED’s cathode to ground through a 220Ω resistor.

2. Pull-Up Resistor Demo

Goal: Show that the input defaults to HIGH (LED on) and goes LOW (LED off) when the button is pressed.

Circuit Setup:

1. Connect +5V to one side of a 10kΩ resistor.
2. Connect the other side of the 10kΩ resistor to a breadboard row (input node).
3. Connect one side of the button from the input node to ground.
4. Connect the input node to the LED’s anode.
5. Connect the LED’s cathode to ground through a 220Ω resistor.

I guess a point here is to avoid something like

with no pull-up or pull-down resistor. When the button is pressed, we do have 5V or HIGH to digital input but we do not have LOW or 0V when it is not pressed. In this case it is floating and the digital input will randomly detect a 1 or a 0.

In the next entry, I will start working on the fundamental logic gates.

Entry 1: Building a computer

Computer 

I asked Grok for the main components of a "simple" computer. I have "curated" the answer, removing part I felt were unnecessary or that I did not understand and I have updated certain parts to make them more clear.

1. Processor (CPU)

• Purpose: Executes instructions by performing the fetch-decode-execute cycle.
• Minimal Features:
  • Arithmetic Logic Unit (ALU): Performs basic operations (e.g., add, AND).
  • Control Unit: Interprets instructions and generates control signals.
  • Register: At least one general-purpose register (e.g., an accumulator) for data.
  • Program Counter (PC): Tracks the current instruction’s memory address.

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2. Memory

• Purpose: Stores both instructions (program) and data.
• Role: Holds the program (e.g., "add 5 to accumulator") and data (e.g., the number 5).

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3. Input/Output (I/O)

• Purpose: Allows the computer to receive input and produce output.
• Role: Provides basic interaction (e.g., input a number, output if it’s positive).

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4. Bus System

• Purpose: Connects the CPU, memory, and I/O for data and address transfer.

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5. Clock

• Purpose: Synchronizes operations by providing a timing pulse.
• Simplest Form: A push-button clock (manual pulse) or a basic oscillator (e.g., RC circuit).

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6. Power Supply

• Purpose: Provides electrical power to all components.  

§end§

It seems reasonable to begin with the CPU. 

CPU

Here is Groks (curated) answer to "What are the main parts of a simple as possible CPU":

1. Arithmetic Logic Unit (ALU)

• Purpose: Performs basic arithmetic and logical operations.
• Role: Processes data (e.g., adds two numbers from registers or memory).
• Example: Input 2 + 3, output 5.

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2. Registers

• Purpose: Provide fast, internal storage for data and addresses.

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3. Control Unit (CU)

• Purpose: Directs CPU operations by interpreting instructions and generating control signals.

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4. Instruction Register (IR)

• Purpose: Temporarily holds the current instruction fetched from memory.

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5. Bus Interface

• Purpose: Connects the CPU to external memory for fetching instructions and data.

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§end§

To me, the simplest one to start with seems to be a register.

Register

Most of the answer to the question "What are the main parts of a simple as possible register" was hard to understand but a few things stood out:

Purpose: Hold the binary data (0s and 1s) that the register represents.

Simplest Form: A single D flip-flop (for 1 bit).

As I understand it, a register is an internal (inside the CPU) memory which can store a number, such as "27". A D flip-flop is a memory that can store either a 0 or a 1. A register combines several such D flip-flops, allowing it to store a "bigger number" (not sure how yet).

D flip-flop

Here is Groks (curated) answer to "What is a D flip-flop":

A D flip-flop (Data flip-flop) is a fundamental digital electronic circuit used to store a single bit of data. It’s a type of sequential logic device, meaning its output depends not only on its current inputs but also on its previous state, making it ideal for memory elements like registers in a CPU.

§end§

The rest was hard to understand, but one thing caught my attention: A D flip-flop can be designed from two SR (Set-Reset) latches.

SR latch

Here is Groks (curated) answer to "What is an SR latch":

An SR latch (Set-Reset latch) is a basic digital electronic circuit that serves as a simple memory element, capable of storing a single bit of data (0 or 1). The SR latch is a foundational building block in sequential logic and is often used to construct more complex circuits, like the D flip-flop we discussed earlier.

§end§

Again, the rest was more difficult to understand. It is not clear to me how a D flip-flop is different from an SR latch, but we will get there. Again, one thing caught my attention: An SR latch can be designed from either two NOR gates or two NAND gates. "A NOR gate (short for "NOT OR" gate) is a fundamental digital logic gate".

Fundamental digital logic gates

Here is Groks (curated) answer to "Which are the fundamental digital logic gates?":

The fundamental digital logic gates are the basic building blocks of digital electronics, used to perform Boolean logic operations. These gates take binary inputs (0 or 1) and produce a binary output based on specific logical rules. They are essential for constructing all digital circuits, from simple switches to complex CPUs like the 8085. While there are many derived gates (e.g., XNOR), the following are widely recognized as the fundamental gates due to their simplicity and universal applicability.

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1. NOT Gate (Inverter)

• Function: Inverts the input—outputs 1 if the input is 0, and 0 if the input is 1.

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2. AND Gate

• Function: Outputs 1 only if all inputs are 1; otherwise, outputs 0.

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3. OR Gate

• Function: Outputs 1 if at least one input is 1; outputs 0 only if all inputs are 0.

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Universal Gates (Fundamental Due to Completeness)

The next two gates are often grouped with NOT, AND, and OR as fundamental because they are functionally complete—meaning all logic functions can be built using just one type of these gates.

4. NAND Gate (NOT AND)

• Function: Outputs 0 only if all inputs are 1; otherwise, outputs 1 (inverts an AND gate).

5. NOR Gate (NOT OR)

• Function: Outputs 1 only if all inputs are 0; otherwise, outputs 0 (inverts an OR gate).

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Common Additions (Not Always Fundamental)

While not always classified as fundamental (since they’re derived from the above), these are often included in basic gate sets due to their utility:

• XOR Gate (Exclusive OR):
  • Q = A ⊕ B (1 if inputs differ).
  • Not universal alone but critical for arithmetic (e.g., adders).
• XNOR Gate (Exclusive NOR):
  • Q = ¬(A ⊕ B) (1 if inputs are the same).
  • Useful for equality checking.

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§end§

In the next post, I will look into the logic gates in more detail. And then move on to the SR - latch, the D flip-flop and finally "the register".