CLOCK Module

Episode 2

In this episode we are going to discover the CLOCK of our 20c computer. This system is responsible for synchronizing all the chips and components of our machine by emitting a regular signal over time.

We are going to study all its subsystems and different clock modes that we have available, then we are going to recreate this PCB board from scratch and we will finish with an experiment that will allow us to see how to use our clock step by step to be able to advance instruction by instruction on our computer.

CLOCK Overview

Our CLOCK PCB sets the pace at which everything happens on our 20c, it has three different clocks:

Adjustable speed or continuous pulse clock

Step by step clock

Fixed clock at 1Mhz

When we talk about a clock we are referring to a circuit with the ability to emit pulses. With these three clocks we will be able to carry out all of the 20c experiments. This clock is based on the one Ben Eater built for his 8 bit computer series. If you haven't seen it, it's excellent and you can go to the references in this article to see the links to the videos.

Adjustable speed or continuous pulse clock

Description

Our circuit consists of a speed-adjustable clock, this will allow us, by using a potentiometer, to regulate how fast we want our clock to emit pulses.

The circuit consists of a timer made with a NE555 timer connected in an a-stable or unstable way, which makes it turn on and off continuously, the speed with which it will turn on and off is regulated with a potentiometer.

Connection

For this type of connection we will connect the 555 pins as follows:

Pin 1 and 8 connect to GROUND and +5V respectively
The DISCHARGE 7 and THRESHOLD 6 pins are connected together through a resistor, in our case there are two resistors, the potentiometer R3 of 1 Mega ohm and the resistor R4 of 1K ohm. I used two resistors so that in case the potentiometer reaches zero it never has 0 resistance an damages the 555.
DISCHARGE pin 7 is also connected to +5v through a 1K ohm resistor R1.
THRESHOLD pin 6 also connects to TRIGGER pin 2 directly.
The TRIGGER 2 pin is connected to ground through a 1 uFarad (micro Farad) electrolytic capacitor C2.
We connect the RESET pin 4 to +5v so that it does not activate. If pin 4 is 0v, the Flip-flop is reset.
We connect pin 5 CONTROL to GROUND through a 10 nFarad (nano Farad) capacitor C1. This will allow us to improve the shape of the square wave of the pulses.
The Output of the circuit are the pulses themselves that come out of pin 3 of OUTPUT.

Operation

The 555 has internally two comparators (triangles on the left) and a Flip-Flop (rectangle).

The capacitor C2, the resistors R1, R3 and R4 are those that regulate the frequency of the pulses.

The capacitor is charged through the resistors and when it accumulates more than ⅓ of the voltage that we use, in this case 1.6v since the circuit has 5v, the first comparator is activated and the Flip-Flop is turned on, producing the output pulse at +5v.

When the capacitor reaches ⅔ of the voltage, the second comparator will activate the Flip-Flop Reset, discharging the capacitor through pin 7 DISCHARGE and outputting the OUTPUT pin to 0v.

When the capacitor discharges to 1.6v, comparator 1 re-activates the Flip-Flop SET and turns the OUTPUT back on to +5v.

By reducing resistance R3 by moving the potentiometer we will increase the frequency of the pulses since the capacitor will charge sooner.

The pulse frequency is calculated with the formula:

Frequency = 1.440 / ((R1+(2*R2))*C1) with R1 and R2 in kilo ohms and C1 in uF (microfarads)

In our case R1 is the resistance R1 of 1K and the resistance R2 of the formula is composed of the sum of the resistances R4 and R3. Resistor R1 has a value of 1k ohms. The sum of resistors R4 and R3 range from 1k ohms to 1001k ohms since the potentiometer is 1 mega ohms.

Here we can set 133 Hz for the Pulse Clock on our oscilloscope:

For more details on how the circuit works in astable form, I included the Texas Instrument datasheet in the references as well as a link to a video by Ben Eater that explains it.