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:





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:


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.


Step by step clock

Description


Our circuit consists of a step-by-step clock that allows us to advance at our own pace as we press a button, each time we press this button a clock pulse is emitted.

The circuit consists of a timer made with a NE555 timer connected in a mono-stable manner, which makes it turn on for a fixed time after the button is pressed. 


If I only used a button without a 555, if I pressed it many times, the internal metal contacts would bounce and it would give me many pulses instead of just one, so if I wanted to advance the clock step by step it might sometimes give me two or three pulses because of the bouncing. When using a 555 in monostable mode it will release a single pulse through the OUTPUT pin.


Connection


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


Operation


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

When turning on our circuit, it finds itself with the OUTPUT off since in the COMPARATOR 1 circuit the voltage brought by TRIGGER pin 2 is 5v, which is greater than ⅓ of the voltage of the entire 5v circuit, which would be 1.67v. The entire circuit is discharging to GROUND internally through pin 7 THRESHOLD.


The KEY1 button that is connected in pull-up mode will make TRIGGER pin 2 go from +5v to 0v when pressed. This will make COMPARATOR 1 compare 1.67v with 0v of the TRIGGER pin and since 1.67 is greater, it will activate the flip-flop and give a pulse through pin 3 OUTPUT.


When the output is activated, the DISCHARGE pin transistor is deactivated, so it stops discharging GROUND and the capacitor C1SMOOTHINGCAPACITOR2 begins to charge through resistor R9 when this capacitor exceeds the value of ⅔ of the total voltage or 3.3v of COMPARATOR 2 it will be activated by resetting the flip-flop and cutting the pulse. That is why the duration of the pulse is determined by the resistor R9 and the capacitor. 


The capacitor discharges immediately and our circuit will be ready to perform a new cycle once we press the button again.


The values ​​we use will make the pulse last a tenth of a second, so any rebound that happens during that tenth of a second will be able to be eliminated with this method.


The time that the pulse stays on is calculated with the formula:


Time = R ohms * C farads* 1.1 with R in kilo ohms and C in microfarads. 


In our case R9 is the 1 mega ohm (1000 k ohms) resistor R and the 100 nF (nano farads) capacitor C2. If we do the calculations our pulse lasts:


T = 1000000 ohms * 0.0000001 farads * 1.1 = 0.11 seconds or one tenth of a second.


T= 1000 k ohms * 0.1 micro farads * 0.0011 = 0.11 seconds or one tenth of a second.


Here we can see how the pulses are recorded on our oscilloscope every time we press the button:

For more details on how the circuit works in monostable form, I include the Texas Instrument datasheet about in the references.


Clock at 1 Mhz

Description


This circuit is very simple, it has a quartz crystal with an oscillator that vibrates at 1 Mhz and a switch that we can use turn it on or off depending on whether we are using it. It will continuously emit 1 million pulses per second.

The circuit consists of a crystal oscillator that emits pulses at the frequency of 1 Mhz. It operates at 5 Volts, when the crystal receives this voltage it expands and contracts at a fixed speed determined by the size of the crystal and how it was cut.

Connection


The crystal is connected to its output pin directly to the OUT_1MHZ Output Header. We include a power off switch connected to the 5v connection of the crystal to turn it off when not in use so that it does not generate electronic noise to the other two clocks.

Operation


The native signal of a quartz crystal is not a square wave but a triangular one, the same integrated circuit has an oscillator that transforms that triangular or analog sine wave of the quartz crystal into the square wave that the computer expects to operate.


To use it, we only connect the OUT_1MHZ header to the device that needs a 1Mhz clock signal.


Here we can see how the pulses are recorded on our oscilloscope at 1 million times per second:

For more details on how the crystal works, I include the datasheet in the references.


Anti bouncing or anti rebound circuit


Description


When we press a button or activate a switch mechanically, the contact is not always perfect and many mini contacts called bounces can occur. To prevent this bounce from occurring when choosing between the pulse or continuous clock function, we use an anti-bouncing circuit.

This consists of a circuit made with a NE555 timer connected in a bi-stable manner, which means that it remains with its output at one value or another but does not oscillate between the two. 


Connection


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


Operation


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

The SW1 button is what regulates what state the circuit is in by connecting the TRIGGER pin or the RESET pin to ground. 


If the TRIGGER pin is not connected to GROUND through switch SW1, it is at +5v because it is connected in a pull-up manner, therefore the COMPARATOR 1 of the flip-flop has +5v at its TRIGGER pin input and because this value is greater than ⅓ of the total voltage of 5v or 1.67v, it is disabled. If we connect the TRIGGER pin to GROUND through switch SW1 we have that 1.67v is greater than 0v and this is why the flip-flip on the SET pin is activated causing the OUTPUT pin to always remain active.


Since the Threshold pin is at 0v, which is less than ⅔ of the total voltage of 5v, that is, 3.3v, it will never activate COMPARATOR 2.


If the RESET pin is not connected to GROUND through switch SW1, it will be at +5v because it is connected in a pull-up manner, and it does nothing. Pressing the switch and connecting it to GROUND activates the R1 reset input of the flip-flop and deactivates it by cutting the output of the OUTPUT pin.


The duration of the pulse is how long we keep what SW1 closed or open in the position we want. This switch only does not select whether to use the continuous pulse clock or the step-by-step clock as output, but for that we will use TTL logic along with the output of this circuit.



TTL logic

Description


We are going to use 3 gates

Connection


AND gate



NOT gate



OR gate


Header Connector


Operation


The AND, NOT and OR gates have 3 very different functionalities which we are going to observe by looking at their truth table.


AND


It needs both inputs to be HIGH or +5v to give a result of HIGH or +5v.

NOT


Inverts the result of its input if it receives a HIGH or +5v it results in a LOW or 0v.

OR


Needs either of two inputs to be HIGH or +5v to result in a HIGH or +5v.

The Pulse Clock signal called OUT_POT is connected to an AND gate with the output signal of the 555 flip-flop OUT_BI when both are 1 this generates the output AND_OUT_POT_BI_31 which represents the pulse value already filtered for the continuous pulse clock.

The step by step Clock signal called OUT_PUSH is connected to an AND gate with the output signal of the 555 bistable OUT_BI when it is off, but since it is an AND gate we need a 1 when it is disconnected instead of a 0 so we pass it through the inverter firt and then we generate the NOT_OUT_BIT signal. Now when both signals are 1 this generates the output AND_OUT_NOT_BI_PUSH_62 which represents the pulse value already filtered for the step clock.

The Step by Step Clock (AND_OUT_NOT_BI_PUSH_62) and Pulse Clock (AND_OUT_POT_BI_31) signals, both already filtered by the 555 bistable, are connected to the OR gate where the signal of one or the other appears, outputting OR_OUT_39 on pin 11.

The signal from both OR_OUT_39 outputs is ANDed as pin 2, with the negative of the HLT signal ( NOT_HLT_810) or HALT passing through an inverter to generate the clock signal CLK as pin 1. 

In this manner, if HLT is connected to 0v when it passes through the inverter it will be +5v when going to the AND gate, each time it receives a clock pulse it will execute the AND operation with the +5v of the inverse of HLT (NOT_HLT_810) and produce a pulse clock on pin 3 of the AND gate called CLK.


With this we have our three clock outputs. In the Header OUT_555 that of the Pulse Clock or that of the step by step CLK signal and in the Header OUT_1MHZ that of the 1Mhz crystal oscillator, both completely independent.


20c CLOCK Board Circuit


The following graphic shows the circuit that explains the operation of the CLOCK board.

The circuit has the following elements:


SOCKETs 555


We have three 8-pin Sockets in dip format to place the 3 NE555 timers, it is important to align pin 1 with the value of pin 1 that we will find written on the PCB board.


SOCKETS AND, NOT, OR


These are three 16-pin dip type sockets to house our AND 74HC08, NOT 74HC04 and OR 74HC32 TTL chips. It is important to align pin 1 with the value of pin 1 that we will find written on the PCB board.

 


HEADER OUT_555


This is an 8-pin female header which has a ground connection to connect to the HLT or halt port so that our clock starts, three CLK pins with the clock pulse and 3 NOT CLK pins with the inverted clock pulse.


HEADER OUT_1MHZ


This is an 8-pin female header which has the output of our 1Mhz oscillator


SWITCHES ON/OFF


We have two single pole double throw switches. One is used to select between the Pulse Clock and the Step by Step Clock and the other is used to turn the 1Mhz crystal oscillator on or off. 


HEADER POWER y GROUND


This header has two pins and they are connected to the 5V and GROUND layers respectively. We have four of these connectors, one in each corner of the board.


BUTTON


We have a button to be able to mark the pulses manually when we use the step by step clock. 


POTENTIOMETER


For our speed-adjustable clock we use a 1 Mega Ohm potentiometer that gives us a variable resistance which allows us to increase or decrease the speed of our clock pulses.


RESISTANCES


We use various resistors to regulate our NE 555 timers.


CAPACITORS


We use various capacitors to regulate our NE 555 timers.


HOLES for SCREWS


To be able to fix our PCB to different places, it has 6 holes for 3mm or M3 screws.


NETLIST


The components are connected to each other through an object called netlist which allows us to have cleaner diagrams.

 

This works in such a way that everything that has the same name belongs to the same netlist and must be connected together when we make the PCB (with traces). 


In our example all CLK connections will be connected by traces on the PCB

20c CLOCK Board PCB


The following PCB explains how the elements and traces are located to connect our circuit. The PCB is 4 layers, having in the layer called TOP all the connections of our netlist through traces, which are thin layers of copper that connect them as if they were cables.

We have two special layers called 5v and GROUND where we will connect the power and ground connections. By using these connections on separate layers we save many centimeters of traces that should be connected to pins with the 5V and GROUND voltages and we can directly connect these pins to the corresponding layer.

Another advantage of having an entire layer for GROUND is being able to isolate electrical noise from the connections between components.


Lastly we have the BOTTOM layer where our components are soldered and we have additional connections.

3D modeling


When we finish making our PCB we must proceed to 3D modeling it to see how it looks and if we like the location where we placed each component.

Source file generation


In order to send our PCB to be manufactured we must have at least 3 files:


1. gerber file where we tell the manufacturer how to assemble our board

2. BOM or Bill of materials file where all the components that make up our board are located with the manufacturer's product code.

3. Pick and Place file where we tell the manufacturer how to place the components on our PCB board.


You will find these files previously generated in the Osolabs github. 


https://github.com/osolabstech/The20c/tree/main/Episode2_CLOCK/v03_CLOCK_4Plane 


We also include files to be able to edit the circuit with EasyEDA or Altium Designer software.


With these three files we will be able to manufacture our board:


Gerber_v03_CLOCK_4Plane_2025-01-28.zip

This file contains the blueprint to generate our PCB board.


BOM_v03_CLOCK_4Plane_v03_CLOCK_4Plane_2025-01-28.xlsx

It contains all the components of our board and the manufacturer's code that identifies that component.


PickAndPlace_v03_CLOCK_4Plane_2025-01-28.xlsx

It indicates where on the PCB each component goes and what layer it should be soldered to.


The following files are provided to be able to edit the circuit: 


3DModel_CLOCK_FRONT.png

It is an image with the front of our PCB modeled in 3D


3DModel_CLOCK_BACK.png

It is an image with the back of our PCB modeled in 3D


Altium_Circuit_1_v03_CLOCK_4Plane_Page1.schdoc

Contains the circuit design editable with the Altium Design program.


Altium_Circuit_2_v03_CLOCK_4Plane_Page2.schdoc

Contains the circuit design editable with the Altium Desig program.


Altium_PCB_v03_CLOCK_4Plane.pcbdoc

Contains the PCB design editable with the Altium Desig program.


PCB_v03_CLOCK_4Plane_2025-01-28.pdf

It is a PDF with a drawing of each layer of the PCB


ProDoc_v03_CLOCK_4Plane_2025-01-28.epro

File for EasyEDAPro with the circuit and the PCB which can be edited.


SCH_v03_CLOCK_4Plane_2025-01-28.pdf

It is a PDF with the logic circuits of the board.


Preparation of CLOCK PCBs


Upon receiving our PCB from the factory and before starting to use it as soon as we take it out of the box, there are some tasks that we must do in order to be able to make the most of it:

1. Remove handling edges from PCB

2. Clean PCB with isopropyl alcohol

3. Put 555 chips in the sockets


4. Put AND, NOT, OR chips in the corresponding sockets.


5. Put screws to support the PCB so that it is raised above the surface.

20c CLOCK board Connection Guide


We are going to explain how to connect the different components of the board to leave it ready to work in our 20c computer. This is a configuration with all the PCBs that we are going to use even if we have not seen them all yet in our articles. You can keep this section as a reference to be used when you assemble the entire 20c.


CLOCK


To configure the 6502 CPU board we are going to make the following connections.


1. We connect the ground connector of the CLOCK_555_OUT header to the HLT pin, this is to turn on our clock as the HLT pin is used to turn the clock on and off programmatically and it needs to be connected to ground to start.


2. Connect the CLK pin of the CLOCK_555_OUT header of our CLOCK board to the EXPANSION BUS header of another 20c board that needs clock signaling (for example the 6502 CPU board) in this manner we can give pulses with the clock adjustable speed or step by step formats. 


3. Alternatively, connect the OUT_1MHZ pin of the CLOCK_OUT_1MHZ header of our CLOCK board to the EXPANSION BUS header of another 20c board in this manner we can give pulses with the 1 Mhz clock.


4. Connect the 5v and GROUND pins to a 5V and 0v source, respectively.

Experiment - Testing CLOCK with the RESET Process and the NOP instruction


We are going to carry out this experiment again testing our CLOCK but we will vary its speeds. We will go over what the NOP instruction setup looks like and the basis of the experiment.


We are going to hardcode our 6502 CPU to execute the non-operation instruction NOP, it is coded as EA in the 6502 machine language instructions.

EA is translated into binary as 1110 1010 and this is what our processor will wait on its data pins to be able to read that instruction. In order to do this we are going to connect 8 resistors to our data pins as follows:

If we run a reset sequence by touching the RESET button on the CPU 6502 board and observe the result, we will see how it executes the reset in 7 clock cycles and goes to the memory positions FFFC and FFFD where the RESERT vector is (another memory position ) it has the addresses of the first instruction to execute. It takes 7 cycles since a RESET it is a BRK or BREAK instruction loaded by hardware through the RESET pin.


 The reset must be held at 0v for at least two clock cycles to be recognized.



Experiment 1 - Step by Step Clock








Experiment 2 - Speed ​​adjustable clock







Experiment 3 - 1 Mhz Clock







Conclusions


The CLOCK Board will give us the rhythm to execute the instructions of our 20c and synchronize all the chips that we connect to it, memories, cpu peripherals, etc. 


The CLOCK board in operation


To visually see how to connect the CLOCK board to the 20c, as always, I leave you with a detailed video of the series. In this we are going to see how the pins are connected, and see our experiment in a visual manner. Find the video of Episode 2 


https://www.youtube.com/playlist?list=PL1OpW6G6pEzMtTNEjPg9Uvo_5qGXiU20o 

References

Below I leave you some links where you can delve deeper into the topic:


WEBSITE

Here is the OsoLabs site with all the videos and articles.


OsoLabs.tech 


VIDEOS

Here is the video corresponding to this chapter and the list of all the videos in this series.


https://www.youtube.com/playlist?list=PL1OpW6G6pEzMtTNEjPg9Uvo_5qGXiU20o 


https://en.osolabs.tech/the20c 


ARTICLES

Here you can find all the articles about 20c:


https://blog.espaciotec.com.ar/la-20c-indice-de-episodios/ 


https://en.osolabs.tech/the20c 


CODE and PCB BOARD DESIGN

All the code examples in this article can be found at:


https://github.com/osolabstech/The20c/ 


OTHER RESOURCES

And as always, Ben Eater's series from 6502 is excellent:


Build a 6502 computer | Ben Eater 


Astable 555 timer - 8-bit computer clock - part 1 


Monostable 555 timer - 8-bit computer clock - part 2 


Bistable 555 - 8-bit computer clock - part 3 


Clock logic - 8-bit computer clock - part 4 



And the series of my creation comparing the 6502 vs 6510 both in video and in articles


https://www.osolabs.tech/6502vs6510 


6502 vs 6510 detailed study and comparison - YouTube 


C64 in depth – Index


PAPERS / Datasheets


NE555P Datasheet 


1 Mhz Crystal Oscillator 


SNx4HC08 Quadruple 2-Input AND Gates datasheet (Rev. I) 


SNx4HC04 Hex Inverters datasheet (Rev. H)


SNx4HC32 Quadruple 2-Input OR Gates datasheet (Rev. F)  


SNx4HC00 Quadruple 2-Input NAND Gates datasheet (Rev. H)