picoclock

The idea is to use a Raspberry Pico board to generate a clock signal that could be directly connected to the PHI2 pin of a 6502 microprocessor.

It is a square wave generator that can generate frequency from 1 Hz to 127 Mhz, the maximum frequency you can get from a Raspberry Pico GPIO pin configured as PWM.

Picoclock will have a four-digit, seven-segment display to inform the user about the current frequency and operational messages.

Picoclock will have six push buttons:

• Reset: reset picoclock to the default configuration. The configuration is stored in the main.py file.
• Cycle: Cycle the first digit from 0 to 9.
• Shift: Multiply the current sequence by ten. Combined with the Cycle button, this mechanism will allow you to enter frequencies from 0 to 9999.
• Set: This button will configure picoclock with the current displayed frequency and start the clock.
• Start/Stop: Start and stop the clock to allow the user to freeze the 6502 in the current state. This will work only if you use a modern 6502 with a fully static design (e.g., W65C02S6TPLG-14 from The Western Design Center, Inc.)
• Pulse: Pushing this button will generate a transition from low to high and high to low on the clock output pin. This allows you to single-step the 6502 microprocessor.

I have decided to go with MicroPython since I use Python for most of my projects.

The main.py file contains the following configuration:

# GPIO pin mapping

PIN_CLOCK = 0                # Output pin for generated clock signal
PIN_BUTTON_RESET = 16.       # Reset button
PIN_BUTTON_CYCLE = 17        # Cycle button
PIN_BUTTON_SHIFT = 18        # Shift button
PIN_BUTTON_SET = 19          # Set button
PIN_BUTTON_START_STOP = 20.  # Start/Stop button
PIN_BUTTON_PULSE = 21        # Pulse button
PIN_LED = 25                 # On board led (on if the clock is running)
PIN_DISPLAY_CLK = 2          # i2c CLK Pin (Mapped on i2c1 on Pico)
PIN_DISPLAY_DIO = 3          # i2c DIO Pin (Mapped on i2ci on Pico)

You can modify these according to your taste.

DEFAULT_CLOCK_FREQUENCY = 1 # Frequency in Hz
DEFAULT_CLOCK_MULTIPLIER = 1 # Multiplier for PWM frequency

These are three global variables that declare: DEFAULT_CLOCK_FREQUENCY: The starting frequency value to be displayed and used upon board startup. DEFAULT_CLOCK_MULTIPLIER: When setting or starting and stopping the clock, the clock frequency will be multiplied by the value of this variable.

I use picoclock with a minimal frequency and have always stayed within 1 Mhz. It should work :-).

This is it. I have put this together in an hour or so, and I am no expert at all in circuit design. The Python code is 'quick and dirty' and needs to be optimized.

Schematics

This is the circuit I have designed. My scarce knowledge of Kicad did not allow me to do any better. You will find the Kicad files for this "project" in the kicad directory.

Bill of materials

This is what you will need to build a picoclock:

• One 840 points solderless breadboard. Please buy a high-quality board to avoid issues.
• One Raspberry Pi Pico (SKU:  RPI-PICO).
• Six button switches. Choose some that can fit on the breadboard easily.
• One four-digit 7-segment display with an i2c interface.
• Six 220 ohm resistors.

Picoclock assembly

The circuit is so simple that you will not need help putting it together. Below is a picture of my final assembly (if you can say "final" to any personal project).

Software

You will need to set up the Raspberry Pi Pico for MicroPython development. You can find detailed instructions here: MicroPython

I have used Visual Studio Code for development and testing. Even if I prefer to work with PyCharm, the Visual Studio Code MicroPython extension is much better than the one you will find on PyCharm. The Visual Studio Code extension I have used is MicroPico.

Just upload the project to your Raspberry Pico, and you will be ready.

Development note

While writing the software, I discovered that the Pico PWM can not generate frequencies below 8 Hz. This is why I had to use PWM for frequencies greater than 10 Hz and a Timer for frequencies below or equal to 10 Hz. Interestingly enough, I did not have the same problem using a Raspberry Pi Model 3b.

Connecting to the 6502 microprocessor

In order to connect picoclock to a 6502 board, that does not already have a clock, you need to follow two simple steps:

• Connect the Raspberry Pi Pico GP0 pin to the 6502 PHI2 pin. (PHI2 pin is pin 37 on the 6502 microprocessor)
• Connect picoclock ground to the 6502 ground.

Final words

This project is not up to par with others built by much more experienced people out there. It is something that works for me and that I wanted to share.

It is also expensive compared to other square wave generators you can build or buy. I always use these kind of projects to learn something.

License

This work is licensed under the MIT License.