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Shift Register Displays

The 74hc595 is a chip that takes a clock and data stream in and turns many LEDs on or off. 74hc595 chips can be connected in series to control 7 segments on 4 digits.

I'll create a detailed walkthrough of the clock-driver.py program, breaking it down into digestible sections with accompanying labs. This will help students understand both the code and the underlying concepts.

1. Core Components and Imports

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from machine import Pin, RTC 
from sr74hc595 import SR74HC595_BITBANG
from utime import sleep, localtime, ticks_ms

This section imports the necessary libraries. The program uses: - machine.Pin: Controls individual GPIO pins on the Pico - RTC: Real-Time Clock for keeping time - sr74hc595: Manages the shift register that controls the display - utime: Provides timing functions

Lab 1: Understanding GPIO Pins

Have students create a simple LED blink program:

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from machine import Pin
from utime import sleep

led = Pin(25, Pin.OUT)  # Built-in LED on Pico
while True:
    led.value(1)  # Turn on
    sleep(1)
    led.value(0)  # Turn off
    sleep(1)

2. RTC Initialization

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def wait_for_rtc():
    print("Waiting for RTC to be ready...")
    rtc = RTC()
    while True:
        current_time = rtc.datetime()
        if current_time[0] != 2021 or current_time[1] != 1 or current_time[2] != 1:
            print("RTC is ready!")
            return
        print("RTC not ready yet...")
        sleep(1)

This function ensures the RTC has been set to a valid time before proceeding.

Lab 2: RTC Basics

Have students experiment with reading and setting the RTC:

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from machine import RTC
from utime import sleep

rtc = RTC()
# Set the time (year, month, day, weekday, hour, minute, second, subsecond)
rtc.datetime((2024, 12, 27, 5, 14, 30, 0, 0))

while True:
    current_time = rtc.datetime()
    print(f"Current time: {current_time[4]:02d}:{current_time[5]:02d}:{current_time[6]:02d}")
    sleep(1)

3. Seven-Segment Display Setup

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class FourDigitClock:
    ALL_OFF = 0xFF  # All segments off (inverted logic)

    def __init__(self, ser_pin, srclk_pin, rclk_pin):
        self.ser = Pin(ser_pin, Pin.OUT)
        self.srclk = Pin(srclk_pin, Pin.OUT) 
        self.rclk = Pin(rclk_pin, Pin.OUT)
This class manages the four-digit display. It uses three pins to control the shift register.

Lab 3: Seven-Segment Pattern Display

Have students create a simple program to display a single digit: 

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from machine import Pin
from utime import sleep

# Create a simple version that lights up segments manually
segments = {
    'a': Pin(2, Pin.OUT),
    'b': Pin(3, Pin.OUT),
    'c': Pin(4, Pin.OUT),
    # ... add more segments
}

def display_number(number):
    # Pattern for number 1
    if number == 1:
        segments['b'].value(1)
        segments['c'].value(1)
    # Add more numbers...

4. Digit Patterns

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self.SEGMENTS = {
    'a': 4, 'b': 3, 'c': 2, 
    'd': 7, 'e': 6, 'f': 5, 'g': 1
}
self.DIGIT_SEGMENTS = {
    0: 'abcdef', 1: 'bc', 2: 'abged', 3: 'abgcd',
    4: 'fbcg', 5: 'afgcd', 6: 'afedcg', 
    7: 'abc', 8: 'abcdefg', 9: 'abfgcd'
}
This section defines which segments should be lit for each number.

Lab 4: Pattern Design

Have students draw and design their own custom characters using the seven segments. They can create: - Letters (A, b, C, d, E, F) - Custom symbols - Animated patterns

5. Time Display Logic

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def display_time(self, hour, minute, colon_state):
    # Convert to 12-hour format
    if hour > 12:
        hour -= 12
    elif hour == 0:  
        hour = 12

Lab 5: Time Format Conversion

Have students write a program that converts between 24-hour and 12-hour time formats:

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def convert_time(hour24):
    if hour24 > 12:
        return hour24 - 12, "PM"
    elif hour24 == 0:
        return 12, "AM"
    elif hour24 == 12:
        return 12, "PM"
    else:
        return hour24, "AM"

Advanced Labs and Extensions:

1. Alarm Clock Lab

Modify the clock to add alarm functionality: - Add a button to set alarm time - Add a buzzer for the alarm - Implement snooze functionality

2. Temperature Display Lab

Alternate between showing time and temperature: - Add a temperature sensor - Display temperature for 3 seconds every minute - Add a button to toggle between time and temperature

3. Custom Animation Lab

Create animations for the display: - Make digits spin when changing - Create a "snake" animation for the top of each hour - Design transitions between numbers

4. World Clock Lab

Modify the clock to show multiple time zones: - Add buttons to cycle through different time zones - Show timezone abbreviation - Store favorite time zones

5. Stopwatch Lab

Add stopwatch functionality: - Use buttons to start/stop/reset - Display tenths of seconds - Store lap times

Summary

These labs progressively build upon the base code while introducing new concepts and challenges. Each lab reinforces different programming concepts:

  • Variables and data types
  • Control structures (if/else, loops)
  • Functions and methods
  • Object-oriented programming
  • Hardware interaction
  • Time and date handling
  • User input processing

This allows students to learn both programming and hardware concepts in a hands-on, engaging way while creating something practical and visible.