Pico W four-digit display counts seconds

# Time counter: a four-digit display shows the seconds since start. The
# 74HC595 (DS GP18, STcp GP19, SHcp GP20) sets the segments, and pulling
# a digit's select pin (GP10 to GP13) low lights that digit.
from machine import Pin
import time

DIGITS = [0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F]
ds = Pin(18, Pin.OUT)
latch = Pin(19, Pin.OUT)
clock = Pin(20, Pin.OUT)
selects = [Pin(n, Pin.OUT, value=1) for n in (10, 11, 12, 13)]   # digit 1 to 4

def segments(byte):
    for i in range(7, -1, -1):
        ds.value((byte >> i) & 1)
        clock.value(1)
        clock.value(0)
    latch.value(1)
    latch.value(0)

def show(text):
    # text is four characters, digits or spaces, with an optional "." after
    # one of them. Each digit is lit for a moment in turn, fast enough
    # that the eye sees all four.
    cells = []
    for ch in text:
        if ch == "." and cells:
            cells[-1] |= 0x80
        else:
            cells.append(DIGITS[int(ch)] if ch.isdigit() else 0)
    for n, byte in enumerate(cells[-4:]):
        segments(byte)
        selects[n].value(0)
        time.sleep_ms(2)
        selects[n].value(1)

start = time.ticks_ms()
shown = -1

while True:
    seconds = time.ticks_diff(time.ticks_ms(), start) // 1000
    if seconds != shown:
        shown = seconds
        print("seconds", seconds)
    show("%4d" % (seconds % 10000))