Lab 15: Stopwatch

Time a race, a breath-hold, or a trip around the block. This stopwatch shows minutes, seconds and hundredths of a second. It also keeps lap times, the time each trip around a track takes. Press the knob each time you pass the start line.

Control	What it does
K0	Start and stop
Press the knob	While running: record a lap. While stopped: reset to zero.
Turn the knob	Nothing in this lab

The top line of the screen always says what the controls do right now. It calls a press of the knob PUSH.

Welcome to Lab 15

On your marks! This stopwatch times laps the way a coach's does, and it hides a timing trick that real stopwatches use. Let's make time tick!

What You Will Learn

• how a stopwatch keeps time without counting
• how to find the fastest lap and the average lap
• how one file can hold parts that many programs share

Run It

1. Open 15-stopwatch.py in Thonny and click Run. The screen shows 00:00.00 and the top line says K0 start.
2. Press K0. The digits start to run and the top line turns green.
3. Press the knob straight down. A lap time appears under the digits.
4. Press the knob two more times, a few seconds apart. The fastest lap turns green, and Best and Average appear.
5. Press K0 to stop. The top line turns yellow.
6. Press the knob to reset to zero, or press K0 to keep going.

Reset only works while the stopwatch is stopped. That way a bump of the knob can't wipe out a run.

It Remembers When It Started

You might guess that a stopwatch counts: add a little time, again and again. It doesn't. Each trip around the loop takes a slightly different time, so the small errors would pile up.

Think of a cook who puts a cake in the oven. The cook doesn't count seconds out loud for an hour. The cook writes "in at 3:15" on a note, and looks at the wall clock later.

The Pico has a wall clock too: a counter of milliseconds, or thousandths of a second, that never stops. When you press K0, the stopwatch writes down the counter's reading. To find the time so far, it reads the counter again:

def elapsed_ms(now):
    if state == RUNNING:
        return banked_ms + time.ticks_diff(now, run_started)
    return banked_ms

• now is the newest reading of the counter.
• run_started is the reading from when you pressed K0.
• time.ticks_diff(now, run_started) is how far apart those two readings are.
• banked_ms is time saved up from before the last stop. It starts at 0.

Follow the numbers through one run:

What happens	Counter reads	run_started	banked_ms	Stopwatch shows
You press K0 to start	20,000	20,000	0	00:00.00
A moment later	25,890	20,000	0	0 + 5,890, so 00:05.89
You press K0 to stop	27,000	20,000	7,000	00:07.00
You press K0 to go on	60,000	60,000	7,000	00:07.00
A moment later	61,500	60,000	7,000	7,000 + 1,500, so 00:08.50

The program reads the counter with time.ticks_ms(). For a reminder, see the timing functions lab in Learning MicroPython.

Which Moment to Remember?

When a program must measure time, ask which moment it should write down. The stopwatch writes down its start. In the next lab, the timer writes down the moment it will end.

Laps: Fastest and Average

A lap is the time since you last pressed the knob. This code runs each time you press it:

    elif push_pressed and state == RUNNING:     # lap
        total = min(elapsed_ms(now), MAX_MS)
        laps.append(total - lap_started_ms)
        lap_started_ms = total
        show_laps()

• total is the stopwatch time right now. MAX_MS stops it at 99:59.99, the most the digits can show.
• total - lap_started_ms is this lap: the time now, minus the time when the lap began.
• laps.append(...) adds the lap to a list named laps.
• lap_started_ms = total starts the next lap from here.

Then show_laps() does some arithmetic on the list. Python's min() finds the smallest number, which is the fastest lap. For the average, it adds up all the laps with sum(), then divides by how many there are.

Check the picture. The list has room for the two newest laps, 1.50 and 2.00 seconds. Lap 1 took 2.00 and has scrolled off. The fastest is 1.50, so lap 2 is green. The average is 5.50 ÷ 3, which is about 1.83.

Parts Packed in One File

This program is short because it borrows its parts. The file lib/watchparts.py holds the parts from earlier labs, packed in one file. A file like that is called a module, and import brings its parts into your program:

from watchparts import Digit, TickFrame, TextLine, Button, DIGITS

Part	What it is	Where you met the idea
Digit	a seven-bar digit that repaints only the pieces that change	lab 13
TickFrame	the 60 ticks around the edge of the screen	lab 13
TextLine	a line of text that repaints only the letters that change	lab 13
Button	a push button that ignores bounce	lab 12
Knob	the knob, counted in clicks	labs 06 and 07

DIGITS is lab 13's table of bit patterns. This lab doesn't turn the knob, so Knob waits for the next lab.

Try This

1. Press the knob quickly for lap 1. Take your time on laps 2 and 3. Lap 1 has scrolled off the list. Does Best still remember it?
2. Change FASTEST_COLOR = config.GREEN to config.MAGENTA. Which two things change color?
3. In the HINTS line, change "K0 start" to "Ready? Press K0!". Keep it to 18 characters or fewer, or the end gets cut off.
4. Change LAP_ROWS = 2 to LAP_ROWS = 1. What moves?

If It Doesn't Work

• Pressing the knob does nothing. A lap only works while the stopwatch runs. Reset only works while it is stopped. Read the top line of the screen.
• Best and Average are blank. They appear after your second lap.
• Thonny says ImportError: no module named 'watchparts'. The lib folder is missing from the Pico. Copy the kit's files again, as the kit's main page shows.

No Counting Needed!

You built a stopwatch that remembers a moment instead of counting. You used min() and an average to rank your laps. Next, you'll flip the idea around and count down to an alarm.

Next: Lab 16: Countdown Timer