1. Data logging csv

For the microbit utime module
see: https://microbit-micropython.readthedocs.io/en/v2-docs/utime.html
For opening a file on the microbit
see: https://microbit-micropython.readthedocs.io/en/v2-docs/filesystem.html
For reading the light level via display
see: https://microbit-micropython.readthedocs.io/en/v2-docs/display.html

1.1. Write to microbit

Below is code to record light readings to the microbit as a csv file.

# measure light readings for about (read_interval * number_of_readings /1000) seconds.
# alter read_interval, number_of_readings to suit usage.
from microbit import *
import utime


def writeFile(fileName, data):
    with open(fileName, "w") as f:
        data = str(data)
        f.write(data)


def saveData(fileName, number_of_readings, read_interval):
    log_data = "Time,Light_reading\r"
    start_time = utime.ticks_ms()
    for x in range(number_of_readings):
        if button_b.is_pressed():
            display.show(Image.ARROW_W)
            break
        reading = str(display.read_light_level())
        curr_time = utime.ticks_ms()
        time_from_start = round(utime.ticks_diff(curr_time, start_time)/1000, 2)
        new_data = str(time_from_start) + "," + reading + "\r"
        log_data += new_data
        sleep(read_interval)
    writeFile(fileName, log_data)


read_interval = 100
number_of_readings = 80
fileName = "light_readings.csv"

display.show(Image.ARROW_W)
while True:
    if button_a.was_pressed():
        display.show(Image.YES)
        saveData(fileName, number_of_readings, read_interval)
        display.show(Image.ARROW_W)
    else:
        sleep(1000)

The light readings are obtained by display.read_light_level().
The time between readings is set to 100 milliseconds by read_interval.
The the number of readings is set to 80 by number_of_readings.
The file on the microbit to hold the readings is set by fileName = "light_readings.csv".
save_data saves the log_data to the microbit.
log_data = "Time,Light_reading\r" makes the headings for the readings.
new_data = str(time_from_start) + "," + reading + "\r" is used to collect the line string to be added to the log_data.

The start time, start_time, for the data collection is set by start_time = utime.ticks_ms()
After a sleep set by read_interval, the current time is set by curr_time = utime.ticks_ms().
time_from_start = round(utime.ticks_diff(curr_time, start_time)/1000, 2) rounds the time since start_time to 2 dp.

1.2. File transfer from microbit to computer

To transfer the “light_readings.csv” file to the computer click on the Files button in Mu editor.
Click and drag the file on the left, “light_readings.csv”, across to the right to “files on your computer” which is the mu_code folder on the computer.
The mu_code folder will be located at: C:\Users\username\mu_code, depending on the username.

The file can then be used to display a chart in excel or in python using plotly or matplotlib.

1.3. Display elapsed seconds

It can be useful to give user feedback by displaying the elapsed seconds on the microbit.
count_seconds is used to display the elapsed seconds on the microbit as the readings occur.
count_seconds takes 2 parameters: time_from_start, time_displayed.
sec_from_start = int(time_from_start) rounds down the time_from_start.
if sec_from_start > time_displayed will be true once time_from_start has increased by at least 1 second above time_displayed.
time_displayed is then updated and used for display.

Tasks

Complete the count_seconds function, by replacing the XXX stings.

def count_seconds(XXX, XXX):
    sec_from_start = int(time_from_start)
    if XXX > time_displayed:
        XXX = sec_from_start
    return time_displayed

In the completed code:
time_displayed is set to 0 before the for-loop.
time_displayed = count_seconds(time_from_start, time_displayed) updates the time_displayed value.
display.show(time_displayed, wait=False) shows the time in seconds from the start of taking readings.

Tasks

Complete the code to display the elapsed seconds on the microbit.