Poking at hardware live

Situation

You want to find out how a piece of hardware behaves before writing a program around it: does this pin drive the LED, what does the sensor read, how fast does the value change. Running a whole file each time is slow, and typing into the REPL by hand loses your work. Instead, write the code in a buffer and send it to the REPL a line or a block at a time, keeping what works.

What you’ll use

• :MP repl to open the REPL in a split
• :MP send to send the current line, or a selection
• :MP send_buffer to send the whole file
• :MP interrupt to interrupt a loop without losing your variables

The suggested keymaps make each of these one keystroke.

The example reads the Pico’s built-in temperature sensor and blinks its LED. Put it in a file in your project, such as scratch/live.py:

from machine import Pin, ADC
from time import sleep

led = Pin("LED", Pin.OUT)
led.on()
led.off()

sensor = ADC(4)
sensor.read_u16()

def temperature():
    volts = sensor.read_u16() * 3.3 / 65535
    return 27 - (volts - 0.706) / 0.001721

print(temperature())

while True:
    led.toggle()
    print(round(temperature(), 1))
    sleep(1)

Steps

1. Open the REPL

:MP repl

You should see a split at the bottom of the screen with the MicroPython >>> prompt, and the cursor in it. Press <Esc><Esc> to leave terminal mode, then Ctrl-w k to go back to your code.

2. Send lines one at a time

Put the cursor on the first line, from machine import Pin, ADC, and run:

:MP send

You should see the line appear at the REPL prompt and run, as if you’d typed it. Your cursor stays in your buffer. Do the same for the sleep import, led = Pin(...), then led.on().

You should see the LED come on. Send led.off() and it goes off.

3. Look at a value

Send sensor = ADC(4), then sensor.read_u16().

You should see a number such as 14000 in the REPL. A single expression sent on its own is echoed, just like typing it at the prompt. Send sensor.read_u16() a few more times and watch it change, or put your finger on the chip.

4. Send a block

Select the def temperature(): function, from the def line to its return, with V, then run:

:'<,'>MP send

You should see the function sent as one block. Nothing is printed: defining a function has no output. Send print(temperature()).

You should see the temperature in °C, something like 22.4.

Several lines are sent in paste mode, which keeps their indentation as it is. See :MP send for the details.

5. Start a loop, then stop it

Select the while True: loop and send it.

You should see the LED blinking and a new temperature printed every second.

The loop never ends, so stop it from your buffer:

:MP interrupt

You should see a KeyboardInterrupt traceback, then the >>> prompt again. Interrupting stops the code but keeps everything you defined: send led.off() or print(temperature()) and they still work.

6. Change something and send the whole file

Change the loop, for example to sleep(0.2), then send the whole file:

:MP send_buffer

You should see the file run from the top, then the faster loop. Stop it with :MP interrupt.

7. Finish

When you’re done, go to the REPL split and press Ctrl-]. This quits the REPL and frees the device’s port for other commands. Keep the lines that worked: they’re already in your buffer, ready to move into main.py.

What can go wrong

Another command fails with an error about the port. While the REPL is open it holds the port. :MP run and :MP run_main run through the REPL instead, but other commands, such as uploads and :MP files, need it for themselves. (:MP flash is the exception: it closes the REPL itself.) Quit the REPL with Ctrl-] first. Closing the split’s window isn’t enough: the REPL keeps running in the background.

The REPL won’t open, or shows an error about the port. Something else holds it: a :MP run terminal that’s still running, a mount, or another program such as Thonny. Stop or close it, then run :MP repl again.

A value isn’t printed when you send several lines. In paste mode the REPL doesn’t echo expressions. Send the expression on its own line, or wrap it in print().

SyntaxError or IndentationError after sending part of a block. A block has to be sent whole: select from its first line to its last. Lines inside a loop or an if can be sent on their own, because shared indentation is removed.

NameError for something you defined earlier. The REPL’s state was cleared, by a soft reset (Ctrl-D in the REPL, or :MP reset) or a hard reset. Select everything above the loop and send it again.

The REPL is not open. Run :MP repl first. :MP interrupt only works in the REPL. Code started with :MP run while the REPL was closed runs in its own terminal: press Ctrl-C there to stop it.

ValueError on Pin("LED") or ADC(4). These are the Pico’s names for its LED and temperature sensor. On other boards, use the pin numbers from your board’s pinout.