1. A monitor for realtime MicroPython code

This library provides a means of examining the behaviour of a running system.
It was initially designed to characterise uasyncio programs but may also find
use to study any code whose behaviour may change dynamically such as threaded
code or applications using interrupts.

The device under test (DUT) is linked to a Raspberry Pico. The latter displays
the behaviour of the DUT by pin changes and optional print statements. A logic
analyser or scope provides a view of the realtime behaviour of the code.
Valuable information can also be gleaned at the Pico command line.

Where an application runs multiple concurrent tasks it can be difficult to
identify a task which is hogging CPU time. Long blocking periods can also occur
when several tasks each block for a period. If, on occasion, these are
scheduled in succession, the times will add. The monitor issues a trigger pulse
when the blocking period exceeds a threshold. The threshold can be a fixed time
or the current maximum blocking period. A logic analyser enables the state at
the time of the transient event to be examined.

This image shows the detection of CPU hogging. In this example a task hogs the
CPU for 500ms, causing the scheduler to be unable to schedule other tasks. A
trigger pulse is generated by the Pico 100ms after hogging started. This script
is discussed in more detail in section 6.


The following image shows brief (<4ms) hogging while quick_test.py ran. The
likely cause is garbage collection on the Pyboard D DUT. The monitor was able
to demonstrate that this never exceeded 5ms.


1.1 Concepts

Communication with the Pico may be by UART or SPI, and is uni-directional from
DUT to Pico. If a UART is used only one GPIO pin is needed. SPI requires three,
namely mosi, sck and cs/.

The Pico runs the following:

from monitor_pico import run
run()  # or run(device="spi")

Debug lines are inserted at key points in the DUT code. These cause state
changes on Pico pins. All debug lines are associated with an ident which is a
number where 0 <= ident <= 21. The ident value defines a Pico GPIO pin
according to the mapping in section 5.1.

For example the following will cause a pulse on GPIO6.

import monitor
trig1 = monitor.trigger(1)  # Create a trigger on ident 1

async def test():
    while True:
        await asyncio.sleep_ms(100)
        trig1()  # Pulse appears now

In uasyncio programs a decorator is inserted prior to a coroutine definition.
This causes a Pico pin to go high for the duration every time that coro runs.
Other mechanisms are provided, with special support for measuring cpu hogging.

The Pico can output a trigger pulse on GPIO28 which may be used to trigger a
scope or logic analyser. This can be configured to occur when excessive latency
arises or when a segment of code runs unusually slowly. This enables the cause
of the problem to be identified.

1.2 Pre-requisites

The DUT and the Pico must run firmware V1.17 or later.

1.3 Installation

The file monitor.py must be copied to the DUT filesystem. monitor_pico.py
is copied to the Pico.

1.4 UART connection


Gnd Gnd 3
txd 1 2

The DUT is configured to use a UART by passing an initialised UART with 1MHz
baudrate to monitor.set_device:

from machine import UART
import monitor
monitor.set_device(UART(2, 1_000_000))  # Baudrate MUST be 1MHz.

The Pico run() command assumes a UART by default.

1.5 SPI connection


Gnd Gnd 3
mosi 0 1
sck 1 2
cs 2 4

The DUT is configured to use SPI by passing an initialised SPI instance and a
cs/ Pin instance to set_device:

from machine import Pin, SPI
import monitor
monitor.set_device(SPI(2, baudrate=5_000_000), Pin('X6', Pin.OUT))  # Device under test SPI

The SPI instance must have default args; the one exception being baudrate which
may be any value. I have tested up to 30MHz but there is no benefit in running
above 1MHz. Hard or soft SPI may be used. It should be possible to share the
bus with other devices, although I haven’t tested this.

The Pico should be started with


1.6 Quick start

This example assumes a UART connection. On the Pico issue:

from monitor_pico import run

The Pico should issue “Awaiting communication.”
Adapt the following to match the UART to be used on the DUT and run it.

import uasyncio as asyncio
from machine import UART  # Using a UART for monitoring
import monitor
monitor.set_device(UART(2, 1_000_000))  # Baudrate MUST be 1MHz.

@monitor.asyn(1)  # Assign ident 1 to foo (GPIO 4)
async def foo():
    await asyncio.sleep_ms(100)

async def main():
    monitor.init()  # Initialise Pico state at the start of every run
    while True:
        await foo()  # Pico GPIO4 will go high for duration
        await asyncio.sleep_ms(100)


When this runs the Pico should issue “Got communication.” and a square wave of
period 200ms should be observed on Pico GPIO 4 (pin 6).

Example script quick_test.py provides a usage example. It may be adapted to
use a UART or SPI interface: see commented-out code.

2. Monitoring

An application to be monitored should first define the interface:

from machine import UART  # Using a UART for monitoring
import monitor
monitor.set_device(UART(2, 1_000_000))  # Baudrate MUST be 1MHz.


from machine import Pin, SPI
import monitor
# Pass a configured SPI interface and a cs/ Pin instance.
monitor.set_device(SPI(2, baudrate=5_000_000), Pin('X1', Pin.OUT))

The pin used for cs/ is arbitrary.

Each time the application runs it should issue:

def main():
    # rest of application code

This ensures that the Pico code assumes a known state, even if a prior run
crashed, was interrupted or failed.

2.1 Validation of idents

Re-using idents would lead to confusing behaviour. If an ident is out of range
or is assigned to more than one coroutine an error message is printed and
execution terminates.

3. Monitoring uasyncio code

3.1 Monitoring coroutines

Coroutines to be monitored are prefixed with the @monitor.asyn decorator:

@monitor.asyn(2, 3)
async def my_coro():
    # code

The decorator positional args are as follows:

  1. n A unique ident in range 0 <= ident <= 21 for the code being
    monitored. Determines the pin number on the Pico. See
    section 5.1.
  2. max_instances=1 Defines the maximum number of concurrent instances of the
    task to be independently monitored (default 1).
  3. verbose=True If False suppress the warning which is printed on the DUT
    if the instance count exceeds max_instances.
  4. looping=False Set True if the decorator is called repeatedly e.g.
    decorating a nested function or method. The True value ensures validation of
    the ident occurs once only when the decorator first runs.

Whenever the coroutine runs, a pin on the Pico will go high, and when the code
terminates it will go low. This enables the behaviour of the system to be
viewed on a logic analyser or via console output on the Pico. This behavior
works whether the code terminates normally, is cancelled or has a timeout.

In the example above, when my_coro starts, the pin defined by ident==2
(GPIO 5) will go high. When it ends, the pin will go low. If, while it is
running, a second instance of my_coro is launched, the next pin (GPIO 6) will
go high. Pins will go low when the relevant instance terminates, is cancelled,
or times out. If more instances are started than were specified to the
decorator, a warning will be printed on the DUT. All excess instances will be
associated with the final pin (pins[ident + max_instances - 1]) which will
only go low when all instances associated with that pin have terminated.

Consequently if max_instances=1 and multiple instances are launched, a
warning will appear on the DUT; the pin will go high when the first instance
starts and will not go low until all have ended. The purpose of the warning is
because the existence of multiple instances may be unexpected behaviour in the
application under test – it does not imply a problem with the monitor.

3.2 Detecting CPU hogging

A common cause of problems in asynchronous code is the case where a task blocks
for a period, hogging the CPU, stalling the scheduler and preventing other
tasks from running. Determining the task responsible can be difficult,
especially as excessive latency may only occur when several greedy tasks are
scheduled in succession.

The Pico pin state only indicates that the task is running. A high pin does not
imply CPU hogging. Thus

async def long_time():
    await asyncio.sleep(30)

will cause the pin to go high for 30s, even though the task is consuming no
resources for that period.

To provide a clue about CPU hogging, a hog_detect coroutine is provided. This
has ident=0 and, if used, is monitored on GPIO3. It loops, yielding to the
scheduler. It will therefore be scheduled in round-robin fashion at speed. If
long gaps appear in the pulses on GPIO3, other tasks are hogging the CPU.
Usage of this is optional. To use, issue

import uasyncio as asyncio
import monitor
# code omitted
# code omitted

To aid in detecting the gaps in execution, in its default mode the Pico code
implements a timer. This is retriggered by activity on ident=0. If it times
out, a brief high going pulse is produced on GPIO 28, along with the console
message “Hog”. The pulse can be used to trigger a scope or logic analyser. The
duration of the timer may be adjusted. Other modes of hog detection are also
supported, notably producing a trigger pulse only when the prior maximum was
exceeded. See section 5.

4. Monitoring arbitrary code

The following features may be used to characterise synchronous or asynchronous
applications by causing Pico pin changes at specific points in code execution.

The following are provided:

  • A sync decorator for synchronous functions or methods: like async it
    monitors every call to the function.
  • A mon_call context manager enables function monitoring to be restricted to
    specific calls.
  • A trigger function which issues a brief pulse on the Pico or can set and
    clear the pin on demand.

4.1 The sync decorator

This works as per the @async decorator, but with no max_instances arg. The
following example will activate GPIO 26 (associated with ident 20) for the
duration of every call to sync_func():

def sync_func():

Decorator args:

  1. ident
  2. looping=False Set True if the decorator is called repeatedly e.g. in a
    nested function or method. The True value ensures validation of the ident
    occurs once only when the decorator first runs.

4.2 The mon_call context manager

This may be used to monitor a function only when called from specific points in
the code. Since context managers may be used in a looping construct the ident
is only checked for conflicts when the CM is first instantiated.


def another_sync_func():

with monitor.mon_call(22):

It is advisable not to use the context manager with a function having the
mon_func decorator. The behaviour of pins and reports are confusing.

4.3 The trigger timing marker

The trigger closure is intended for timing blocks of code. A closure instance
is created by passing the ident. If the instance is run with no args a brief
(~80μs) pulse will occur on the Pico pin. If True is passed, the pin will go
high until False is passed.

The closure should be instantiated once only in the outermost scope.

trig = monitor.trigger(10)  # Associate trig with ident 10.

def foo():
    trig()  # Pulse ident 10, GPIO 13

def bar():
    trig(True)  # set pin high
    # code omitted
    trig(False)  # set pin low

4.4 Timing of code segments

It can be useful to time the execution of a specific block of code especially
if the duration varies in real time. It is possible to cause a message to be
printed and a trigger pulse to be generated whenever the execution time exceeds
the prior maximum. A scope or logic analyser may be triggered by this pulse
allowing the state of other components of the system to be checked.

This is done by re-purposing ident 0 as follows:

trig = monitor.trigger(0)
def foo():
    # code omitted
    trig(True)  # Start of code block
    # code omitted

See section 5.5 for the Pico usage
and demo syn_time.py.

5. Pico

5.1 Pico pin mapping

The Pico GPIO numbers used by idents start at 3 and have a gap where the Pico
uses GPIO’s for particular purposes. This is the mapping between ident GPIO
no. and Pico PCB pin. Pins for the trigger and the UART/SPI link are also

ident GPIO pin
nc/mosi 0 1
rxd/sck 1 2
nc/cs/ 2 4
0 3 5
1 4 6
2 5 7
3 6 9
4 7 10
5 8 11
6 9 12
7 10 14
8 11 15
9 12 16
10 13 17
11 14 19
12 15 20
13 16 21
14 17 22
15 18 24
16 19 25
17 20 26
18 21 27
19 22 29
20 26 31
21 27 32
trigger 28 34

5.2 The Pico code

Monitoring via the UART with default behaviour is started as follows:

from monitor_pico import run

By default the Pico retriggers a timer every time ident 0 becomes active. If
the timer times out, a pulse appears on GPIO28 which may be used to trigger a
scope or logic analyser. This is intended for use with the hog_detect coro,
with the pulse occurring when excessive latency is encountered.

5.3 The Pico run function

Arguments to run() can select the interface and modify the default behaviour.

  1. period=100 Define the hog_detect timer period in ms. A 2-tuple may also
    be passed for specialised reporting, see below.
  2. verbose=() A list or tuple of ident values which should produce console
    output. A passed ident will produce console output each time that task starts
    or ends.
  3. device="uart" Set to "spi" for an SPI interface.
  4. vb=True By default the Pico issues console messages reporting on initial
    communication status, repeated each time the application under test restarts.
    Set False to disable these messages.

Thus to run such that idents 4 and 7 produce console output, with hogging
reported if blocking is for more than 60ms, issue

from monitor_pico import run
run(60, (4, 7))

Hog reporting is as follows. If ident 0 is inactive for more than the specified
time, “Timeout” is issued. If ident 0 occurs after this, “Hog Nms” is issued
where N is the duration of the outage. If the outage is longer than the prior
maximum, “Max hog Nms” is also issued.

This means that if the application under test terminates, throws an exception
or crashes, “Timeout” will be issued.

5.4 Advanced hog detection

The detection of rare instances of high latency is a key requirement and other
modes are available. There are two aims: providing information to users lacking
test equipment and enhancing the ability to detect infrequent cases. Modes
affect the timing of the trigger pulse and the frequency of reports.

Modes are invoked by passing a 2-tuple as the period arg.

  • period[0] The period (ms): outages shorter than this time will be ignored.
  • period[1] is the mode: constants SOON, LATE and MAX are exported.

The mode has the following effect on the trigger pulse:

  • SOON Default behaviour: pulse occurs early at time period[0] ms after
    the last trigger.
  • LATE Pulse occurs when the outage ends.
  • MAX Pulse occurs when the outage ends and its duration exceeds the prior

The mode also affects reporting. The effect of mode is as follows:

  • SOON Default behaviour as described in section 4.
  • LATE As above, but no “Timeout” message: reporting occurs at the end of an
    outage only.
  • MAX Report at end of outage but only when prior maximum exceeded. This
    ensures worst-case is not missed.

Running the following produce instructive console output:

from monitor_pico import run, MAX
run((1, MAX))

5.5 Timing of code segments

This may be done by issuing:

from monitor_pico import run, WIDTH
run((20, WIDTH))  # Ignore widths < 20ms. 

Assuming that ident 0 is used as described in
section 5.5 a trigger pulse on GPIO28
will occur each time the time taken exceeds both 20ms and its prior maximum. A
message with the actual width is also printed whenever this occurs.

6. Test and demo scripts

The following image shows the quick_test.py code being monitored at the point
when a task hogs the CPU. The top line 00 shows the “hog detect” trigger. Line
01 shows the fast running hog_detect task which cannot run at the time of the
trigger because another task is hogging the CPU. Lines 02 and 04 show the foo
and bar tasks. Line 03 shows the hog task and line 05 is a trigger issued
by hog() when it starts monopolising the CPU. The Pico issues the “hog
detect” trigger 100ms after hogging starts.


full_test.py Tests task timeout and cancellation, also the handling of
multiple task instances. If the Pico is run with run((1, MAX)) it reveals
the maximum time the DUT hogs the CPU. On a Pyboard D I measured 5ms.

The sequence here is a trigger is issued on ident 4. The task on ident 1 is
started, but times out after 100ms. 100ms later, five instances of the task on
ident 1 are started, at 100ms intervals. They are then cancelled at 100ms
intervals. Because 3 idents are allocated for multiple instances, these show up
on idents 1, 2, and 3 with ident 3 representing 3 instances. Ident 3 therefore
only goes low when the last of these three instances is cancelled.


latency.py Measures latency between the start of a monitored task and the
Pico pin going high. In the image below the sequence starts when the DUT
pulses a pin (ident 6). The Pico pin monitoring the task then goes high (ident
1 after ~20μs). Then the trigger on ident 2 occurs 112μs after the pin pulse.


syn_test.py Demonstrates two instances of a bound method along with the ways
of monitoring synchronous code. The trigger on ident 5 marks the start of the
sequence. The foo1.pause method on ident 1 starts and runs foo1.wait1 on
ident 3. 100ms after this ends, foo.wait2 on ident 4 is triggered. 100ms
after this ends, foo1.pause on ident 1 ends. The second instance of .pause
(foo2.pause) on ident 2 repeats this sequence shifted by 50ms. The 10ms gaps
in hog_detect show the periods of deliberate CPU hogging.


syn_time.py Demonstrates timing of a specific code segment with a trigger
pulse being generated every time the period exceeds its prior maximum.


7. Internals

7.1 Performance and design notes

Using a UART the latency between a monitored coroutine starting to run and the
Pico pin going high is about 23μs. With SPI I measured -12μs. This isn’t as
absurd as it sounds: a negative latency is the effect of the decorator which
sends the character before the coroutine starts. These values are small in the
context of uasyncio: scheduling delays are on the order of 150μs or greater
depending on the platform. See tests/latency.py for a way to measure latency.

The use of decorators eases debugging: they are readily turned on and off by
commenting out.

The Pico was chosen for extremely low cost. It has plenty of GPIO pins and no
underlying OS to introduce timing uncertainties. The PIO enables a simple SPI

Symbols transmitted by the UART are printable ASCII characters to ease
debugging. A single byte protocol simplifies and speeds the Pico code.

The baudrate of 1Mbps was chosen to minimise latency (10μs per character is
fast in the context of uasyncio). It also ensures that tasks like hog_detect,
which can be scheduled at a high rate, can’t overflow the UART buffer. The
1Mbps rate seems widely supported.

7.2 How it works

This is for anyone wanting to modify the code. Each ident is associated with
two bytes, 0x40 + ident and 0x60 + ident. These are upper and lower case
printable ASCII characters (aside from ident 0 which is @ paired with the
backtick character). When an ident becomes active (e.g. at the start of a
coroutine), uppercase is transmitted, when it becomes inactive lowercase is

The Pico maintains a list pins indexed by ident. Each entry is a 3-list

  • The Pin object associated with that ident.
  • An instance counter.
  • A verbose boolean defaulting False.

When a character arrives, the ident value is recovered. If it is uppercase
the pin goes high and the instance count is incremented. If it is lowercase the
instance count is decremented: if it becomes 0 the pin goes low.

The init function on the DUT sends b"z" to the Pico. This sets each pin
in pins low and clears its instance counter (the program under test may have
previously failed, leaving instance counters non-zero). The Pico also clears
variables used to measure hogging. In the case of SPI communication, before
sending the b"z", a 0 character is sent with cs/ high. The Pico implements
a basic SPI slave using the PIO. This may have been left in an invalid state by
a crashing DUT. The slave is designed to reset to a “ready” state if it
receives any character with cs/ high.

The ident @ (0x40) is assumed to be used by the hog_detect() function. When
the Pico receives it, processing occurs to aid in hog detection and creating a
trigger on GPIO28. Behaviour depends on the mode passed to the run() command.
In the following, thresh is the time passed to run() in period[0].

  • SOON This retriggers a timer with period thresh. Timeout causes a
  • LATE Trigger occurs if the period since the last @ exceeds thresh. The
    trigger happens when the next @ is received.
  • MAX Trigger occurs if period exceeds thresh and also exceeds the prior

7.3 ESP8266 note

ESP8266 applications can be monitored using the transmit-only UART 1.

I was expecting problems: on boot the ESP8266 transmits data on both UARTs at
75Kbaud. In practice monitor_pico.py ignores this data for the following

A bit at 75Kbaud corresponds to 13.3 bits at 1Mbaud. The receiving UART will
see a transmitted 1 as 13 consecutive 1 bits. In the absence of a start bit, it
will ignore the idle level. An incoming 0 will be interpreted as a framing
error because of the absence of a stop bit. In practice the Pico UART returns
b'\x00' when this occurs; monitor.py ignores such characters. A monitored
ESP8266 behaves identically to other platforms and can be rebooted at will.

8. A hardware implementation

I expect to use this a great deal, so I designed a PCB. In the image below the
device under test is on the right, linked to the Pico board by means of a UART.


I can supply a schematic and PCB details if anyone is interested.

This project was inspired by
this GitHub thread.


View Github