Technic Hub freezes when using ble.broadcast()

[akiyuki-k]

I really appreciate all the great work on Pybricks.

I'm currently facing an issue where the Technic Hub freezes when calling Bluetooth functions, and I haven't been able to resolve it on my own. I would greatly appreciate any help or guidance.

The program I am using is a simple Bluetooth send-and-receive loop. It includes a receive operation, but in my testing, no messages are ever received (the received message is always empty).

Environment where the issue is reproducible:

• Pybricks v3.6.1 (Pybricks Code v2.6.0)
• One Technic Hub with nothing connected to any ports
• The following program is transferred to the Hub
• After unpairing the Bluetooth connection between the programming PC and the Hub, the program is executed → the Hub freezes
• This occurs consistently: 20 out of 20 attempts result in a freeze
• The time to freeze varies between 1 minute and 150 minutes

When the Hub freezes, the status light remains yellow. Based on this, I suspect the issue is related to the hub.ble.broadcast(senddata) call.

As a reference, when I keep the Bluetooth pairing between the programming PC and the Hub, the same program runs for more than 10 hours without freezing (confirmed twice).

Program:

from pybricks.hubs import TechnicHub
from pybricks.pupdevices import Motor
from pybricks.parameters import Port, Stop, Direction, Button, Color
from pybricks.tools import wait, StopWatch
from umath import pi, sin, cos, radians

hub = TechnicHub(broadcast_channel=1, observe_channels=[2])

# Receive a message every 100ms. Send a message every 1s.
# The indicator switches to red when receiving and yellow when sending.
count = 0
while True:
    hub.light.on(Color.RED)
    data = hub.ble.observe(2)
    if count > 10:
        hub.light.on(Color.YELLOW)
        senddata = ('send', 1)
        hub.ble.broadcast(senddata)
        count = 0
    wait(100)
    count += 1

Please let me know if there's anything else I can try or provide to help diagnose the issue. Thank you in advance!

[dlech]

Do you actually need to broadcast and observe on the same hub? If you can do only one or the other, the Bluetooth chip will be happier.

Other than that, I think we need to dust of the parts of pybricks/pybricks-micropython#80 where we transparently don't broadcast and observe at the same time on the Bluetooth chip and have some random timing to avoid aliasing issues.

[akiyuki-k]

@dlech
Thank you for your reply.
I want to run multiple hubs in sync, so I need some kind of two-way communication.

I had not yet looked at that #80 issue. I will take a look.
I understand that I need to separate sending and receiving. Which library should I use if I want to control Bluetooth in such a detailed manner?

[dlech]

FYI, #80 is not the same as pybricks/pybricks-micropython#80.

The change I am referring to is something that would have to be done in firmware. pybricks/pybricks-micropython#80 is a pull request that implemented this already, but we ended up doing it a different way because it seemed to be working ok after quite a bit of testing. So we could take the ideas and code from that existing pull request to try to fix the issue.

In particular, this file has the code for the CC2640 Bluetooth chip that is used on Technic hub and contains the code I am referring to.

[akiyuki-k]

@dlech
I see, so that's something that can only be handled on the firmware side. That’s not something I can try on my own.
As a request for a future firmware version, I would really appreciate it if it could support both sending and receiving.
For now, I’ll try to see if I can achieve my goal by structuring things so that it only does either sending or receiving.

Thank you very much.

[laurensvalk]

Thank you @akiyuki-k for filing this issue. I'm a big fan of your work!

Even though we've looked at this at various times, we'd still like to find a better solution or workaround than we have today. I guess you have inspired us to take another look 😄 . For context, a somewhat recent attempt is pybricks/pybricks-micropython#269, but it wasn't quite satisfactory.

As a reference, when I keep the Bluetooth pairing between the programming PC and the Hub, the same program runs for more than 10 hours without freezing (confirmed twice).

This is really interesting and got me curious. Could you try and repeat this without the PC connected, but having remote = Remote() to keep the remote connected during your experiment? Not ideal perhaps, but possibly an interesting workaround.

One Technic Hub with nothing connected to any ports

Have you been able to reproduce it on City Hub? I am able to reproduce it with your example on the Technic Hub but my City Hub has now been running for several hours without getting stuck. Even though its Bluetooth chip is the same, it has a different firmware version.

something that can only be handled on the firmware side.

Due to recent unrelated improvements related to debugging, we now have the ability to analyse the problem in a bit more detail. There does seem to be a specific pattern of internal Bluetooth operations that always shows up when it gets stuck. If that turns out to be true, maybe there is a way we can work around it. Stay tuned...

[BertLindeman]

Ran the original program from this issue

All tests:

• no PC connection
• no attatchments

Test on a technichub (My HubB) at version ('technichub', '3.6.0b4', 'ci-release-83-v3.6.0b4 on 2025-02-14')
Results in a hang after ~10 minutes status: Yellow.
Hub powered down by long button press.

Another hub (HubA) with the program below (included as Laurens suggested: remote = Remote())
on firmware from build-3821 at git hash dfbcfa8.
Still running.

The adapted program:

from pybricks.hubs import TechnicHub
from pybricks.pupdevices import Motor, Remote
from pybricks.parameters import Port, Stop, Direction, Button, Color
from pybricks.tools import wait, StopWatch
from umath import pi, sin, cos, radians
C1 = 1
C2 = 2
hub = TechnicHub(broadcast_channel=C1, observe_channels=[C2])
remote = Remote()
# Receive a message every 100ms. Send a message every 1s.
# The indicator switches to red when receiving and yellow when sending.
count = 0
while True:
    hub.light.on(Color.RED)
    data = hub.ble.observe(C2)
    if data:
        hub.light.on(Color.VIOLET)
    if count > 10:
        hub.light.on(Color.YELLOW)
        senddata = ('send', 1)
        hub.ble.broadcast(senddata)
        count = 0
    wait(100)
    count += 1

Restarted HubB with this program too. (I have two remotes 😄 )
Both tests still running ~ half an hour now.

Will report later (Hope all batteries last long enough)

Bert

[laurensvalk]

@BertLindeman , would you like to try the original on CityHub also? Thanks!