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Susano , 04-07-2026, 12:01 AM

I'm wondering if there's any examples or tips/tricks of routing multiple I2c slaves cause I feel like I'm creating stubs everywhere with pull ups, branches going to different chips ...etc

QDrives , 04-07-2026, 01:04 PM

1) What are the lengths?
2) What is the rising edge time?

Susano , 04-07-2026, 01:35 PM

the SCL and SDA lines are around the 80mm ish range
I'm using 2.2K pull up resistors and the board is for an IMU glove housing about 16-17 IMUs (only one IMU on board the rest are connected through a flex PCB on the bottom mezzanine connector) so the lengths might actually double taking into account the other flex PCB traces (this board is to rest on the back wrist of the glove)

the bus also houses a power mangement IC a body temperature sensor and one sp02 and pulse sensor totalling about 20 slave I guess assuming a 10pf per device I think this puts the trise well above the 400khz i2c spec so I'm sticking to the 100Khz for now

it's a 4-layer board I did a year or so back that I'm redesigning (didn't make it to production yet) and the first thing I noticed is the ugly i2c tracks to me
my first thought was to probably divide the load between multiple i2c peripherals on the MCU but it's already on maximum capacity this means I need to consider 64 pins version or something

also let me know if you have any further comments on the design that I didn't catch yet

thanks

QDrives , 04-07-2026, 01:40 PM

You might want to consider using something like a redriver for going off-board (mezzanine connector).

Susano , 04-07-2026, 01:43 PM

alright noted what about the routing of the tracks itself isn't this branching left and right problematic to some degree ?

Susano , 04-07-2026, 01:43 PM

any tips one routing those kind of busses without creating stubs everywhere ?

Susano , 04-07-2026, 01:46 PM

also highlights on any obvious mistakes or so I need to take into account will be greatly appreciated

thanks

QDrives , 04-07-2026, 02:08 PM

So have I got the I2C bus correctly shown in violet in the modifed screen capture?
If so, the red sqaure is for a redriver. Then most of the rest is almost like a bus. The remaning stubs will not be a problem.

Susano , 04-07-2026, 02:28 PM

Yes the violet is the scl and sda line correct
however as you see the tracks branch right (the ones you highlighted) but also left (removed it in the screenshot you see direct lines going for unrouted tracks on the left that I removed I'm worried this branching left and right from the middle are a problem

QDrives , 04-07-2026, 02:38 PM

Yes, I had seen them. They too are colored violet.
As I said, add a redriver to the off-board connection.
https://www.digikey.nl/en/products/filter/interface/signal-buffers-repeaters-splitters/756?s=N4IgjCBcoGwJxVAYygMwIYBsDOBTANCAPZQDaIALAAxwDMdIAuoQA4AuUIAymwE4CWAOwDmIAL6FqAVkQgUkDDgLEyIAOxwNtGCEK0p8MAA5dII1QBMVc6Y1GpVKqfNWjEQmqkWKVpqw6Q3HxComJhQA

Susano , 04-07-2026, 02:45 PM

Yes I got this part for adding a redriver thanks alot for that

However I'm just wondering
Is there's a better/recommended way to routing bus lines than this ?

Also any comments on the rest of the board ?

QDrives , 04-07-2026, 03:30 PM

I2C is not a high speed bus, so you do not care so much on the length.
The pull-up make the rise time slow, but the open collector make the fall time fast. That is more a EMC issue then a SI issue.
Going off-board there is the ESD issue more then on the board itself. Another reason for a redriver.

As for the rest of the board, do not route traces under a metal connector (USB). Solder resist is **not** an insulator (for that).

Susano , 04-07-2026, 03:35 PM

I was actually worried about this for a second that the heat or pressure might peel it off with time however this very practice of placing the CC resistors on the sides and routing under the connector I've seen done by phil himself so I thought it would be fine

QDrives , 04-08-2026, 03:42 PM

It depends on what material is on the bottom of the connector.
If it is metal on the bottom and it 'touches' the trace, then no, do not do it. Even if Phil is doing it too.
However, if it is plastic or not touching the trace, then it may not be a problem.