Go Down

Topic: Shared components for I2C devices (Read 500 times) previous topic - next topic

point5

Hi, I think I'm right in saying that multiple I2C devices can share the pullup resistors from the SDA & SCL lines but wondering if the decoupling capacitor may also be shared across multiple devices on the same PCB or is it better for each device to be decoupled seperately?

RandallR

It is my understanding that they to may be shared.  Especially if they are on the same board.  If you are wired to a second board you may need caps on that board but can still use the same pull-up resistors.

MarkT

The lines should share pull-ups - if you keep adding more pull-ups then at some point one of the chips will no longer be able to pull them low correctly - pull ups of 4k7 are usually recommended.

Decoupling is not really related to the I2C bus.  The requirement for decoupling is universal for digital logic and it is important that decoupling is physically close to the chip being decoupled, since any significant stray inductance between load and capacitor will make it ineffective in controlling supply voltage at the chip.  Generally that means 0.1uF right next to each chip, but datasheets often give guidance.

Note that most modules/breakouts already have decoupling on-board, unlike ICs, but check the datasheet.
[ I won't respond to messages, use the forum please ]

Grumpy_Mike

Quote
is it better for each device to be decoupled seperately

Yes each device should be decoupled separately. see:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

point5

Wow, thanks guys, the replies were coming in so fast that each time I replied it was out of date :-)

Thanks for clarifying and link to the tutorial - I will have a read.  If the sesnors are very close to one another on the board, say the same position on each side of the PCB, would it still be advisable for each to have its own cap if the distance from decvice to cap is both short and equal?

Grumpy_Mike

Yes it is advisable to have one fitted to each chip, as close to the chip as possible, with the leads as short as possible.

point5

Thanks Mike - good advice (and article) which I will follow :-)

Go Up