5 replies to this topic

[Miha Markic]

Hi guys,

I am going to read from barometric pressure sensor through I2C and I am wondering whether I need pull-up resistors or not?
http://www.sparkfun.com/tutorials/253'>http://www.sparkfun.com/tutorials/253

The tutorial suggests that those aren't needed.

Thanks

[Miha Markic]

OTOH I guess the BMP085 has already pull-up reistors on-board, correct?

[CW2]

I am going to read from barometric pressure sensor through I2C and I am wondering whether I need pull-up resistors or not?
http://www.sparkfun.com/tutorials/253

The pull-ups are mandatory as per the I2C specification, so if you use the sensor alone, you'd have to add them, if you use Sparkfun's breakout board, it already has them (those two small black 4k7 resistors).

[Miha Markic]

The pull-ups are mandatory as per the I2C specification, so if you use the sensor alone, you'd have to add them, if you use Sparkfun's breakout board, it already has them (those two small black 4k7 resistors).

Thanks, I'll use the breakout board so I am fine. Out of curiosity, what happens if I want to use another board with pull up resistors mounted as well?

[Spork]

Here's some reading:

A thread about BM085, i2c, and pull-up resistors: here.

A really good web page about pull-up resistors and i2c: here.

In a nutshell, you need to choose pull-up resistors to match the characteristics of the bus and all the devices on it, but it seems like i2c is pretty forgiving. If you keep adding breakouts with built-in pull-ups, the total pull-up resistance will be LOWER than the LOWEST value on any given breakout (resistors in parallel rule). I eventually ended up cutting some resistors off a breakout to solve this problem.

[Miha Markic]

Thanks for a clear explanation. I guess when it comes to it, I'll just add slaves and if problems start then I'll try to figure something out.