Multiple i2c devices that have their own pull ups.

[Dan Moos]

Say I want to use a bunch (6 or more) LCDs with those i2c backpacks that are pretty common. The back packs have their own pull ups. I imagine this puts a limit on how many of these devices I can have on one bus before the parallel resistance of all the pull ups becomes too small.  I haven't actually looked to see what value pull ups are on the things.

For my idea to work, do I need to ditch the backpacks, and just get the individual expander chips?

[Mr.B]

Perhaps desolder the pullups on all and add the pullups at your MCU.
Typical value is 4K7 but it can vary a bit.

Edit: Forgot to say... 6 backpacks? Do they support 6 different I2C addresses...?

[Dan Moos]

The pins for setting addresses are brought out  on pads. You don't get the full range of addresses that are possible though. I think it's more than six, but I can't remember how many address pins are available. It's definitely a possible limiting factor on my plan.

EDIT: three address pins. My math says five combinations. So much for six screens!

[Mr.B]

3 bit address.
My math says 8 addresses.
Thats not to say the device supports all 8.

URL for spec sheet?
Photo?

[Mr.B]

Depending upon the device it could be a lot more than 8 - although unlikely for an LCD.
Some devices these days are using a sort of tri-state address pin combination.
e.g. one pin supports three addresses - High, Low and Floating.

[Dan Moos]

Yer right, three makes eight.

Pretty sure no tri state. The chip is a pcf8574. The board brings the address pins out on pads. There are also three corresponding pads that are go to either pull ups, or pull downs (can't remember as I sit away from the lab) so you can conveniently bridge different combos to get the address you want. I would assume having low/high/tri state would require a more elaborate setup. 

[Mr.B]

One last piece of advice...
Six devices on an I2C bus can be problematic.
Keep your bus lengths as short as possible. 

[Neilm]

One last piece of advice...
Six devices on an I2C bus can be problematic.
Keep your bus lengths as short as possible.

Just make sure that your rise times are in spec for the chips - this might mean putting the pull up resistors somewhere you didn't didn't expect to. I have had five on a bus that was about 1m long. Fortunately, it went over several boards, so the one in the middle ended up with the pull up resistors.