edit - also see Ii bus board, v2, with pullups
for those not following the TT expander thread, we’ve found that putting a large number of devices on the i2c bus will add sufficient capacitance to break some things. timeouts are long and TT crashes, etc.
unfortunately the TT circuit has built-in pullups of 10k. these are not easy to change short of taking the unit apart and SMD soldering on new resistors. (see below for how to do this)
so, @sam proposed a bus board with pullups. but it needs 3v3, so it’ll need power.
2.2k pullups in parallel with the 10k on TT would give about 1.8k, which seems right. http://www.digikey.com/product-detail/en/yageo/RC0603JR-072K2L/311-2.2KGRCT-ND/729676
here’s a cheap, widely available, easy to solder 3v3 reg: http://www.digikey.com/product-detail/en/diodes-incorporated/AZ1117CH-3.3TRG1/AZ1117CH-3.3TRG1DICT-ND/4505206
just threw this together in 15 minutes… board and schem attached.
tt-i2c-v2.zip (16.4 KB)
here’s an osh order link. note, this is totally untested, but it’s so simple i can’t imagine there being a problem. if somebody wants to test this right away, be sure to volt-meter the levels before plugging in i2c, to ensure the 3v3 is correct. i’m saying this because kelli and i will be away for a bit soon, so those of you in a hurry wanting to take the torch on this, please do.
$9 for three boards!
that said, if you’d rather just swap the resistors on your TT, here they are:
replace with 0603 package resistors. 2.2k or around there will be fine.