Two questions concerning power

Another set of newbie questions, I hope you don’t mind…

What is exactly the function of the power jumper? I am not sure when to use which setting… For example, I plan to power my board from USB and the imp from my board (through VIN). I suppose I should set the jumper to battery then? Is there any danger of leaving the jumper in incorrect position?

The second question: I plan to use an I2C chip (digital pot). As I understand it, I cannot pull the resistors to 5V as that would damage the imp? Also, why is the value of the resistors given in the i2c example 4.7K? Would the value change if e.g. I use a 5V microcontroller?

The schematic makes this clearer:

  • the imp DCDC runs from the VIN pin on the edge of the breakout board.
  • the jumper allows this VIN pin to be connected either to the mini-USB socket (jumper to USB) or to the battery pads (jumper to BATT).

If you are powering the imp board through VIN, the jumper position is immaterial. You could even remove it totally.

Ideally, you ought to use a 3.3v digital pot. If not, the pot may support 3.3v I2C even though it’s powered from 5v - it’s worth checking the datasheet (or just trying it). For this, power the pot through 5v, but connect your I2C pullups to 3.3v.

I2C pull-up resistor values depend on bus capacitance and the speed you want to run. I’ve seen everything from 1k to 10k used, 4k7 is a typical number people go for.

Thanks a lot for the explanations!

One more question, if I may - unfortunately, the pullups are already connected to the 5V input from USB and rewiring them could be a bit problematic… What if I use this product: between the Imp and SCl/SDA pulled up to 5V - would that save the Imp from overvoltage?

That product should work, in theory. I’ve not tried it though.