avrresistorssignalusb
I'm currently trying to make a USB device with the ATMega16U2.
On page 187 of the datasheet, there are unspecified resistors between the USB port and the pins of the microcontroller
The Arduino Uno (R3) similarly uses these resistors in its design, with a value of 22 Ohms.
What are these for, and why are they needed with such low values?
Bonus question: The wiring between UDP, UDM, D+ and D- in the figure on page 187 is a mistake, right? On the next page, they are reversed.
If I am not getting confused, this system of resistors is required by iPhones and other i-Devices to charge at different current levels. Otherwise an error message will pop up. If you don't have an iPhone you can quite safely remove these and replace them.
Apple stopped being as 'lax' with the charging interface and started being very picky about having the official chargers. We still doubted that there was an enumeration chip inside each charger - too expensive and complex. So there must be something else going on in those data lines. Time to sacrifice an official Apple iPhone 3Gs charger!
Taking it apart, desoldering the 4 data line resistors and measuring them on our multimeter, we found the following as shown in the schematic:
Although I don't have direct experience with this, from what I've read, IC's that perform this function usually sample the D+/D- lines on application of power with some sink on the D+/D- to detect the charger type, and disconnect their detection logic once a determination has been made of the connected source, AC adapter or USB port. More information is available from a TI app note here.
I sure from my previous part searches that some battery charger IC's have built in charger type detection, but I couldn't find an example part when searching just now.
There's the MAX14578 which also does the job, but it may not fit your criteria of low cost.
Best Answer
They are "termination" resistors. for impedance matching purpose.
Per USB 2.0 standard, when using a full speed (12Mbps) buffer in a USB 2.0 compliant system, it is necessary to place a series resistor on the D+ and D− pins external to the transceiver. This placement is a result of the need to properly match the impedance of the specified transmission line having a differential characteristic impedance (\$Z_{O}\$) of 90Ω and a single ended impedance of 45Ω. To match this load, it requires that the full speed transceiver output impedance must be in the range 28Ω to 44Ω. So if the transceiver has a output impedance less than this (many output drivers has a low output impedance), you may need a external termination resistor.