On the Arduino Diecimila you had to choose between using the USB to supply power or using the external barrel jack via a jumper. Since the Duemilanove the Arduino can choose automatically. How does it do that?
there is a jumper setting in the board which selects the power supply through usb or battery ...
betadecay95:
On the Arduino Diecimila you had to choose between using the USB to supply power or using the external barrel jack via a jumper. Since the Duemilanove the Arduino can choose automatically. How does it do that?
Via an auto-voltage selector circuit on the board. It has an op-amp stage set up as a comparator that monitors the voltage on the Vin pin (external DC voltage) and if it's above a minimum amount it switches a FET switch off that isolates the USB power from the board. If both USB and external power are available the circuit uses the external power to power the board via the on-board +5vdc voltage regulator that is powered via external DC power.
Lefty
Alright I understand that. I looked at the Duemilanove schematic and found what you were talking about. I do have one more question though:
Looking at the portion of the schematic attached: I understand comparing half of the Vin to the 3.3V from VCC30 to raise the comparator IC5A high, but what is the point of the IC5B op-amp?
betadecay95:
Alright I understand that. I looked at the Duemilanove schematic and found what you were talking about. I do have one more question though:Looking at the portion of the schematic attached: I understand comparing half of the Vin to the 3.3V from VCC30 to raise the comparator IC5A high, but what is the point of the IC5B op-amp?
Just a buffer, in fact in the latest rev 3 boards they don't use that buffer stage there but rather reused it to buffer drive the pin 13 led.
Lefty