Hi !! this is my first post here so I´ll just keep it short and simple, I have an arduino diecimila board so far I´ve only powered via USB and I want to start using it with a 9v battery, the first problem I encountered trying to do so was that nothing happened once I plugged the battery, I moved the power selector jumper to external and still nothing, I read the arduino troubleshooting guide, the answer to the first question suggests connecting the Rx pin to ground with a 10k resistor, I did that and it worked just fine however it also suggests connecting it to the Tx pin and I was wondering if I´d also have to connect it with a 10k resistor between them since I prefer the second option because it looks nicer.
Thanks in advance!
It should just work...
9V battery > DC power jack (using adapter) OR wires to GND and VIN.
The Arduino Diecimila can be powered via the USB connection or with an external power supply. The power source is selected by the PWR_SEL jumper: to power the board from the USB connection, place it on the two pins closest to the USB connector, for an external power supply, the two pins closest to the external power jack.
External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack. Leads from a battery can be inserted in the Gnd and Vin pin headers of the POWER connector. A low dropout regulator provides improved energy efficiency.
The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.
The power pins are as follows:
?VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.
?5V. The regulated power supply used to power the microcontroller and other components on the board. This can come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply.
?3V3. A 3.3 volt supply generated by the on-board FTDI chip. Maximum current draw is 50 mA.
?GND. Ground pins.
Because the RX pin is unconnected, the bootloader on the board may be seeing garbage data coming in, meaning that it never times out and starts your sketch. Try tying the RX pin to ground with a 10K resistor (or connecting it to the TX pin).
Because the RX pin is unconnected, the bootloader on the board may be seeing garbage data coming in, meaning that it never times out and starts your sketch. Try tying the RX pin to ground with a 10K resistor (or connecting it to the TX pin).
I thought the newer bootloaders have some features to minimize this.
No, you don't need one on TX. The bootloader is apparently receiving garbage, causing it to hang. A pull up/down on RX solves that problem. The TX line is not involved at all.
-j
I quoted all that stuff from the FAQ, but I took "nothing happened once I plugged in the battery." as the power not even coming on. I guess he'll have to clarify.
Hello everyone and thanks for your responses but it looks like I didn´t make myself clear so here you go again: what I meant was that I have already made my board work with the 9v battery(using the 2.5mm plug for the battery and connecting the Rx pin to ground with a 10k resistor), my question is If I wanted to power the board the same way except now I want to connect the Rx pin to Tx as opposed to connecting it to ground would I also have to do so with a 10k resistor??, thats all 
!!
would I also have to do so with a 10k resistor?
No.