Hello All,
I am interfacing Arduino Mega, adafruit TFT_LCD, Nema 17 Stepper motor, adafruit motor shield version 1 & DIY Matrix Keypad. My aim is to set stepper RPM from Matrix Keypad and the same input should be displayed on LCD and then afterwards the stepper should move according to data entered. My problem is when I am doing the same motor moves for few seconds then slowly slowly the TFT_LCD dims and the whole system gets restarted. 
I think the power supply is not sufficient enough. But when I am supplying Arduino Board from USB of laptop and simultaneously connecting Supply of 12V/500mA to the external supply pin of Arduino Mega. Its working perfectly. So my question is how can I achieve it by supplying single power supply.
Hi,
Your USB port can supply over 1Amp at 5v.
If you use the 12V 500mA it is not able to provide that level of power, also when you use the 12V you are relying on the 5V regulator in the arduino to supply 5V power, it cannot supply that current level.
You need to provide an external 5V supply for all your output devices, use a regulator to drop the 12V to 5V, also I would think you will need a bigger power supply.
Do not try and power the stepper using the 5V supply coming out of the Mega.
Tom... 
"Your USB port can supply over 1Amp at 5v."
Hmm, more likely it cannot.
And if it could, the 500mA fuse after the USB connector on the Mega would open.
"But when I am supplying Arduino Board from USB of laptop and simultaneously connecting Supply of 12V/500mA to the external supply pin of Arduino Mega. "
If you have 12V connected to the barrel jack, then the autovoltage select circtuit should kick and prevent current from being supplied via the USB port.
I would make a splitter or Y cable and power the devices separately.
Hi,
Your USB port can supply over 1Amp at 5v.
Sorry to late a night. up to 900mA with USB 3.0.
Tom....