Ethernet, SD, LCD, and pin 10.

I have a Mega2560 with an Ardiuno ethernet shield with an LCD shield all stacked. The LCD is an HD44780 found here: Gravity: 1602 LCD Keypad Shield For Arduino - DFRobot

I have the following code:

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(8,9,4,5,6,7);

const int chipSelect = 4;

void setup() 
  lcd.begin(16, 2);
  lcd.print("Initializing SD card...");
  // make sure that the default chip select pin is set to
  // output, even if you don't use it:
  pinMode(10, OUTPUT);
  // see if the card is present and can be initialized:
  if (!SD.begin(chipSelect)) {
    lcd.print("Card failed, or not present");
    // don't do anything more:
  lcd.print("card initialized.");

Everything is working great until I set pin 10 and then my LCD turns off. As near as I can tell, the LCD doesn't use pin 10.

The example says that pin 10 is a chip select. Can you only use the ethernet or the SD at a time? If so, do you/can you switch back and forth at runtime?

Also, I am running the hardware solely on USB (no power source) and have experienced some voltage sag before, could I be exceeding the available power on the USB? I have no idea how much current these things draw.

I believe you need one Slave Select pin per SPI device, the Arduino uses the SS pin on a device to tell that device it's the one being talked to. Without the SS pin triggered the SPI devices assume the Arduino is talking to someone else, and ignores the communications.