Stellaris LM4F120XL assigning ADC to GPIO

Quickly - several things:

 a) appears you've correctly enabled the ports bearing potential ADC channels - but never enable the ADC Peripheral itself!  (i.e. SysCtlPeripheralEnable(SYSCTL_PERIPH_ADC0);  // one suspects ADC0 is reasonable choice

b) #include "driverlib/adc.h" should make some appearance - its absence notable...

c) ADC seeks 16MHz System Clock - minimum.  Are you in compliance?

d) your early code does not yet, "ADCSequenceConfigure(), nor ADCSequenceStepConfigure(), nor ADCSequenceEnable()" - all likely required to properly transact w/the ADC.

Such "mixed signal devices" (your MCU) have a fairly complex input circuit arrangement in which a "holding capacitor" charges w/ sampling voltage - which may result from any/all channels which see a voltage introduction.  Thus - your report of some "voltage presence" - upon a "potential" ADC input pin - may not be cause for alarm.

Perhaps a better test would be to complete the ADC initialization process.   (single-ended.c) w/in Examples\Peripherals\ADC\ (my suggestion)  Insure that any/all external voltages you apply are w/in ADC input min/max range - and ideally arise/fall with application of proper power to your MCU...

Thanks, but I initialise the ADC in another part of code before I initialise the GPIO.

int main(void) {
State = Waiting;

// Enable lazy stacking for interrupt handlers. This allows floating-point
// instructions to be used within interrupt handlers, but at the expense of
// extra stack usage.

ROM_FPULazyStackingEnable ();
ROM_FPUEnable ();

//Set up system clock to run at 50MHz
SysClockInit();

//--Initialise screen and display test image
ScreenInit();

//initialise UART0
UARTinit();

//initialise ADC0
ADCinit();

//initialise GPIO peripheral
GPIOinit2();

With GPIO PD0 it's as if there is already a voltage on it before initialisation.

I have just noticed that 3.3V appears on the PD0 pin after I initialise the UART. The code for the UART initialisation is below. I don't see how this should affect pin PD0.

It's all very confusing.

#include "inc/hw_types.h"
#include "inc/hw_ints.h"
#include "inc/hw_memmap.h"
#include "driverlib/sysctl.h"
#include "driverlib/gpio.h"
#include "driverlib/rom.h"
#include "utils/uartstdio.h"
#include "inc/hw_uart.h"
#include "driverlib/interrupt.h"
#include "driverlib/uart.h"
extern char OutBuf[80];
#define byte unsigned char
extern byte OutPtr;

//#define GPIO_PA0_U0RX 0x00000001
//#define GPIO_PA1_U0TX 0x00000401

void UARTinit(void)
{

//initialise UART0
SysCtlPeripheralEnable(SYSCTL_PERIPH_UART0);
SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);

GPIOPinConfigure(GPIO_PA0_U0RX);
GPIOPinConfigure(GPIO_PA1_U0TX);
GPIOPinTypeUART(GPIO_PORTA_BASE, GPIO_PIN_0 | GPIO_PIN_1);

SysCtlPeripheralEnable (SYSCTL_PERIPH_GPIOF);
UARTConfigSetExpClk (UART0_BASE, ROM_SysCtlClockGet (), 115200,
(UART_CONFIG_WLEN_8 | UART_CONFIG_STOP_ONE | UART_CONFIG_PAR_NONE));

UARTFIFOLevelSet(UART0_BASE, UART_FIFO_TX4_8, UART_FIFO_RX4_8); // FIFO 8 chars
UARTFIFOEnable(UART0_BASE); //enable FIFOs

IntEnable(INT_UART0); //enable UART interrupt

UARTIntEnable(UART0_BASE, UART_INT_TX);

UARTStdioInit(0); //tells uartstdio functions to use UART0
}

Your board schematic - identifying the "cross pin" connections (2 of them) should not avoid your attention.

Real thanks results from tick of Verify Answer...  (atop my earlier post - beneficial to future posters who "re-visit" your issue)

Update: 10:56 CST - Verify duly received - acknowledged - and appreciated.  Merci mon ami...