Example of a Program Written in C and running on a ARM7

// Author Richard St-Denis, Universite de Sherbrooke, 2013.
#include "uart.h"

// Addresses of SCB (System Control Block) registers
#define SCB_MEMMAP *((volatile unsigned int*)0xE01FC040)
// interrupt vectors are not re-mapped and reside in flash
#define MEMMAP_USER_FLASH_MODE (1 << 0)
#define SCB_PLLCON *((volatile unsigned int*)0xE01FC080)
#define PLLE 0         // enable PLL (Phase Locked Loop) 
#define PLLC 1         // connect PLL (Phase Locked Loop)
#define SCB_PLLCFG *((volatile unsigned int*)0xE01FC084)
#define PLL_M 4        // multiplier
#define MSEL (PLL_M-1) // multiplier - 1
#define PSEL0 5
#define PSEL1 6
#define SCB_PLLSTAT *((volatile unsigned int*)0xE01FC088)
#define PLOCK 10       // PLOCK bit
#define SCB_PLLFEED *((volatile unsigned int*)0xE01FC08C)
#define PLL_FEED1 0xAA // feed sequence
#define PLL_FEED2 0x55
#define SCB_VPBDIV *((volatile unsigned int*)0xE01FC100)
// VPB should run at full speed
#define VPBDIV_VAL 1

// Addresses of MAM (Memory Accelerator Module) registers
#define MAM_MAMCR *((volatile unsigned int*)0xE01FC000)
#define MAM_MODE 2     // MAM fully enabled
#define MAM_MAMTIM *((volatile unsigned int*)0xE01FC004)
#define MAM_FETCH 3    // 3 cclks are proposed as fetch timing

void systemInit(void)
{
 // Enable and connect the PLL (Phase Locked Loop)
 // 1. Set multiplier and divider
 SCB_PLLCFG = MSEL | (1 << PSEL1) | (0 << PSEL0);
 // 2. Enable PLL
 SCB_PLLCON = (1 << PLLE);
 // 3. Feed sequence
 SCB_PLLFEED = PLL_FEED1;
 SCB_PLLFEED = PLL_FEED2;
 // 4. Wait for PLL lock (PLOCK bit is set if locked)
 while (!(SCB_PLLSTAT & (1 << PLOCK)));
 // 5. Connect (and enable) PLL
 SCB_PLLCON = (1 << PLLE) | (1 << PLLC);
 // 6. Feed sequence
 SCB_PLLFEED = PLL_FEED1;
 SCB_PLLFEED = PLL_FEED2;
	
 // Setup and enable the MAM (Memory Accelerator Module)
 // 1. Start change by turning of the MAM (redundant)
 MAM_MAMCR = 0;	
 // 2. Set MAM-Fetch cycle to 3 cclks as recommended for > 40MHz
 MAM_MAMTIM = MAM_FETCH;
 // 3. Enable MAM 
 MAM_MAMCR = MAM_MODE;

 // --- set VPB speed ---
 SCB_VPBDIV = VPBDIV_VAL;
	
 SCB_MEMMAP = MEMMAP_USER_FLASH_MODE; // according to the Makefile
}
// Addresses of PCB (Pin Connect Block) registers
#define PCB_PINSEL1 *((volatile unsigned int*)0xE002C004)
// Addresses of GPIO (General Purpose I/O) registers
#define GPIO_IOSET *((volatile unsigned int*)0xE0028004)
#define GPIO_IODIR *((volatile unsigned int*)0xE0028008)
#define GPIO_IOCLR *((volatile unsigned int*)0xE002800C)
// GPIO pin associated to the LED
#define LEDPIN 30

void gpioInit(void)
{
 GPIO_IODIR |= (1 << LEDPIN);	// define LED-Pin as output
 PCB_PINSEL1 = 0x0;
 GPIO_IOSET  = (1 << LEDPIN);	// set Bit = LED off (active low)
}

void ledToggle(void)
{
 static unsigned char state=0;

 state = !state;
 if (state)
     GPIO_IOCLR = (1 << LEDPIN);  // set Bit = LED on
 else 
     GPIO_IOSET = (1 << LEDPIN);  // set Bit = LED off
}

// Line Control Register bit definitions
#define ULCR_CHAR_8 (3 << 0)      // 8-bit character length
#define ULCR_STOP_1 (0 << 2)      // 1 stop bit
#define ULCR_PAR_NO (0 << 3)      // no parity

// Definition of a typical UART 'mode' setting
#define UART_8N1 (uint8_t)(ULCR_CHAR_8 + ULCR_PAR_NO + ULCR_STOP_1)

// FIFO Control Register bit definitions
#define UFCR_FIFO_ENABLE (1 << 0)  // FIFO enable
#define UFCR_FIFO_TRIG8  (2 << 6)  // Rx trigger, 8 characters in FIFO

// Definition of a typical UART 'fmode' setting
#define UART_FIFO_8 (uint8_t)(UFCR_FIFO_ENABLE + UFCR_FIFO_TRIG8)

#define BAUD 9600
#define FOSC 14745000
// Baud-Rate is calculated based on pclk (VPB-clock)
// the devisor must be 16 times the desired baudrate
#define UART_BAUD(baud) (uint16_t)(((FOSC*PLL_M/VPBDIV_VAL) / ((baud) * 16.0)) + 0.5)

int main(void)
{
 int ch;

 systemInit();
 gpioInit();
	
 uart0Init(UART_BAUD(BAUD), UART_8N1, UART_FIFO_8);

 uart0Puts("\r\nTesting the USB serial interface of the Olimex DEV-00268 board\r\n");
 uart0Puts("and testing the LED...\r\n");
	
 while(1) {
   if ((ch = uart0Getch()) >= 0) {
       uart0Putch(ch);
       uart0Puts("\r\n");
       ledToggle();
   }
 }	
 return 0;
}

/******************************************************************************
 * Based on software from:
 * Copyright 2004, R O SoftWare
 * No guarantees, warrantees, or promises, implied or otherwise.
 * May be used for hobby or commercial purposes provided copyright
 * notice remains intact.
 * 
 * Reduced to see what has to be done for minimum UART-support
 * by Richard St-Denis, Universite de Sherbrooke, 2013.
 *****************************************************************************/
#ifndef INC_UART_H
#define INC_UART_H

#include "inttypes.h"

void uart0Init(uint16_t baud, uint8_t mode, uint8_t fmode);
int uart0Getch(void);
int uart0Putch(int ch);
const char *uart0Puts(const char *string);

#endif

/******************************************************************************
 * This module provides interface routines to the LPC ARM UARTs.
 * Copyright 2004, R O SoftWare
 * No guarantees, warrantees, or promises, implied or otherwise.
 * May be used for hobby or commercial purposes provided copyright
 * notice remains intact.
 *
 * Reduced to see what has to be done for minimum UART-support
 * by Richard St-Denis, Universite de Sherbrooke, 2013.
 *****************************************************************************/

#include "uart.h"

// Addresses of UART0 registers
#define UART0_RBR *((volatile unsigned int*)0xE000C000)
#define UART0_THR *((volatile unsigned int*)0xE000C000)
#define UART0_IER *((volatile unsigned int*)0xE000C004)
#define UART0_IIR *((volatile unsigned int*)0xE000C008)
#define UART0_FCR *((volatile unsigned int*)0xE000C008)
#define UART0_LCR *((volatile unsigned int*)0xE000C00C)
#define ULCR_DLAB_ENABLE (1 << 7)  // Enable Divisor Latch Access
#define UART0_LSR *((volatile unsigned int*)0xE000C014)
#define ULSR_RDR  (1 << 0)         // Receive Data Ready
#define ULSR_THRE (1 << 5)         // Transmit Holding Register Empty
#define UART0_DLL *((volatile unsigned int*)0xE000C000)
#define UART0_DLM *((volatile unsigned int*)0xE000C004)

// Addresses of PCB (Pin Connect Block) registers
#define PCB_PINSEL0 *((volatile unsigned int*)0xE002C000)

#define PINSEL_BITPIN0 0           // TX-Pin for Pin 0.0
#define PINSEL_BITPIN1 2           // RX-Pin for Pin 0.1
// PINSEL0 has to be set to "UART-Function" = Function "01"
#define PINSEL_FIRST_ALT_FUNC 1
// Values of bits 0-3 in PINSEL to activate UART0
#define UART0_PINSEL ((PINSEL_FIRST_ALT_FUNC << PINSEL_BITPIN0)|(PINSEL_FIRST_ALT_FUNC << PINSEL_BITPIN1))
#define UART0_PINMASK (0x0000000F) // PINSEL0 Mask for UART0

void uart0Init(uint16_t baud, uint8_t mode, uint8_t fmode)
{
 // Setup Pin Function Select Register (Pin Connect Block) 
 // Make sure old values of Bits 0-4 are masked out and
 // Set them according to UART0-Pin-Selection
 PCB_PINSEL0 = (PCB_PINSEL0 & ~UART0_PINMASK) | UART0_PINSEL;

 UART0_IER = 0x00;   // clear Interrupt Enable Register
 UART0_IIR = 0x00;   // clear Interrupt Identification Register
 UART0_LSR = 0x00;   // clear Line Status Register

 // Set the baud rate - DLAB must be set to access DLL/DLM
 UART0_LCR = (ULCR_DLAB_ENABLE);    // set divisor latches (DLAB)
 UART0_DLL = (uint8_t)baud;         // set for baud low byte
 UART0_DLM = (uint8_t)(baud >> 8);  // set for baud high byte

 // Setup Line Control Register (Parity, Stop bit, Word length)
 UART0_LCR = (mode & ~(ULCR_DLAB_ENABLE));   // and clear DLAB 
 // Setup FIFO Control Register (FIFO-Enabled + Rx Trigger) 
 UART0_FCR = fmode;
}

int uart0Getch(void)
{                            // get a character
 if (UART0_LSR & ULSR_RDR)   // check if a character is available
     return UART0_RBR;       // return the character
 return -1;
}

int uart0Putch(int ch)
{                            // put a character
 while (!(UART0_LSR & ULSR_THRE))
   continue;                 // wait for an empty buffer
 UART0_THR = (uint8_t)ch;    // put the character to the Transmit Holding Register
 return (uint8_t)ch;         // return char
}

const char *uart0Puts(const char *string)
{                            // put a string
 char ch;

 while ((ch = *string)) {
   if (uart0Putch(ch)<0) break;
       string++;
 }
 return string;
}