Hello Word ( LED blinker ) on LPC1768 explained. GPIO tutorial

Traditionally, blinking an LED has been the first program written for almost any embedded system and LPC1768 is no exception. Here's a detailed description of all the software tools needed to get started. The LPC1768H schematic is available in the NGX website. The schematics show that TEST_LED is connected to P1.29. Now, for the register description, we'll need the LPC1768 manual.

For using any of the LPC1768 peripherals, the general steps to be followed are

1. Power Up the peripheral to be used.
2. Set the Clock Rate for the peripheral
3. Connect necessary pins using Pin Connect Block.
4. Initialize the registers of the peripheral.

Before proceeding, you'll need to know about bit manipulation in C. Here is a quick tutorial if you need it.

Now lets say "Hello World".

Open the LPC1768 manual, goto chapter 4: Clocking and Power Control, look for register Power Control for Peripherals register (PCONP) . Its in page 63. From the table, bit 15 is PGPIO. Setting the bit to 1 will power up the GPIO ports. Note that the default value is 1, which means GPIO is powered up by default on reset. But the start up code may have modified this and its best not to assume anything.So,

LPC_SC->PCONP |= ( 1 << 15 ); // power up GPIO

Now, to give the right clock to the peripheral. In the same chapter, look for Peripheral Clock Selection register ( PCLKSEL1 ). Its in page 57. Bits 3:2 set the clock divider for GPIO. Since we're not using interrupts, we wont change the default value. Even if it isn't default, it wont matter to our program. So we wont write to this register. But careful attention to these bits is needed for peripherals like timers and SSP.
Just in case someone is confused, PCLK refers to Peripheral Clock and CCLK refers to CPU Clock. PCLK is obtained by dividing CCLK by 1,2,4 etc. (For more details, look at table 42 in page 57).


By default, GPIO pins are connected to the external pins. So there is need to change settings in the pin connect block.

To initialize the port, goto chapter 9 GPIO (page 122). First, P1.29 has to be made an output pin

LPC_GPIO1->FIODIR |= 1 << 29; // puts P1.29 into output mode.

Caveat : Unlike the AVR pins which are in hi-Z after reset, LPC1768 pins are in internal pull-up mode after reset.

To turn ON the LED (assuming the other end is connected to GND via a resistor),

LPC_GPIO1->FIOPIN |= 1 << 29; // make P1.29 high

To turn it OFF

LPC_GPIO1->FIOPIN &= ~( 1 << 29 ); // make P1.29 low

This can more easily be accomplished using FIOSET and FIOCLR. Most of these registers are also byte addressable.

So, lets have the complete code

Assuming you have CodeSourcery g++ Lite, Eclipse installed, to create a project in Eclipse
1. Open Eclipse
2. File->New->C Project
3. In the project type pane, select Makefile Project -> Empty Project. Select Other ToolChain as the toochain. Give a project name, say "LEDBlinky". Click Finish
4. Get the Startup code and extract it somewhere.
5. In eclipse, in the project window, right click the newly created "LEDBlinky" project and import files.
6. Select File System in the Import window. Select the folder to which you extracted the startup code ("starter_code_21Oct2010"). Select all files for import. Click Finish.
7. Now open main.c

#include "LPC17xx.h"

volatile uint32_t temp;

void _delay(uint32_t del);

int main (void) 
{
    LPC_SC->PCONP |= ( 1 << 15 ); // power up GPIO
    LPC_GPIO1->FIODIR |= 1 << 29; // puts P1.29 into output mode.
    while(1)
    {
        LPC_GPIO1->FIOPIN |= 1 << 29; // make P1.29 high
        _delay( 1 << 24 );
        LPC_GPIO1->FIOPIN &= ~( 1 << 29 ); // make P1.29 low
        _delay( 1 << 24 );
    }
    return 0;
  
}
void _delay(uint32_t del)
{
    uint32_t i;
    for(i=0;i < del; i++)
        temp = i;
}

_delay() has been used as a crude software delay producing function. It should produce about a second or so of delay depending on CCLK.
A volatile variable 'temp' has been updated everytime in the loop so that the compiler will not optimize away the loop. Now hit Ctrl+B for build ( Project -> Build All ). You should see this in the console window

**** Build of configuration Default for project LEDBlinky ****

make all
arm-none-eabi-gcc -c -W -Wall -Os --std=gnu99 -fgnu89-inline -mcpu=cortex-m3 -mthumb -ffunction-sections -fdata-sections main.c -o main.o
arm-none-eabi-gcc -c -W -Wall -Os --std=gnu99 -fgnu89-inline -mcpu=cortex-m3 -mthumb -ffunction-sections -fdata-sections system_LPC17xx.c -o system_LPC17xx.o
arm-none-eabi-ld -Map main.map --gc-sections -T LPC17xx.ld startup_LPC17xx.o core_cm3.o main.o system_LPC17xx.o -o main.elf
arm-none-eabi-objdump -x --syms main.elf > main.dump
   text       data        bss        dec        hex    filename
    648          0       2056       2704        a90    main.elf
arm-none-eabi-objcopy  -O binary main.elf main.bin
arm-none-eabi-objcopy  -O ihex main.elf main.hex

Now you can burn the hex file using whatever method you prefer.

The Compiled Eclipse project is here. To download the hex code,use jtag or load serially using flash magic. The flash start has been put as 0x00000000 in the linker file.

If you are using NXP secondary bootloader, then the flash start will have to be changed to 0x00002000 in the linker file ( LPC17xx.ld ). Here is the compiled project with that change.

UPDATE: The latest code base is available here.