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LPCXpresso Experiment Kit - User’s Guide

Page 34

Note that registers GPIO0DIR and GPIO0DATA are accessed as LPC_GPIO0->DIR and

LPC_GPIO0->DATA, respectively.

Below is the two statements needed to first set PIO0_2 to an output and then pull the output low. This

will turn the LED on.

// Set PIO0_2 as an output

LPC_GPIO0->DIR = LPC_GPIO0->DIR | (0x1<<2);

// Turn LED1 on = set PIO0_2 pin low, i.e., clear bit

LPC_GPIO0->DATA = LPC_GPIO0->DATA & ~(0x1<<2);

As seen, each of the registers is first read and then bit #2 is manipulated. In the first statement, bit #2

is set which makes PIO0_2 an output. In the second statement, bit #2 is set to zero. This pulls PIO0_2

low.

Note that all bits in the registers must be read and only the bit of interest shall be manipulated. The

shift operation, (0x1<<2), is a good way of writing code. The “<<2” part indicates clearly that it is bit #2

that is manipulated. It is simpler for a reader of the code to quickly see this than to write the constant

value 0x04.

Below is an alternative, more compact way of writing the statements. This is a common way to write

this kind of statements.

// Set PIO0_2 as an output

LPC_GPIO0->DIR |= (0x1<<2);

// Turn LED1 on = set PIO0_2 pin low, i.e., clear bit

LPC_GPIO0->DATA &= ~(0x1<<2);

In real, professional programs, it is common to use defines to hide details about hardware

manipulation. Below is an example of how this can be done.

// Create defines for simpler access of LED1

#define DIR_REG_LED1 LPC_GPIO0->DIR

#define DATA_REG_LED1 LPC_GPIO0->DATA

#define PIO_PIN_LED1 2

#define LED1_ON DATA_REG_LED1 &= ~(1<<PIO_PIN_LED1)

#define LED1_OFF DATA_REG_LED1 |= (1<<PIO_PIN_LED1)

// Set PIO0_2 as an output

DIR_REG_LED1 |= (0x1<<PIO_PIN_LED1);

// Turn LED1 on

LED1_ON;

It is possible to take the principles further and create general macros for handling all ports and pins.

This was just an example of how to create well-structured, maintainable and professionally looking

code.

Chapter 9 contains a description how to get started with the LPCXpresso IDE. Read this chapter and

follow the guide how to import the projects. Start working with project “lab 1a”, which is the base for

this first experiment.

After compiling and linking without errors, follow the guide how to download and run the project.

In embedded programming it is important to have full control over the variables, more specifically the

number range they can hold. The original C standard was a little vague on the number of bits different

variable types have. It is specified as “at least X number of bits” and there is a specified order between

different types. However in embedded programming the exact number of bits is important to keep track

of. Therefore it is common to have an include file that have created/specified new variable types with

the number of bits exactly specified. We will use this setup in all experiments.