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How to Interface UART with LPC2148 ARM7 starter board

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The ARM7 LPC2148 Starter Board is specifically designed to help students to master the required skills in the area of embedded systems. The kit is designed in such way that all the possible features of the microcontroller will be easily used by the students. The kit supports in system programming (ISP) which is done through serial port.

NXP’s ARM7 (LPC2148 ), ARM Starter Kit is proposed to smooth the progress of developing and debugging of various designs encompassing of High speed 32-bit Microcontrollers.


UART (Universal Asynchronous Receiver Transmitter) are one of the basic interfaces which provide a cost effective simple and reliable communication between one controller to another controller or between a controller and PC.

RS-232 Level Converter

Usually all the digital ICs work on TTL or CMOS voltage levels which cannot be used to communicate over RS-232 protocol. So a voltage or level converter is needed which can convert TTL to RS232 and RS232 to TTL voltage levels. The most commonly used RS-232 level converter is MAX232.

This IC includes charge pump which can generate RS232 voltage levels (-10V and +10V) from 5V power supply. It also includes two receiver and two transmitters and is capable of full-duplex UART/USART communication.

  • RS-232 communication enables point-to-point data transfer. It is commonly used in data acquisition applications, for the transfer of data between the microcontroller and a PC.
  • The voltage levels of a microcontroller and PC are not directly compatible with those of RS-232, a level transition buffer such as MAX232 be used.

Interfacing UART

Fig. 1 shows how to interface the UART to microcontroller. To communicate over UART or USART, we just need three basic signals which are namely, RXD (receive), TXD (transmit), GND (common ground). So to interface UART with Microcontroller, we just need the basic signals.

Interfacing UART to Microcontroller
Fig. 1 Interfacing UART to Microcontroller

Interfacing UART with LPC2148

We now want to display a text in PC from LPC2148 Starter Board by using UART module. In LPC2148 Starter Board contains two serial interfaces that are UART0 & UART1. Here we are using UART0. The Transmitter pins send the data into PC and the receiver pin receives the data from PC. The PC and microcontroller speed are denoted by using baud rate. When the baud rates of both PC and Microcontroller are same, then only the data transmit and receive correctly otherwise not.

Pin Assignment with LPC2148

Pin Assignment with LPC2148

Circuit Diagram to Interface UART0 with LPC2148

 Circuit Diagram to Interface UART0 with LPC2148

Source Code

The Interfacing UART with LPC2148 program is very simple and straight forward, which display a text in PC from LPC2148 Starter Board through UART0. Some delay is occurring when a single data is sent to PC. C programs are written in Keil software. The baud rate of microcontroller is 9600.

C Program to display a text in PC from LPC2148


Title : Program to display a text in PC from LPC2148 through UART0


#define CR     0x0D
void init_serial (void);
int putchar (int ch);
int getchar (void);
unsigned char test;

//<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<	Code Begins Here	>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

int main(void)
	char *Ptr = "*** UART0 Demo ***\n\n\rType Characters to be echoed!!\n\n\r";

	VPBDIV = 0x02;						//Divide Pclk by two

		while (*Ptr)
		putchar(getchar());					//Echo terminal

//<<<<<<<<<<<<<<<<<<<<<<<<< Serial Initialization >>>>>>>>>>>>>>>>>>>>>>>>>>>>

void init_serial (void)			  /* Initialize Serial Interface       */
  PINSEL0 	= 0x00000005;         /* Enable RxD0 and TxD0              */ 
  U0LCR 	= 0x00000083;         /* 8 bits, no Parity, 1 Stop bit     */
  U0DLL 	= 0x000000C3;         /* 9600 Baud Rate @ 30MHz VPB Clock  */
  U0LCR 	= 0x00000003;         /* DLAB = 0                          */

//<<<<<<<<<<<<<<<<<<<<<<<<<<< Putchar Function  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

int putchar (int ch) 			  /* Write character to Serial Port    */

  if (ch == '\n')  {
    while (!(U0LSR & 0x20));
    U0THR = CR;              	   /* output CR */
  while (!(U0LSR & 0x20));
  return (U0THR = ch);

//<<<<<<<<<<<<<<<<<<<<<<<<<<<< Getchar Function >>>>>>>>>>>>>>>>>>>>>>>>>>>>>

int getchar (void) 				/* Read character from Serial Port   */

  while (!(U0LSR & 0x01));
  return (U0RBR);


To compile the above C code you need the KEIL software. They must be properly set up and a project with correct settings must be created in order to compile the code. To compile the above code, the C file must be added to the project.

In Keil, you want to develop or debug the project without any hardware setup. You must compile the code for generating HEX file. In debugging Mode, you want to check the port output without LPC2148 Starter Board .

The Flash Magic software is used to download the hex file into your microcontroller IC LPC2148 through UART0.

Testing the UART0 with LPC2148

Give +3.3V power supply to LPC2148 Starter Board ; the serial cable is connected between the LPC2148 Starter Board and PC. Open the Hyper Terminal screen, select which port you are using and set the default settings. Now the screen should show some text messages.

If you not reading any text from UART0, then you just check the jumper connections & just check the serial cable is working. Otherwise you just check the code with debugging mode in Keil. If you want to see more details about debugging just see the videos in below link.

General Information

  • For proper working use the components of exact values as shown in Circuit file. Wherever possible use new components.
  • Solder everything in a clean way. A major problem arises due to improper soldering, solder jumps and loose joints.
  • Use the exact value crystal shown in schematic.
  • More instructions are available in following articles,

Creating & Debugging a Project in KEIL

User Manual of LPC2148 Starter Board.

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