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LPCXpresso Experiment Kit - User’s Guide

Page 106

7.15.1 Lab 14a: Transmitting and Receiving via the UART

Expand the UARTSendChar() function to UARTSendString(uint8_t *pStr)

function (transmits a zero-terminated string – the terminating zero is not transmitted) and

UARTSendBuffer(uint8_t *pBuf, uint16_t length) functions.

Create a small program that makes use of these transmission functions. Also let the program echo

every received character.

Connect the FTDI cable and verify that the program works as intended.

Test to echo a longer string: “Received: x” (where x is the received char) from the LPC111x.

Type characters on the PC terminal program and observe the echoed response from the LPC111x.

Now test to send a series of 100 characters back to back from the PC. This is for example done by

sending a 100 byte long file. Select Send file in the File menu. What will happen?

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Every received character results in several characters echoed back to the PC. These echoed

characters will take longer time than the time of the (originally) received character. Characters are

received at full speed (back-to-back) and soon both transmit and received FIFOs will be full. Received

characters will start to be missed.

The solution is flow control. A receiver must be able to inform a transmitter that it must wait for a while

before transmitting more characters. One commonly used software solution for this is called Xon/Xoff

flow control, see: http://en.wikipedia.org/wiki/Software_flow_control for more information. A commonly

used hardware solution is called RTS/CTS flow control, see:

http://en.wikipedia.org/wiki/Flow_control_%28data%29#Hardware_flow_control

7.15.2 Lab 14b: Direct printf() to UART

In exercise Lab 4a-4d, semihosting was explored. In this experiment the printf() output will be sent to

the UART communication channel. Remember that the C runtime library had to be of the correct type

for semihosting to work. Have a look at Figure 21 and make sure the project settings select Redlib

(nohost) as the C runtime library for this exercise. There are hooks in Redlib for directing the printf()-

output to any wanted communication channel – the UART in this case.

The two simple functions below is all that is needed to direct the printf()-output to the UART and also to

let scanf()-input come from the UART.

#include "stdio.h"

#include "uart.h"

...

//use UART for printf

int __sys_write(int iFileHandle, char *pcBuffer, int iLength)

{

UARTSendBuffer((uint8_t *)pcBuffer,iLength); //send data buffer to UART

return iLength;

}

int __sys_readc(void)

{

char c;

UARTReceive((uint8_t*)&c, 1, TRUE);

return (int)c;

}

Place the two functions above in a file called retarget.c.