hello jin, i'm having the same problem... i thiink it has to do somthing about clock settings but i can't figure out clock on mbed. please help if u have rsolve the issue .. Thanks a lot

/*******************************************************
            - TXD1 - DIP26
            - RXD1 - DIP25
            - 38400 bps
*******************************************************/
void uart1_init(void)
{
    LPC_SC->PCONP |= (1<<4);                //power up uart1
    LPC_SC->PCLKSEL0 |= (3<<8);             //run uart1 clock at 12MHz (prescale 8)
    
    //baud generation (38400)
    LPC_UART1->LCR |= (1<<7);               //DLAB enable requirement
    LPC_UART1->DLL = (14<<0);               //divl
    LPC_UART1->DLM = (0<<0);                //divm
    LPC_UART1->FDR = (4<<0) | (10<<4);      //fractional baud div
    
    LPC_UART1->FCR |= (0<<6) | (0<<3) | (1<<2) | (1<<1) | (1<<0);            //FIFO enable, TX and RX FIFO reset, no DMA
    
    LPC_PINCON->PINSEL4 |= (2<<0);          //TXD1
    LPC_PINCON->PINSEL4 |= (2<<2);          //RXD1
    LPC_PINCON->PINMODE4 |= (2<<0);         //disable pull-ups or downs
    LPC_PINCON->PINMODE4 |= (2<<2);         //disable pull-ups or downs
    
    //38400-8-n-1 frame format
    LPC_UART1->LCR &= ~(1<<7);              //DLAB disable to enable transmits and receives
    LPC_UART1->IER |= (0<<1) | (1<<0);      //receive interrupt enable only
    LPC_UART1->LCR |= (0<<3) | (0<<2) | (3<<0);             //frame format register
    LPC_UART1->TER |= (1<<7);               //enable transmitter
    
    //enable interrupt
    NVIC_SetPriority(UART1_IRQn, LOW_PR);
    NVIC_EnableIRQ(UART1_IRQn);
}

heres a code that enables UART1 on pin 25 and 26. The mbed runs at 96MHz so you may want to use that as a reference (you can always prescale though). The computation for the baud prescaler is a little tricky but the LPC17xx manual will guide you through it. This code enables the received interrupt on the line so you need the ISR for it. <<code>> /******************************************************* - TXD1 - DIP26 - RXD1 - DIP25 - 38400 bps *******************************************************/ void uart1_init(void) { LPC_SC->PCONP |= (1<<4); //power up uart1 LPC_SC->PCLKSEL0 |= (3<<8); //run uart1 clock at 12MHz (prescale 8) //baud generation (38400) LPC_UART1->LCR |= (1<<7); //DLAB enable requirement LPC_UART1->DLL = (14<<0); //divl LPC_UART1->DLM = (0<<0); //divm LPC_UART1->FDR = (4<<0) | (10<<4); //fractional baud div LPC_UART1->FCR |= (0<<6) | (0<<3) | (1<<2) | (1<<1) | (1<<0); //FIFO enable, TX and RX FIFO reset, no DMA LPC_PINCON->PINSEL4 |= (2<<0); //TXD1 LPC_PINCON->PINSEL4 |= (2<<2); //RXD1 LPC_PINCON->PINMODE4 |= (2<<0); //disable pull-ups or downs LPC_PINCON->PINMODE4 |= (2<<2); //disable pull-ups or downs //38400-8-n-1 frame format LPC_UART1->LCR &= ~(1<<7); //DLAB disable to enable transmits and receives LPC_UART1->IER |= (0<<1) | (1<<0); //receive interrupt enable only LPC_UART1->LCR |= (0<<3) | (0<<2) | (3<<0); //frame format register LPC_UART1->TER |= (1<<7); //enable transmitter //enable interrupt NVIC_SetPriority(UART1_IRQn, LOW_PR); NVIC_EnableIRQ(UART1_IRQn); } <</code>>