File content as of revision 3:fba5fc24faff:

#include "mbed.h"

void usartSetup (void) {
    RCC->IOPENR |= RCC_IOPENR_IOPBEN;
    RCC->APB2ENR |= RCC_APB2ENR_USART1EN;            // enable clock for USART1
    USART1->BRR  = 320000 / 96;              // set baudrate
    GPIOB->MODER &= ~GPIO_MODER_MODE6_0;
    GPIOB->MODER |= GPIO_MODER_MODE6_1;
    
    GPIOB->AFR[1] |= 0x40;
    GPIOB->OTYPER &= ~GPIO_OTYPER_OT_6;
    USART1->CR1 |= (USART_CR1_RE | USART_CR1_TE | USART_CR1_UE);  // RX, TX enable
}

void SendChar(char Array[], int size){
/*    char stringtosend[] = {"Salut ca va ? Oui et toi ? Oh yes ca fonctionne bien ahahahahahahahahahahahah\r\n"};*/
    Serial pc(USBTX, USBRX);
    for(int send = 0; send < size; send++){
        //On attend que la transmission soit terminée
        while((USART1->ISR & USART_ISR_TC) != USART_ISR_TC);
        
        //Si on a tout envoyé
        if(send == size)
        {
            send=0;
            USART1->ICR = USART_ICR_TCCF; /* Clear transfer complete flag */
        }
        else
        {
            //Si le registre de données est vide
            while (!(USART1->ISR & (USART1->ISR | USART_ISR_TXE)));
            USART1->TDR = Array[send];
        }
    }
}

void ReceiveChar(int TimeOut){
    USART1->BRR = 320000 / 96;
    USART1->CR1 = USART_CR1_RXNEIE | USART_CR1_RE | USART_CR1_UE;
    char buffer[300] = {0};
    int counter = 0;
    for(int i = 0; i < TimeOut; i++){
        if((USART1->ISR & USART_ISR_RXNE) == USART_ISR_RXNE){
            buffer[counter] = (uint8_t)(USART1->RDR); // Receive data, clear flag
            counter++;
        }
        if(counter >= sizeof(buffer)){
            break;
        }
    }
    usartSetup();
    int sizeBuffer = sizeof(buffer);
    SendChar(buffer, sizeBuffer);
}

int main() {
    usartSetup();
    char stringtosend[] = {"Salut ca va ? Oui et toi ? Oh yes ca fonctionne bien ahahahahahahahahahahahah\r\n"};
    int sizeArray = sizeof(stringtosend);
    SendChar(stringtosend, sizeArray);
    ReceiveChar(500);
}