5 years, 1 month ago.

Q's about FRDM-K64 USB Serial & UART operation/coupling

I am using a FRDM-K64F board with the GCC compiler (frdm-k64f-gcc 2.0.0) and mbed/yotta, all stock nothing changed. I am attempting to implement a monitor program using USBTX/USBRX to interact with a user and UART0 to send data to an attached device. I have few questions:

1) when using UART0, I noticed that PTA1 and PTA2 are not defined in 'yotta_modules/mbed-hal-frdm-k64f/source/PeripheralPins.c'. I added the following to the respective tables PinMap_UART_TX and PinMap_UART_RX: {PTA2 , UART_0, 3} TX Pin {PTA1, UART_0, 3} RX Pin This appears to rectify the problem of being able to use the pin (at least it compiles cleanly). Is there a reason this definition was omitted?

2) To test that the UART0/PTA2/PTA1 was working, I jumped PTA2 to PTA1 so it was simply looped back onto itself. In my code I am using the following:

#include "mbed-drivers/mbed.h"

Serial uart(PTA2, PTA1);
Serial pc(USBTX, USBRX);

void rxIRQ(void)
  extern microrl_t *prl;

 if( pc.readable() ) {
    while( pc.readable() )
      microrl_insert_char(prl, pc.getc() );

 if( uart.readable() ) {
    while( pc.readable() )

void serialInit(void)
  void rxIRQ(void);


The reason this may not be working is because USBTX/USBRX seems tied to UART0 functionality, is there anyway to decouple the two so that you still have the ability to use USBTX/RX and the UART0 function?

1 Answer

3 years, 11 months ago.

(USBTX,USBRX) is (PTB17,PTB16) = UART0. When you want to use USB-serial, can use only UART1/2/3/4. UART1 is connected open-SDA, but UART1(PTC4,PTC3) is usable. Note.: UART2/3/4 has no buffer.