EK-TM4C1294XL: I2C master only generates interrupts when loopback is active

Hello,

I've found an interesting behavior, and I'm wondering if anyone has an explanation for it.

I've been developing some I2C code on my EK-TM4C1294XL, and I have it mostly up and running. One weird behavior I noticed is that the I2C module doesn't generate any interrupts unless the loopback is enabled. When it's disabled, no interrupts are generated.

In both situations, I do not have the EK-TM4C1294XL connected to any external I2C devices. But I would still expect to get a start or stop interrupt, even if nothing is connected. Anybody know what's going on?

Here's some code to replicate the issue.

/**
 * main.c
 */
#include <stdint.h>
#include <stdbool.h>

#include "inc/hw_memmap.h"
#include "inc/hw_ints.h"
#include "driverlib/rom_map.h"
#include "driverlib/rom.h"
#include "driverlib/pin_map.h"
#include "driverlib/sysctl.h"
#include "driverlib/gpio.h"
#include "driverlib/i2c.h"
#include "driverlib/interrupt.h"


uint32_t g_ui32SysClock;
static bool bInterruptPending;

void I2C_Int_Handler(void)
{
    uint32_t ui32IntStatus;

    ui32IntStatus = MAP_I2CMasterIntStatusEx(I2C2_BASE, true);
    MAP_I2CMasterIntClearEx(I2C2_BASE, ui32IntStatus);

    bInterruptPending = false;
}


int main(void)
{
    g_ui32SysClock = MAP_SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ | SYSCTL_OSC_MAIN | SYSCTL_USE_PLL | SYSCTL_CFG_VCO_480), 120000000);
    MAP_SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOL);
    while(!MAP_SysCtlPeripheralReady(SYSCTL_PERIPH_GPIOL));
    MAP_SysCtlPeripheralEnable(SYSCTL_PERIPH_I2C2);
    while(!MAP_SysCtlPeripheralReady(SYSCTL_PERIPH_I2C2));

    MAP_GPIOPinConfigure(GPIO_PL0_I2C2SDA);
    MAP_GPIOPinConfigure(GPIO_PL1_I2C2SCL);

    MAP_GPIOPinTypeI2C(GPIO_PORTL_BASE, GPIO_PIN_0);
    MAP_GPIOPinTypeI2CSCL(GPIO_PORTL_BASE, GPIO_PIN_1);

    MAP_I2CMasterInitExpClk(I2C2_BASE, g_ui32SysClock, false);
    MAP_I2CMasterIntEnableEx(I2C2_BASE, 0xFFF);

    I2CIntRegister(I2C2_BASE, I2C_Int_Handler);
    MAP_IntEnable(INT_I2C2);

    //if I2CLoopbackEnable is removed, the program hangs on while(bInterruptPending)
    //if I2CLoopbackEnable is kept, the program exits normally
    I2CLoopbackEnable(I2C2_BASE);


    MAP_I2CMasterSlaveAddrSet(I2C2_BASE, 0x03, false);
    bInterruptPending = true;
    MAP_I2CMasterControl(I2C2_BASE, I2C_MASTER_CMD_QUICK_COMMAND);

     while(bInterruptPending);

	return 0;
}