TM4C123GXL QEI0 not working

Hi, I'm using TM4C123GXL evaluation board.

I tried to use QEI with Peripheral Drive Library, but only one QEI module seem to be working.

QEI1 is OK, when I connect encoder channel A and B to PC5 and PC6(QEI module 1), I can see encoder count increment.

However when I connect the same encoder signals to PD6 and PD7(QEI module 0), the encoder count just doesn't change very much(a random number +-1), any suggestion what could be the problem?

I think I'm using almost the same code for QEI 0 and QEI 1.

Thanks.

Here's the code:

	SysCtlClockSet(SYSCTL_SYSDIV_5|SYSCTL_USE_PLL|SYSCTL_OSC_MAIN|SYSCTL_XTAL_16MHZ);
	//--------------
	//     QEI
	//--------------

	SysCtlPeripheralEnable(SYSCTL_PERIPH_QEI0);
	SysCtlPeripheralEnable(SYSCTL_PERIPH_QEI1);

	SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOD);//QEI0
	SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOC);//QEI1

	//
	GPIOPinConfigure(GPIO_PD6_PHA0);
	GPIOPinConfigure(GPIO_PD7_PHB0);
	GPIOPinConfigure(GPIO_PC5_PHA1);
	GPIOPinConfigure(GPIO_PC6_PHB1);

	//
	GPIOPinTypeQEI(GPIO_PORTD_BASE, GPIO_PIN_6);
	GPIOPinTypeQEI(GPIO_PORTD_BASE, GPIO_PIN_7);
	GPIOPinTypeQEI(GPIO_PORTC_BASE, GPIO_PIN_5);
	GPIOPinTypeQEI(GPIO_PORTC_BASE, GPIO_PIN_6);

	//
	QEIConfigure(QEI0_BASE, (QEI_CONFIG_CAPTURE_A_B | QEI_CONFIG_NO_RESET | QEI_CONFIG_QUADRATURE | QEI_CONFIG_NO_SWAP), 25030);
	QEIConfigure(QEI1_BASE, (QEI_CONFIG_CAPTURE_A_B | QEI_CONFIG_NO_RESET | QEI_CONFIG_QUADRATURE | QEI_CONFIG_NO_SWAP), 25030);

	QEIEnable(QEI0_BASE);
	QEIEnable(QEI1_BASE);

	//--------------
	//     UART
	//--------------

	SysCtlPeripheralEnable(SYSCTL_PERIPH_UART0);
	SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);
	GPIOPinConfigure(GPIO_PA0_U0RX);
	GPIOPinConfigure(GPIO_PA1_U0TX);
	GPIOPinTypeUART(GPIO_PORTA_BASE, GPIO_PIN_0 | GPIO_PIN_1);
	UARTConfigSetExpClk(UART0_BASE, SysCtlClockGet(), 115200, (UART_CONFIG_WLEN_8 | UART_CONFIG_STOP_ONE | UART_CONFIG_PAR_NONE));
	UARTEnable(UART0_BASE);
	//--------------
	//  Loop
	//--------------

	uint32_t duty_cycle = 100;
	uint32_t pos1 = 0;
	uint32_t pos2 = 0;
	char buffer[20]={0};

	while(1)
	{
		SysCtlDelay(5000000);

		pos1 = QEIPositionGet(QEI0_BASE);
		pos2 = QEIPositionGet(QEI1_BASE);

		//the following is just to convert pos to char for UART output
		uint32_t n = pos1;

		int i = 0;

		bool isNeg = false;

		unsigned int n1 = isNeg ? -n : n;

		while(n1!=0)
		{
		    buffer[i++] = n1%10+'0';
		    n1=n1/10;
		}

		if(isNeg)
		    buffer[i++] = '-';

		buffer[i] = '\0';
		int t = 0;
		for(;t < i/2;t++)
		{
		    buffer[t] ^= buffer[i-t-1];
		    buffer[i-t-1] ^= buffer[t];
		    buffer[t] ^= buffer[i-t-1];
		}
		//
		i=0;
		while(buffer[i])
		{
			UARTCharPut(UART0_BASE, buffer[i]);
			i++;
		}

		UARTCharPut(UART0_BASE, ' ');
	}