uart可发不可收

问题如下：使用pc端串口调试助手调试am335x的uart4，am335x向外发送数据，PC端可接收到，但是通过PC端调试助手发送给am335x，am335x一直读取数据失败。而且串口调试助手本身会直接收到已发送的数据。

测试代码如下：

#include <stdio.h>
#include     <stdlib.h>     /*标准函数库定义*/
#include     <unistd.h>     /*Unix标准函数定义*/
#include     <sys/types.h>  /**/
#include     <sys/stat.h>   /**/
#include     <fcntl.h>      /*文件控制定义*/
#include     <termios.h>    /*PPSIX终端控制定义*/
#include     <errno.h>      /*错误号定义*/

int fd = -1;


int open_port(char port[])
{
	fd = open( port, O_RDWR | O_NONBLOCK ); 

	if (fd<0) {
	    printf("open serialport failed!\n");
	    return -1;
	}

	struct termios tio;

	tcgetattr(fd, &tio);
	//bzero(&tio, sizeof(tio));
	// these must be set first, don't know why
	tio.c_cflag |= CLOCAL;
	tio.c_cflag |= CREAD;
	tio.c_cflag &= ~CSIZE;
	// baud rate 115200
	if (cfsetispeed(&tio, B115200) != 0)    {
	    printf("cfsetispeed failed\n");
	    return -1;
	}
	if (cfsetospeed(&tio, B115200) != 0)
	{
	    printf("cfsetospeed failed\n");
	    return -1;
	}
	// parity NONE
	tio.c_cflag &= ~PARENB;
	tio.c_iflag &= ~INPCK;
	tio.c_iflag |= IGNBRK;
	// data bit 8
	tio.c_cflag |= CS8;
	// stop bit 1
	tio.c_cflag &= ~CSTOPB;
	// others
	tio.c_cc[VTIME] = 0;
	tio.c_cc[VMIN]  = 0;
	// flush settings
	if(tcflush(fd, TCIOFLUSH) != 0) 
	{
	    printf("open serialport: Flushing %s ERROR!\n\n", port);
	    return -1;
	}

	if(tcsetattr(fd, TCSANOW, &tio) != 0)
	{
	    printf("open serialport: Setting %s ERROR!\n\n", port);
	    return -1;
	}
	return 0;
}


int main(int argc, char* argv[])
{
	char port_num[10] = {0,0,0,0,0,0,0,0,0,0};
	char wr_num[2] = {1,2};
	int ret;

	if(argc != 2)
	{
		printf("%s SerialPortDevFile\n", argv[0]);
		return -1;
	}
	if(open_port(argv[1]) != 0)
	{
		printf("Fail to open %s!\n", argv[1]);
		return -2;
	}
	else 
	{
		while(1)
		{
			printf( "start read %s\n", argv[1] );
			ret = read( fd, (void *)port_num, 9 );
			if( ret == -1 )
			printf( "ret = -1\n" );
			else
			{
				printf("ret = %d\n", ret);
				printf( "%s received : %s\n", argv[1], port_num );
			}
			printf( "start write %s\n", argv[1] );
			ret = write( fd, (void *)wr_num, 2 );
			sleep(5);
		}

		close_com();

		return 0;
	}
}

很简单的程序，只是循环接收发送。dts中配置uart4如下：

&uart4 {
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&uart4_pins_default>;
	pinctrl-1 = <&uart4_pins_sleep>;

	status = "okay";
};

uart4_pins_default: pinmux_uart4_pins_default {
		pinctrl-single,pins = <
			0x168 (PIN_INPUT_PULLUP | MUX_MODE1)	/* uart0_ctsn.uart4_rxd */ 
			0x16C (PIN_OUTPUT_PULLDOWN | MUX_MODE1) 	/* uart0_rtsn.uart4_txd */
		>;
	};

	uart4_pins_sleep: pinmux_uart4_pins_sleep {
		pinctrl-single,pins = <
			0x168 (PIN_INPUT_PULLDOWN | MUX_MODE7)
			0x16C (PIN_INPUT_PULLDOWN | MUX_MODE7)
		>;
	};

硬件上，uart4的收发信号线 直接 接到 PC端 usb转串口（232）线，信号线上没有连接任何上下拉等。