如何进入U-BOOT的下载模式?

我已经设置好了SECURECRT可是打开后一片空白，没有像教材中出现的选项，请问应该怎样操作?

串口线连接上，拨码拨到电源一侧，上电

我这样做了，可是为什么没出现以下的内容呢，怎样才能出现下面内容
##### EmbedSky BIOS for SKY2440/TQ2440 #####
Press Space key to Download Mode !

#####    Boot for Nor Flash Main Menu   #####
#####     EmbedSky USB download mode     #####

[1] Download u-boot or STEPLDR.nb1 or other bootloader to Nand Flash
[2] Download Eboot (eboot.nb0) to Nand Flash
[3] Download Linux Kernel (zImage.bin) to Nand Flash
[5] Download CRAMFS image to Nand Flash
[6] Download YAFFS image (root.bin) to Nand Flash
[7] Download Program (uCOS-II or TQ2440_Test) to SDRAM and Run it
[8] Boot the system
[9] Format the Nand Flash
[0] Set the boot parameters
[a] Download User Program (eg: uCOS-II or TQ2440_Test)
[b] Download LOGO Picture (.bin) to Nand  Flash
[l] Set LCD Parameters
[n] Enter TFTP download mode menu
[o] Download u-boot to Nor Flash
[r] Reboot u-boot
[t] Test Linux Image (zImage)
[q] quit from menu
Enter your selection: 7