对SDRAM进行读写测试按下KEY0，将发送测试数据，LED1\2将常量，放开KEY0，LED1\2闪烁，大约25秒后，LED1常量，读写成功。-Read and write tests on the SDRAM press KEY0, will send test data, LED1 \ 2 will be constant, release KEY0, LED1 \ 2 flash, about 25 seconds later, LED1 constant, read and write