先在.s文件 里面添加FSMC_SRAM_Init()函数，实现外部SDRAM的初始化如下图：
然后在初始化的时候先调用
rt_system_heap_init((void *)HEAP_BEGIN, (void *)HEAP_END);
初始化内部SRAM，然后调用
rt_memheap_init(&system_heap,"sdram",(void *)0x68000000,0x100000);
将外部SDRAM挂载到heap_map里面。编译下载后，就可以用rt_malloc等接口了。
FSMC_SRAM_Init()如下：
void FSMC_SRAM_Init(void){/*-- GPIOs Configuration -----------------------------------------------------*//* Enable GPIOD, GPIOE, GPIOF and GPIOG interface clock */RCC->AHB1ENR |= 0x00000078;//PD0,1,4,5,8~15 100M AF OUT_PP UP//PE0,1,7~15 100M AF OUT_PP UP//PF0~5,12~15 100M AF OUT_PP UP//PG0~5,10 100M AF OUT_PP UP/* Connect PDx pins to FMC Alternate function */GPIOD->AFR[0] = 0x00CC00CC;GPIOD->AFR[1] = 0xCCCCCCCC;/* Configure PDx pins in Alternate function mode */GPIOD->MODER = 0xAAAA0A0A;/* Configure PDx pins speed to 100 MHz */GPIOD->OSPEEDR = 0xFFFF0F0F;/* Configure PDx pins Output type to push-pull */GPIOD->OTYPER = 0x00000000;/* pull-up for PDx pins */GPIOD->PUPDR = 0x55550505;/* Connect PEx pins to FMC Alternate function */GPIOE->AFR[0] = 0xC00000CC;GPIOE->AFR[1] = 0xCCCCCCCC;/* Configure PEx pins in Alternate function mode */GPIOE->MODER = 0xAAAA800A;/* Configure PEx pins speed to 100 MHz */GPIOE->OSPEEDR = 0xFFFFC00F;/* Configure PEx pins Output type to push-pull */GPIOE->OTYPER = 0x00000000;/* pull-up for PEx pins */GPIOE->PUPDR = 0x55554005;/* Connect PFx pins to FMC Alternate function */GPIOF->AFR[0] = 0x00CCCCCC;GPIOF->AFR[1] = 0xCCCC0000;/* Configure PFx pins in Alternate function mode */GPIOF->MODER = 0xAA000AAA;/* Configure PFx pins speed to 100 MHz */GPIOF->OSPEEDR = 0xFF000FFF;/* Configure PFx pins Output type to push-pull */GPIOF->OTYPER = 0x00000000;/* pull-up for PFx pins */GPIOF->PUPDR = 0x55000555;/* Connect PGx pins to FMC Alternate function */GPIOG->AFR[0] = 0x00CCCCCC;GPIOG->AFR[1] = 0x00000C00;/* Configure PGx pins in Alternate function mode */GPIOG->MODER = 0x00200AAA;/* Configure PGx pins speed to 100 MHz */GPIOG->OSPEEDR = 0x00300FFF;/* Configure PGx pins Output type to push-pull */GPIOG->OTYPER = 0x00000000;/* pull-up for PGx pins */GPIOG->PUPDR = 0x00100555;/*-- FMC/FSMC Configuration --------------------------------------------------*//* Enable the FMC/FSMC interface clock */RCC->AHB3ENR |= 0x00000001;//寄存器清零//bank1有NE1~4,每一个有一个BCR+TCR，所以总共八个寄存器。//这里我们使用NE3 ，也就对应BTCR[4],[5]。FSMC_Bank1->BTCR[4]=0X00000000;FSMC_Bank1->BTCR[5]=0X00000000;FSMC_Bank1E->BWTR[4]=0X00000000;//操作BCR寄存器 使用异步模式,模式A(读写共用一个时序寄存器)//BTCR[偶数]:BCR寄存器;BTCR[奇数]:BTR寄存器FSMC_Bank1->BTCR[4]|=1<<12;//存储器写使能FSMC_Bank1->BTCR[4]|=1<<4; //存储器数据宽度为16bit//操作BTR寄存器 （HCLK=168M, 1个HCLK=6nsFSMC_Bank1->BTCR[5]|=8<<8; //数据保持时间（DATAST）为9个HCLK 6*9=54nsFSMC_Bank1->BTCR[5]|=0<<4; //地址保持时间（ADDHLD）未用到FSMC_Bank1->BTCR[5]|=0<<0; //地址建立时间（ADDSET）为0个HCLK 0ns//闪存写时序寄存器FSMC_Bank1E->BWTR[4]=0x0FFFFFFF;//默认值//使能BANK1区域3FSMC_Bank1->BTCR[4]|=1<<0;}
问 外部内存分配SDRAM