本例演示用的软硬件：
- 片内外设驱动库：STM32CubeF41.24.1的HAL库1.7.6，2019年4月12日
- IDE：MDK-ARM 5.28.0.0，2019年5月
- 开发板：片外SRAM挂在FSMC_NORPSRAM3，16bit×2=1MiB
本例的目的是让编程人员使用片外SRAM就像使用片内SRAM一样，即不用把任何变量声明到指定的RAM地址、连接器也能自动地把片外SRAM作为变量的存储空间
如果把所有需要被放到片外SRAM的变量用__attribute__((at()))、指针等声明到片外SRAM，那么完全不用像本例这样
执行main()前执行片内Flash上初始化FSMC及其GPIO的指令：
- 有些指令由启动文件的汇编代码生成，例如对于本例的STM32F407ZG来说这个启动文件就是startup_stm32f407xx.s(默认版本位于STM32CubeF4\Drivers\CMSIS\Device\ST\STM32F4xx\Source\Templates\arm)
- 有些函数在system_stm32f4xx.c(位于STM32CubeF4\Drivers\CMSIS\Device\ST\STM32F4xx\Source\Templates\)
对于本例的STM32F407来说修改上述的2个源文件的方法是：
1.换用官方的使用片外SRAM作运行内存的例程的启动文件
例如STM32CubeF4\Projects\STM324xG_EVAL\Examples\FSMC\FSMC_SRAM_DataMemory\MDK-ARM\startup_stm32f407xx.s
可以看到适用于使用片外SRAM运存的启动文件相较于默认版本的启动文件变化的地方有
- 第52行、第75行：定义用于初始化片外SRAM的栈
- 第191行~第192行：在执行main()前、执行完启动文件定义的指令后将栈顶指针恢复为默认值
__initial_spTop EQU 0x20000400 ; stack used for SystemInit & SystemInit_ExtMemCtl
__Vectors DCD __initial_spTop ; Top of Stack
__Vectors_End
__Vectors_Size EQU __Vectors_End - __Vectors
AREA |.text|, CODE, READONLY
; Reset handler
Reset_Handler PROC
EXPORT Reset_Handler [WEAK]
IMPORT SystemInit
IMPORT __main
LDR R0, =SystemInit
BLX R0
LDR R0, =__initial_sp ; restore original stack pointer
MSR MSP, R0
LDR R0, =__main
BX R0
ENDP2.取消注释system_stm32f4xx.c的第96行
/************************* Miscellaneous Configuration ************************/
/*!< Uncomment the following line if you need to use external SRAM or SDRAM as data memory */
#if defined(STM32F405xx) || defined(STM32F415xx) || defined(STM32F407xx) || defined(STM32F417xx)\
|| defined(STM32F427xx) || defined(STM32F437xx) || defined(STM32F429xx) || defined(STM32F439xx)\
|| defined(STM32F469xx) || defined(STM32F479xx) || defined(STM32F412Zx) || defined(STM32F412Vx)
/* #define DATA_IN_ExtSRAM */
#endif /* STM32F40xxx || STM32F41xxx || STM32F42xxx || STM32F43xxx || STM32F469xx || STM32F479xx ||\
STM32F412Zx || STM32F412Vx */
那么system_stm32f4xx.c的
- 第662行~第716行
- 开启FSMC模块用到的GPIO的时钟(第662行)
- 配置相应GPIO口的模式、速度等信息(第667行~第712行)
- 开启FSMC模块的时钟(第716行))
- 第737行~第741行
- 配置FSMC_NORPSRAM的控制、时序(第739行~第741行)
都会被执行
#if defined(STM32F405xx) || defined(STM32F415xx) || defined(STM32F407xx) || defined(STM32F417xx)\
|| defined(STM32F427xx) || defined(STM32F437xx) || defined(STM32F429xx) || defined(STM32F439xx)\
|| defined(STM32F469xx) || defined(STM32F479xx) || defined(STM32F412Zx) || defined(STM32F412Vx) #if defined(DATA_IN_ExtSRAM)
/*-- GPIOs Configuration -----------------------------------------------------*/
/* Enable GPIOD, GPIOE, GPIOF and GPIOG interface clock */
RCC->AHB1ENR |= 0x00000078;
/* Delay after an RCC peripheral clock enabling */
tmp = READ_BIT(RCC->AHB1ENR, RCC_AHB1ENR_GPIODEN); /* Connect PDx pins to FMC Alternate function */
GPIOD->AFR[] = 0x00CCC0CC;
GPIOD->AFR[] = 0xCCCCCCCC;
/* Configure PDx pins in Alternate function mode */
GPIOD->MODER = 0xAAAA0A8A;
/* Configure PDx pins speed to 100 MHz */
GPIOD->OSPEEDR = 0xFFFF0FCF;
/* Configure PDx pins Output type to push-pull */
GPIOD->OTYPER = 0x00000000;
/* No pull-up, pull-down for PDx pins */
GPIOD->PUPDR = 0x00000000; /* Connect PEx pins to FMC Alternate function */
GPIOE->AFR[] = 0xC00CC0CC;
GPIOE->AFR[] = 0xCCCCCCCC;
/* Configure PEx pins in Alternate function mode */
GPIOE->MODER = 0xAAAA828A;
/* Configure PEx pins speed to 100 MHz */
GPIOE->OSPEEDR = 0xFFFFC3CF;
/* Configure PEx pins Output type to push-pull */
GPIOE->OTYPER = 0x00000000;
/* No pull-up, pull-down for PEx pins */
GPIOE->PUPDR = 0x00000000; /* Connect PFx pins to FMC Alternate function */
GPIOF->AFR[] = 0x00CCCCCC;
GPIOF->AFR[] = 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;
/* No pull-up, pull-down for PFx pins */
GPIOF->PUPDR = 0x00000000; /* Connect PGx pins to FMC Alternate function */
GPIOG->AFR[] = 0x00CCCCCC;
GPIOG->AFR[] = 0x000000C0;
/* Configure PGx pins in Alternate function mode */
GPIOG->MODER = 0x00085AAA;
/* Configure PGx pins speed to 100 MHz */
GPIOG->OSPEEDR = 0x000CAFFF;
/* Configure PGx pins Output type to push-pull */
GPIOG->OTYPER = 0x00000000;
/* No pull-up, pull-down for PGx pins */
GPIOG->PUPDR = 0x00000000; /*-- FMC/FSMC Configuration --------------------------------------------------*/
/* Enable the FMC/FSMC interface clock */
RCC->AHB3ENR |= 0x00000001;
#if defined(STM32F405xx) || defined(STM32F415xx) || defined(STM32F407xx)|| defined(STM32F417xx)\
|| defined(STM32F412Zx) || defined(STM32F412Vx)
/* Delay after an RCC peripheral clock enabling */
tmp = READ_BIT(RCC->AHB3ENR, RCC_AHB3ENR_FSMCEN);
/* Configure and enable Bank1_SRAM2 */
FSMC_Bank1->BTCR[] = 0x00001011;
FSMC_Bank1->BTCR[] = 0x00000201;
FSMC_Bank1E->BWTR[] = 0x0FFFFFFF;
从system_stm32f4xx.c的第738行的注释可知，第739行~第741行分别配置的是FSMC_NORPSRAM2的控制寄存器、读写时序寄存器、写时序寄存器，而我使用的开发板的片外SRAM挂在FSMC_NORPSRAM3，所以需要修改system_stm32f4xx.c上述的写GPIO寄存器、FSMC寄存器的代码，获取正确的寄存器值的方法是核外片内外设只开启FSMC_NORPSRAM3及其GPIO，再在硬件调试过程中复制出相应的寄存器值
3.更正system_stm32f4xx.c的上述代码
(再次提醒没仔细读题的读者：下文的代码只被保证适用于( (STM32F407ZG) && (片外SRAM挂在FSMC_NORPSRAM3) && (片外SRAM是16bit×219=1MiB) )的情况，且应该根据你用的SRAM芯片、STM32的AHB总线时钟频率等信息修改第739行~第741行配置FSMC寄存器用的值。用上一段提到的方法获取适用于你的开发板的寄存器值，用本文初提到的方法获取适用于你的单片机的启动文件)：
#if defined(STM32F405xx) || defined(STM32F415xx) || defined(STM32F407xx) || defined(STM32F417xx)\
|| defined(STM32F427xx) || defined(STM32F437xx) || defined(STM32F429xx) || defined(STM32F439xx)\
|| defined(STM32F469xx) || defined(STM32F479xx) || defined(STM32F412Zx) || defined(STM32F412Vx) #if defined(DATA_IN_ExtSRAM)
/*-- GPIOs Configuration -----------------------------------------------------*/
/* Enable GPIOD, GPIOE, GPIOF and GPIOG interface clock */
RCC->AHB1ENR |= 0x00000078;
/* Delay after an RCC peripheral clock enabling */
tmp = READ_BIT(RCC->AHB1ENR, RCC_AHB1ENR_GPIODEN); /* Connect PDx pins to FMC Alternate function */
GPIOD->AFR[] = 0x00CC00CC;
GPIOD->AFR[] = 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;
/* No pull-up, pull-down for PDx pins */
GPIOD->PUPDR = 0x00000000; /* Connect PEx pins to FMC Alternate function */
GPIOE->AFR[] = 0xC00000CC;
GPIOE->AFR[] = 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;
/* No pull-up, pull-down for PEx pins */
GPIOE->PUPDR = 0x00000000; /* Connect PFx pins to FMC Alternate function */
GPIOF->AFR[] = 0x00CCCCCC;
GPIOF->AFR[] = 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;
/* No pull-up, pull-down for PFx pins */
GPIOF->PUPDR = 0x00000000; /* Connect PGx pins to FMC Alternate function */
GPIOG->AFR[] = 0x00CCCCCC;
GPIOG->AFR[] = 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;
/* No pull-up, pull-down for PGx pins */
GPIOG->PUPDR = 0x00000000; /*-- FMC/FSMC Configuration --------------------------------------------------*/
/* Enable the FMC/FSMC interface clock */
RCC->AHB3ENR |= 0x00000001;
#if defined(STM32F405xx) || defined(STM32F415xx) || defined(STM32F407xx)|| defined(STM32F417xx)\
|| defined(STM32F412Zx) || defined(STM32F412Vx)
/* Delay after an RCC peripheral clock enabling */
tmp = READ_BIT(RCC->AHB3ENR, RCC_AHB3ENR_FSMCEN);
/* Configure and enable Bank1_SRAM3 */
FSMC_Bank1->BTCR[4] = 0x00001091;
FSMC_Bank1->BTCR[5] = 0x00100222;
4.在MDK把FSMC_NORPSRAM3映射的地址范围0x68000000~0x6BFFFFFF的首1M设为运行内存
把0x68000000、0x100000分别填入下图窗口右下角的"Read/Write Memory Areas"的”off-chip“的任一行：
5.修改完后Reuild
(注意：如果你用MDK-ARM以外的软件修改了前述的2个源文件，那么MDK-ARM可能不知道你修改了那2个文件，所以如果仅Build，那么IDE可能会拿编译旧版本的源文件得到的目标文件进行连接
或者你手动让MDK-ARM知道你修改过那些文件再仅Build，比如在MDK-ARM打开前述的2个源文件，在里面随便找个地方加个字再删掉那个字了再Build)
从MAP文件可以看到，变量被分配到了片外SRAM映射的地址范围0x68000000~0x68100000中，且单片机程序能正常
Exec Addr Load Addr Size Type Attr Idx E Section Name Object 0x68000000 0x0800628c 0x00000008 Data RW .data main.o
0x68000008 0x08006294 0x00000008 Data RW .data stm32f4xx_hal_msp.o
0x68000010 0x0800629c 0x0000000c Data RW .data stm32f4xx_hal.o
0x6800001c 0x080062a8 0x00000004 Data RW .data system_stm32f4xx.o
0x68000020 0x080062ac 0x00000004 Data RW .data tftlcd.o
0x68000024 - 0x00000050 Zero RW .bss main.o
0x68000074 - 0x0000000e Zero RW .bss tftlcd.o
0x68000082 0x080062b0 0x00000006 PAD
0x68000088 - 0x00000400 Zero RW STACK startup_stm32f407xx.o
希望CubeMX以后的版本能自动将片外RAM设为运存