Nuttx当中有很多系统的组件都会用到内存中堆（heap）的资源，在终端上通过键入“free”指令可以查看到当前内存中活动的用户堆的实时用量，当应用在需要大量的堆开销的情况下，本身芯片内部集成的小容量内存就显得捉襟见肘了（我使用过MCU目前最大的内存容量也就是128KB+16KB+368KB =512KB）;估计有疑问说为什么是3块，再移植前，我们先来捋一捋一些基础知识。
日常中接触到的cortex-M4及其一下系列内核的单片机，一般在内存上大都是一块内存，我接触到最多的ST的单片机一般规格多为“32KB、64KB、256KB、320KB”等等，这里的内存就是我们所说的sram。例如下图： 但是在F7系列上确实这样内存变成了512KB+16KB： 这个是因为在F7系列上sram还分为了ITCM和DTCM（TCM=Tightly Coupled Memory，是一种高速缓存，据说是被直接集成在CPU芯片中。DS有两种TCM，分别是ITCM（Instruction TCM）和DTCM（Data TCM）。由于是高速缓存，所以这两块内存区域被当做特殊的用途。比如某些对时间要求非常严格的代码，就可以被放到ITCM中执行。这可以有效地提高运行速度。某些需要频繁存取的数据，也可以放到DTCM中以节省存取时间。）
既然板载的内存不够用，外扩的内存怎么加呢？这里就需要用到Flexible memory controller (FMC)，来实现外扩内存。首先需要确定外扩的SDRAM的规格和接口类型；
- 内部规格
SDRAM内部包含的存储阵列，可以把它理解成一张表格，数据就填在这张表格上。和表格查找一样，指定一个行地址和列地址，就可以精确地找到目标单元格，这是SDRAM芯片寻址的基本原理。这样的每个单元格被称为存储单元，而这样的表则被称为存储阵列(Bank)，目前设计的SDRAM芯片基本上内部都包含有4个这样的Bank，寻址时指定Bank号以及行地址，然后再指定列地址即可寻找到目标存储单元。所及行地址、列地址以及bank数基本就决定了我们SDRAM的大小了。
- 硬件接口 大概了解了SDRAM的内部结构，接下来看看常见的SDRAM的硬件接口：
CLK，同步时钟信号，所有输入信号都在CLK为上升沿的时候被采集；CKE，时钟使能信号，禁止时钟信号时SDRAM会启动自刷新操作；CS，片选信号，低电平有效；CAS#，列地址选通，为低电平时地址线表示的是列地址；RAS#，行地址选通，为低电平时地址线表示的是行地址；WE#，写入使能，低电平有效；DQM[0:1]，数据输入/输出掩码信号，表示DQ信号线的有效部分；BA[0:1]，Bank地址输入，选择要控制的Bank；A[0:11]，地址输入；DQ[0:15]，数据输入输出信号 ；（同种IC有16bits、32bits位宽等多种规格） 具体的时序可以参考对应使用的芯片使用手册；与一般的驱动不一样的是，Nuttx下外扩SDRAM直接就挂在地址总线上了，以本次移植为例，采用STM32F746系列的单片机；
- 外设驱动FMC 在配置的板子的src文件下比如我的板子是stm32f746g-disco，找到stm32_extmem.c文件<…/nuttx/configs/stm32f746g-disco/src>
/************************************************************************************ * configs/stm32f746g-disco/src/stm32_extmem.c * * Copyright (C) 2018 Marcin Wyrwas. All rights reserved. * Author: Marcin Wyrwas <mvp1@wp.pl> * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in * the documentation and/or other materials provided with the * distribution. * 3. Neither the name NuttX nor the names of its contributors may be * used to endorse or promote products derived from this software * without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE * POSSIBILITY OF SUCH DAMAGE. * ************************************************************************************/ /************************************************************************************ * Included Files ************************************************************************************/ #include <nuttx/config.h> #include <stdint.h> #include <assert.h> #include <debug.h> #include "chip.h" #include "up_arch.h" #include "stm32_fmc.h" #include "stm32_gpio.h" #include "stm32_rcc.h" #include "stm32f746g-disco.h" #include <arch/board/board.h> /************************************************************************************ * Pre-processor Definitions ************************************************************************************/ #ifndef CONFIG_STM32F7_FMC # warning "FMC is not enabled" #endif #if STM32F7_NGPIO < 7 # error "Required GPIO ports not enabled" #endif #define STM32_FMC_NADDRCONFIGS 22 #define STM32_FMC_NDATACONFIGS 16 #define STM32_SDRAM_CLKEN FMC_SDRAM_MODE_CMD_CLK_ENABLE | FMC_SDRAM_CMD_BANK_1 #define STM32_SDRAM_PALL FMC_SDRAM_MODE_CMD_PALL | FMC_SDRAM_CMD_BANK_1 #define STM32_SDRAM_REFRESH FMC_SDRAM_MODE_CMD_AUTO_REFRESH | FMC_SDRAM_CMD_BANK_1 |\ (7 << FMC_SDRAM_AUTO_REFRESH_SHIFT) #define STM32_SDRAM_MODEREG FMC_SDRAM_MODE_CMD_LOAD_MODE | FMC_SDRAM_CMD_BANK_1 |\ FMC_SDRAM_MODEREG_BURST_LENGTH_1 | \ FMC_SDRAM_MODEREG_BURST_TYPE_SEQUENTIAL |\ FMC_SDRAM_MODEREG_CAS_LATENCY_3 |\ FMC_SDRAM_MODEREG_WRITEBURST_MODE_SINGLE /************************************************************************************ * Public Data ************************************************************************************/ /* GPIO configurations common to most external memories */ **//注意1：这里需要根据硬件IO连接修改对应的IO口** static const uint32_t g_addressconfig[STM32_FMC_NADDRCONFIGS] = { GPIO_FMC_A0, GPIO_FMC_A1, GPIO_FMC_A2, GPIO_FMC_A3, GPIO_FMC_A4 , GPIO_FMC_A5, GPIO_FMC_A6, GPIO_FMC_A7, GPIO_FMC_A8, GPIO_FMC_A9, GPIO_FMC_A10, GPIO_FMC_A11, GPIO_FMC_SDCKE0_1, GPIO_FMC_SDNE0_3, GPIO_FMC_SDNWE_3, GPIO_FMC_NBL0, GPIO_FMC_SDNRAS, GPIO_FMC_NBL1, GPIO_FMC_BA0, GPIO_FMC_BA1, GPIO_FMC_SDCLK, GPIO_FMC_SDNCAS }; static const uint32_t g_dataconfig[STM32_FMC_NDATACONFIGS] = { GPIO_FMC_D0, GPIO_FMC_D1 , GPIO_FMC_D2, GPIO_FMC_D3, GPIO_FMC_D4 , GPIO_FMC_D5, GPIO_FMC_D6, GPIO_FMC_D7, GPIO_FMC_D8, GPIO_FMC_D9, GPIO_FMC_D10, GPIO_FMC_D11, GPIO_FMC_D12, GPIO_FMC_D13, GPIO_FMC_D14, GPIO_FMC_D15 }; /************************************************************************************ * Private Data ************************************************************************************/ /************************************************************************************ * Private Functions ************************************************************************************/ /************************************************************************************ * Public Functions ************************************************************************************/ /************************************************************************************ * Name: stm32_extmemgpios * * Description: * Initialize GPIOs for external memory usage * ************************************************************************************/ static void stm32_extmemgpios(const uint32_t *gpios, int ngpios) { int i; /* Configure GPIOs */ for (i = 0; i < ngpios; i++) { stm32_configgpio(gpios[i]); } } /************************************************************************************ * Name: stm32_sdramcommand * * Description: * Initialize data line GPIOs for external memory access * ************************************************************************************/ static void stm32_sdramcommand(uint32_t command) { uint32_t regval; volatile uint32_t timeout = 0xFFFF; regval = getreg32( STM32_FMC_SDSR ) & 0x00000020; while ((regval != 0) && timeout-- > 0) { regval = getreg32( STM32_FMC_SDSR ) & 0x00000020; } putreg32(command, STM32_FMC_SDCMR); timeout = 0xFFFF; regval = getreg32( STM32_FMC_SDSR ) & 0x00000020; while ((regval != 0) && timeout-- > 0) { regval = getreg32( STM32_FMC_SDSR ) & 0x00000020; } } /************************************************************************************ * Name: stm32_enablefmc * * Description: * enable clocking to the FMC module * ************************************************************************************/ void stm32_enablefmc(void) { uint32_t regval; volatile int count; /* Enable GPIOs as FMC / memory pins */ stm32_extmemgpios(g_addressconfig, STM32_FMC_NADDRCONFIGS); stm32_extmemgpios(g_dataconfig, STM32_FMC_NDATACONFIGS); /* Enable AHB clocking to the FMC */ regval = getreg32( STM32_RCC_AHB3ENR); regval |= RCC_AHB3ENR_FMCEN; putreg32(regval, STM32_RCC_AHB3ENR); /* Configure and enable the SDRAM bank1 * * FMC clock = 216MHz/2 = 108MHz * 108MHz = 9,26 ns * All timings from the datasheet for Speedgrade -6A (=6ns) */ putreg32(FMC_SDRAM_CR_RPIPE_0 | FMC_SDRAM_CR_BURST_READ | FMC_SDRAM_CR_SDCLK_2X | FMC_SDRAM_CR_CASLAT_3 | FMC_SDRAM_CR_BANKS_4 | //根据所使用的IC驱动BANK数量一般为2或4 FMC_SDRAM_CR_WIDTH_16 | //设置数据位宽，硬件连了16根数据线就16，连了32根就是32 FMC_SDRAM_CR_ROWBITS_12 | //行地址位宽（参看IC手册确定） FMC_SDRAM_CR_COLBITS_8, //列地址位宽（参看IC手册确定） STM32_FMC_SDCR1); putreg32((1 << FMC_SDRAM_TR_TRCD_SHIFT) | /* tRCD min = 18ns */ (1 << FMC_SDRAM_TR_TRP_SHIFT) | /* tRP min = 18ns */ (1 << FMC_SDRAM_TR_TWR_SHIFT) | /* tWR = 2CLK */ (6 << FMC_SDRAM_TR_TRC_SHIFT) | /* tRC min = 64ns */ (4 << FMC_SDRAM_TR_TRAS_SHIFT) | /* tRAS min = 46ns */ (7 << FMC_SDRAM_TR_TXSR_SHIFT) | /* tXSR min = 74ns */ (1 << FMC_SDRAM_TR_TMRD_SHIFT), /* tMRD = 2CLK */ STM32_FMC_SDTR1); /* SDRAM Initialization sequence */ stm32_sdramcommand(STM32_SDRAM_CLKEN); /* Clock enable command */ for (count = 0; count < 10000; count++) ; /* Delay */ stm32_sdramcommand(STM32_SDRAM_PALL); /* Precharge ALL command */ stm32_sdramcommand(STM32_SDRAM_REFRESH); /* Auto refresh command */ stm32_sdramcommand(STM32_SDRAM_MODEREG); /* Mode Register program */ /* Set refresh count * * FMC_CLK = 108MHz * Refresh_Rate = 64ms / 4096 rows = 15.63us * Counter = (FMC_CLK * Refresh_Rate) - 20 */ putreg32(1668 << 1, STM32_FMC_SDRTR); } /************************************************************************************ * Name: stm32_disablefmc * * Description: * enable clocking to the FMC module * ************************************************************************************/ void stm32_disablefmc(void) { uint32_t regval; /* Disable AHB clocking to the FMC */ regval = getreg32(STM32_RCC_AHB3ENR); regval &= ~(uint32_t)RCC_AHB3ENR_FMCEN; putreg32(regval, STM32_RCC_AHB3ENR); }改好驱动了以后，我们需要把文件添加到makefile中，然后呢找到stm32_boot.c文件<…/nuttx/configs/stm32f746g-disco/src> 在如下位置调用初始化接口：
void stm32_boardinitialize(void) { #if defined(CONFIG_STM32F7_SPI1) || defined(CONFIG_STM32F7_SPI2) || \ defined(CONFIG_STM32F7_SPI3) || defined(CONFIG_STM32F7_SPI4) || \ defined(CONFIG_STM32F7_SPI5) /* Configure SPI chip selects if 1) SPI is not disabled, and 2) the weak function * stm32_spidev_initialize() has been brought into the link. */ if (stm32_spidev_initialize) { stm32_spidev_initialize(); } #endif #ifdef CONFIG_SPORADIC_INSTRUMENTATION /* This configuration has been used for evaluating the NuttX sporadic scheduler. * The following caqll initializes the sporadic scheduler monitor. */ arch_sporadic_initialize(); #endif #ifdef CONFIG_ARCH_LEDS /* Configure on-board LEDs if LED support has been selected. */ board_autoled_initialize(); #endif **//在这里添加我们的硬件驱动初始化代码，注意记得在stm32f746g-disco.h头文件里面去声明函数，不然编译会有警告。** #ifdef CONFIG_STM32F7_FMC /* Initialize sdram via fmc.*/ stm32_enablefmc(); #endif } .config配置外设驱动好了以后，接下来就是配置编译选项：
通过make menuconfig指令打开配置页面System Type > STM32 Peripheral Support [] FMC Memory Management [] Second user heap (0xC0000000) Start address of second user heap region // 这个地址要看单片机划分的区域来定，哪段能用 (16777216) Size of the second user heap region //实际是多大内存就填写多大内存 保存
编写flash.ld 文件的路径<…/nuttx/configs/stm32f746g-disco/scripts>，添加最后一行代码，确保地址和大小一致； MEMORY { itcm (rwx) : ORIGIN = 0x00200000, LENGTH = 2048K flash (rx) : ORIGIN = 0x08000000, LENGTH = 2048K dtcm (rwx) : ORIGIN = 0x20000000, LENGTH = 128K sram1 (rwx) : ORIGIN = 0x20020000, LENGTH = 368K sram2 (rwx) : ORIGIN = 0x2007c000, LENGTH = 16K sdram (rwx) : ORIGIN = 0xC0000000, LENGTH = 16384K } 保存编译 一切就绪，编译内核代码并烧录到目标板中，然后在终端中键入free： 未加SDRAM： nsh> free total used free largest Umem: 358592 51216 307376 208944加入SDRAM后：
nsh> free total used free largest Umem: 17004736 75248 16929488 16777200