FM25Q128上电初始化流程如下图所示，要使能芯片的QPI模式，需要使能状态寄存器Status Register－2中的bit1位QE，然后使用0x38命令使能进入QPI模式。退出QPI模式可以发送0xFF命令或者发送软件复位序列0x66 ＋ 0x99，如下图5所示。
图5 FM25Q125上电初始化流程
如上所述，初次上电后，Flash芯片处于单线SPI模式，WP＃和HOLD＃管脚可能影响状态寄存器的写入操作，实际调试底层驱动代码的过程中，笔者也发现了这个问题，由于HOLD＃没有外接上拉电阻，在SPI模式下Flash芯片的HOLD＃脚电平不稳，导致读写状态寄存器时而正常，时而失败，开发板正常，客户的板子失败等现象，调试了很长时间才发现问题根源在于此。我们可以将Apollo3芯片的MSPI接口配置成SPI模式，并将连接到FM25Q128的DQ2（WP＃）和DQ3（HOLD＃／RESET＃）的管脚GP4和GP23配置成GPIO输出，使能内部上拉电阻，并且同时输出高电平，让WP＃和HOLD＃管脚处于非激活状态。
我们先通过阅读Datasheet来了解一下Apollo3的GPIO口结构，以及内部上拉电阻的配置。
Apollo3的所有的GPIO都可以通过软件来配置（PADnPULL）使能内部上拉电阻（除了GPIO20为下拉），上拉电阻默认状态是禁用的。另外，包含有I2C功能的GPIO的内部上拉电阻是可以选择四种不同阻值的。我们用到的GP4和GP23不包含I2C功能，因此不能选择上拉电阻的阻值。经过与AmbiqMicro原厂工程师沟通，了解到内部通用上拉电阻的阻值大约为63KΩ，这个内部的弱上拉足可以保证在SPI模式初始化SPI Flash时，WP＃和HOLD＃／RESET＃管脚稳定为高电平状态。参考代码如下：
＃define AM＿BSP＿GPIO＿GP23 23
＃define AM＿BSP＿GPIO＿GP4 4
const am＿hal＿gpio＿pincfg＿t g＿AM＿BSP＿GPIO＿GP23＿HOLD ＝
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．uFuncSel ＝ AM＿HAL＿PIN＿23＿GPIO，
．eDriveStrength ＝ AM＿HAL＿GPIO＿PIN＿DRIVESTRENGTH＿8MA，
．ePullup ＝ AM＿HAL＿GPIO＿PIN＿PULLUP＿WEAK，
．eGPOutcfg ＝ AM＿HAL＿GPIO＿PIN＿OUTCFG＿PUSHPULL，
．eGPInput ＝ AM＿HAL＿GPIO＿PIN＿INPUT＿NONE
｝；
const am＿hal＿gpio＿pincfg＿t g＿AM＿BSP＿GPIO＿GP4＿WP ＝
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．uFuncSel ＝ AM＿HAL＿PIN＿4＿GPIO，
．eDriveStrength ＝ AM＿HAL＿GPIO＿PIN＿DRIVESTRENGTH＿8MA，
．ePullup ＝ AM＿HAL＿GPIO＿PIN＿PULLUP＿WEAK，
．eGPOutcfg ＝ AM＿HAL＿GPIO＿PIN＿OUTCFG＿PUSHPULL，
．eGPInput ＝ AM＿HAL＿GPIO＿PIN＿INPUT＿NONE
｝；
static void ConfigGP23AsGpioOutputForPullHighHoldPin （void）
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／／
／／ Configure the pin as a push－pull GPIO output．
／／
am＿hal＿gpio＿pinconfig（AM＿BSP＿GPIO＿GP23，g＿AM＿BSP＿GPIO＿GP23＿HOLD）；
／／
／／ Disable the output driver， and set the output value to the high level
／／ state． Note that for Apollo3 GPIOs in push－pull mode， the output
／／ enable， normally a tri－state control， instead functions as an enable
／／ for Fast GPIO． Its state does not matter on previous chips， so for
／／ normal GPIO usage on Apollo3， it must be disabled．
／／
am＿hal＿gpio＿state＿write（AM＿BSP＿GPIO＿GP23，AM＿HAL＿GPIO＿OUTPUT＿TRISTATE＿DISABLE）；
am＿hal＿gpio＿state＿write（AM＿BSP＿GPIO＿GP23，AM＿HAL＿GPIO＿OUTPUT＿SET）； ／／ pull high
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static void ConfigGP4AsGpioOutputForPullHighWPPin （void）
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／／
／／ Configure the pin as a push－pull GPIO output．
／／
am＿hal＿gpio＿pinconfig（AM＿BSP＿GPIO＿GP4，g＿AM＿BSP＿GPIO＿GP4＿WP）；
／／
／／ Disable the output driver， and set the output value to the high level
／／ state． Note that for Apollo3 GPIOs in push－pull mode， the output
／／ enable， normally a tri－state control， instead functions as an enable
／／ for Fast GPIO． Its state does not matter on previous chips， so for
／／ normal GPIO usage on Apollo3， it must be disabled．
／／
am＿hal＿gpio＿state＿write（AM＿BSP＿GPIO＿GP4，AM＿HAL＿GPIO＿OUTPUT＿TRISTATE＿DISABLE）；
am＿hal＿gpio＿state＿write（AM＿BSP＿GPIO＿GP4，AM＿HAL＿GPIO＿OUTPUT＿SET）； ／／ pull high
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代码详见SDK v2．0目录下的示例工程：mspi＿quad＿example，
文件am＿devices＿mspi＿flash．c。
5、在SDK v2．0示例代码mspi＿quad＿example的基础上添加FM25Q128的支持
am＿devices＿mspi＿flash．h文件中增加器件型号宏定义：＃define FUDAN＿FM25Q128
以及器件支持的相关操作命令，ID码以及容量宏定义
＃if defined （FUDAN＿FM25Q128）
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
／／
／／ Device specific identification．
／／
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
＃define AM＿DEVICES＿MSPI＿FLASH＿ID 0x001840A1
＃define AM＿DEVICES＿MSPI＿FLASH＿ID＿MASK 0x00FFFFFF
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
／／
／／ Device specific definitions for flash commands
／／
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
＃defineAM＿DEVICES＿MSPI＿FLASH＿READ＿STATUS＿REGISTER1 0x05 ／／ RDSR1， read STATUS＿REGISTER 1
＃define AM＿DEVICES＿MSPI＿FLASH＿READ＿STATUS＿REGISTER2 0x35 ／／ RDSR2， read STATUS＿REGISTER 2
＃define AM＿DEVICES＿MSPI＿FLASH＿READ＿STATUS＿REGISTER3 0x15 ／／ RDSR3， read STATUS＿REGISTER 3
＃define AM＿DEVICES＿MSPI＿FLASH＿WRITE＿STATUS＿REGISTER1 0x01 ／／ WRSR1， write STATUS＿REGISTER 1
＃define AM＿DEVICES＿MSPI＿FLASH＿WRITE＿STATUS＿REGISTER2 0x31 ／／ WRSR2， write STATUS＿REGISTER 2
＃define AM＿DEVICES＿MSPI＿FLASH＿WRITE＿STATUS＿REGISTER3 0x11 ／／ WRSR3， write STATUS＿REGISTER 3
＃define AM＿DEVICES＿MSPI＿FLASH＿ENABLE＿QPI＿MODE 0x38 ／／ EQIO， Enable QPI
＃define AM＿DEVICES＿MSPI＿FLASH＿DISABLE＿QPI＿MODE 0xFF ／／ Disable QPI
＃define AM＿DEVICES＿MSPI＿FLASH＿ENABLE＿RESET 0x66 ／／ Enable Reset
＃define AM＿DEVICES＿MSPI＿FLASH＿RESET 0x99 ／／ Reset
＃define AM＿DEVICES＿MSPI＿FLASH＿WRITE＿ENABLE＿VOLATILE 0x50 ／／ Write Enable for Volatile Status Register
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
／／
／／ Device specific definitions for the Configuration register（s）
／／
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
＃define AM＿DEVICES＿MSPI＿CLEAR＿STATUS＿REGISTER1 （0x00） ／／ the value to clear the status register－1
＃define
AM＿DEVICES＿MSPI＿ENABLE＿QUAD＿STATUS＿REGISTER2 （0xF2） ／／ LC ＝ 11b， dummy cycles ＝ 4 for quad enable， 0 for spi mdode
＃define AM＿DEVICES＿MSPI＿CLEAR＿STATUS＿REGISTER3 （0xFC） ／／ to clear the status register－3， the reserved bits will be bypassed
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
／／
／／ Device specific definitions for the flash size information
／／
／／＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊
＃define AM＿DEVICES＿MSPI＿FLASH＿PAGE＿SIZE 0x100
／／ 256 bytes， minimum program unit
＃define AM＿DEVICES＿MSPI＿FLASH＿SECTOR＿SIZE 0x1000
／／ 4K bytes
＃define AM＿DEVICES＿MSPI＿FLASH＿BLOCK＿SIZE 0x10000
／／ 64K bytes．
＃define AM＿DEVICES＿MSPI＿FLASH＿MAX＿SECTORS 4096
／／ Sectors within 3－byte address range． 4096 ＊ 4KB ＝ 16MB
＃define AM＿DEVICES＿MSPI＿FLASH＿MAX＿BLOCKS 256
／／ 256 ＊ 64KB ＝ 16MB
＃endif
上电后初始化先清除SR1和SR3中的相应的状态位和错误标志位。