几个月前，intel提供的开发资料少之又少，但随edison用户的逐步增多，intel在吐槽声中也增加了不少资料。阅读这些资料，有如醍醐灌顶，让我了解到edison真正的牛逼之处。这里介绍的edison mcu sdk的开发也是源自intel的开发文档：https://software.intel.com/en-us/node/557537
以下为我阅读文档后的理解与复述，可能有不正确之处，望回复告知，谢谢。
组成intel Edison组成主要为两部分，Atom的核心 和 quark的MCU
（不知道为何intel不说Quark，而只是称作MCU了，大家可以回帖揣测下）
这是一个双CPU，双系统的模块，Atom核心运行着yocto版的linux，而quark的MCU部分则运行着Viper RTOS，这是风河提供了一套系统，包含了线程调度、内存管理、中断调度等等功能。
MCU应用运行在Viper内核上，并独立控制着连接MCU的外围设备，我们使用GPIO读写传感器就是由MCU完成的，然后MCU再和Atom处理器进行通信，传回数据，用以完成更多功能。MCU还可以用于省电，他能促使Atom核心休眠，等到需要的时候再唤醒。
MCU驱动提供了IPC协议支持，使得可以下载应用到我们的设备上。
MCU子系统
MCU子系统基于486架构，对奔腾IA ISA兼容，集成了 I/O (IPC, I2C, GPIO, HSU, DMA)和SRAM。其主要特性如下：
- 在fully active D0 device power状态，CPU频率是100MHz，在S0ix “active idle”休眠状态，CPU会降频到38.4 MHz。
- SRAM: 192 KB SRAM（包括代码和数据）
- IPC: 这个很重要，MCU提供了控制子系统的IPC机制，MCU和核心的thread-based通信也通过此进行。
- I2C: I2C 8和I2C 9 in the system controller unit (SCU) cluster are assigned for MCU usage.
- UART: MCU能访问UART1、UART2，which may be used as a Linux* kernel console as well.
- GPIO: MCU能访问所有GPIO口，MCU和核心共用所有GPIO。
- PWM: MCU和核心共用所有PWM
- MCU 提供了微秒级 (1 µs)高精度延时API。详见：Auxiliary API部分。
- MCU提供了不同等级的追踪能力，用以调试程序。
- The MCU SDK是一套基于Eclipse的SDK，方便用户创建、编译、下载和调试MCU应用，并且他支持多系统。
图片是SDK包含的东西：
MCU应用
MCU应用运行在Viper上，它的二进制文件存储在linux RootFS中，在linux内核启动期间，会通过MCU driver将二进制文件下载到MCU上。MCU应用能独立控制外设、和atom核心通信，也能进入低功耗模式。通过intel提供的SDK就可以开发MCU应用。
核心端接口
以下列出了核心上暴露出的一些TTY通道和多个sysfs节点，这些接口都是用于核心和MCU间通信的。
TTY接口
- /dev/ttymcu0: 核心与MCU的传输通道。核心和MCU间能通过这个接口相互发送和接受数据。
- /dev/ttymcu1: 用于获取MCU的log信息
Sysfs接口
- /sys/devices/platform/intel_mcu/control: A write-only control node to load the MCU application. Currently this node is for internal use only.
- /sys/devices/platform/intel_mcu/fw_version: The version of the MCU SDK that is used to build MCU applications.
- /sys/devices/platform/intel_mcu/log_level: A read/write node to set and get the current MCU application log level. Supported input strings include fatal, error, warning, info, and debug.
MCU SDK
MCU SDK主要是一个Eclipse plugin，它包含一个预建工具，能用于编译、下载和调试MCU应用。同时当创建项目时，会使用到其中包含的模板源码。MCU SDK主要有以下几个特性：
- 创建MCU项目
- 建立MCU应用
- 下载MCU应用到目标设备
- 显示调试信息
以下是一些intel官方提到的注意事项，主要为edison硬件的局限性说明，不做翻译了：
- The maximum message size for interprocess communication between the CPU and the MCU is currently limited to 255 bytes.
- The current Viper OS release does not support unloading an MCU user application after it is loaded. The user must reboot the device to unload the downloaded binary.
- The MCU application update requires a Linux reboot.
- Maximum MCU binary size is limited to 120 KB currently.
- Due to the limitations of the current Viper OS release, the MCU OS tick value is 10 ms.
- You cannot install the SDK in any directory that has spaces in the path name. (For example, Program Files or My Documents is not allowed.)
- There is no floating point support in MCU. (You can build a floating point code using MCU, but it will generate runtime exceptions.)
- The MCU is limited to 2xI2C.
- The MCU application is limited to single-thread usage.
- You cannot boot the MCU without first booting Linux.
- There is no access coordination between the Intel Atom processor and the MCU. Both can configure the same I/O at once. This can potentially cause conflicts and must be user-managed.
- The MCU SDK (Eclipse) is different and separate from the C/C++ Eclipse SDK for the Intel Atom processor.
- The MCU I/O API is not aligned with the API for the Intel Atom processor. You must rewrite code to move from the Intel Atom processor to the MCU.
- The MCU does not have a watchdog timer.
- Currently, the SPI feature is not supported for the MCU.
- It may take a long time to launch the MCU SDK on Windows under some network configurations due to Eclipse or Cygwin limitations.
- Pin muxing (Arduino extension board configuration) must be done through the Linux interface before starting the MCU. For details, see Setting the Intel® Edison board up to test system on a chip (SoC) GPIOs.
- PWM runtime power management must be disabled from the Linux side in order to access this module. For details, seeControlling PWM ports.
- There are no programmable timer interrupts available on the MCU.
