emmc要怎么取代emmc（eMMC深入浅出第五章）

We are going to talk about Tuning today.我们今天来讲讲Tuning的问题这里仍然用英文单词Tuning，因为翻译之后很难找到表意准确的说法，我来为大家科普一下关于emmc要怎么取代emmc?以下内容希望对你有帮助!

emmc要怎么取代emmc

Section 2 Tuning第二节 Tuning

We are going to talk about Tuning today.

我们今天来讲讲Tuning的问题。这里仍然用英文单词Tuning，因为翻译之后很难找到表意准确的说法。

First question, why eMMC need tuning? Because the bus frequency is high to 200Mhz, and the clock and host read data direction are opposite.

第一个问题，eMMC为什么需要Tuning? 因为总线频率达到了200Mhz，而且时钟和主机读取数据是反方向的。

Second Question, who need this tuning? It’s Host, to be more specifically, it is Host receiver which need it for the optimizing the sampling point at HS200 (200Mhz SDR) mode read data. eMMC device do not need this.

第二个问题，谁需要Tuning？主机，更准确的说是主机的接收器需要在HS200（200Mhz 单沿采样）模式下读数据找到最安全的采样点。eMMC器件的接收器是不需要这个的。

Then comes the third question, why device do not need tuning? Two reasons, the bus frequency is not that high（only 200Mhz）, plus the clock and data input（host write data） are the same direction from host to device, the skew between input clock and data in are neglectable and tolerable from device point of view.

那么第三个问题来了，为什么eMMC器件不需要tuning？两个原因，总线频率还不是那么高（才200Mhz），再加上输入时钟和eMMC输入数据（主机写的数据）的方向是相同的，从eMMC器件来看，时钟和数据线之间的信号偏差是可以忽略和容忍的。

The last question, why HS200 need tuning, while HS400 don’t? That’s because in HS400 mode, a new hardware signal is introduced in eMMC 5.0 Specification, it is DS (Data Strobe, a.k.a. R-Clock which means reference clock or returned clock, my understanding more likely the former one). This DS is the same direction as eMMC device data output（Host read data), it ensures the Host receiver to get a reliable reference clock in the high HS400（200Mhz DDR) sampling for the data bus.

最后一个问题是为什么只有HS200需要tuning，而HS400不需要？那是因为eMMC 5.0规范在HS400模式引入了一根新的信号线，DS（data strobe数据选通，又叫R-Clock，可以理解成参考时钟或者返回时钟，我理解是参考时钟更合适）。这个DS的方向是和eMMC输出数据方向(主机读数据)相同，DS保证了主机接收器在数据信号总线工作在HS400（200Mhz 双沿采样）时采样有可靠的参考时钟。

But, the Command/Response line still work in HS200 mode when data lines are in HS400, which means no DS reference for the response on the Command/Response line for the Host receiver. This is why when switching to HS400, it must go through HS200 tuning process first, this tuning is actually for the Command/Response line Response signal only.

但是，Command/Response信号线仍然工作在HS200模式，这也就意味着在Command/Response线上没有DS参考时钟给到主机接收器。这就是为什么当切换到HS400模式之前需要先经过HS200的tuning，这个tuning实际上仅仅是给Command/Response线上的Response应答信号用的。

Unless, the host and device both support a new feature Enhanced Strobe, which is introduced in eMMC Specification 5.1, this feature allows the bus goes to HS400 without any tuning simply because host will use DS as the synchronization reference clock for both eMMC data out signal and eMMC response signal.

除非，主机和eMMC都支持eMMC 5.1规范新引入的特性，enhanced DS增强型的数据选通。这个新特性允许切换到HS400模式之前不需要任何tuning，原因很简单，因为主机会用DS信号同时作为Data数据信号和Response应答信号的参考时钟。

Beyond above，there are still some more concepts need to be noted.

除此之外，仍有一些需要注意一些概念。

First of all is the CMD21 and tuning block. Tuning could be implemented in different way, while CMD21 with the predefined tuning block is one of them, and this is recommended in the eMMC Specification,

最紧要的是CMD21和tuning block（tuning块）。Tuning实现的方式很多，CMD21和预先定义tuning块是其中一种，而且是eMMC规范推荐的。

however not use it is not violation of the tuning concept. For example, Host could use some redundant read operation and optimize the read sampling points by the read data CRC （Cyclic Redundancy Check）verification.

但是，不用这种方式并不违背tuning的概念。比如，主机可以用一些冗余的读操作，通过校验读数据的CRC（循环冗余校验）去优化采样点。

Then what’s the advantage of the CMD21 way? Because of the tuning block. The tuning block is combined by some pre-defined data pattern that is stored in the eMMC specific inaccessible area by any other Command except CMD21, total 128Bytes in 8-bit mode (64 Bytes if 4-bit mode).

那么用CMD21的方式有什么好处呢？因为Tuning块。Tuning块是由一些预先定义好的数据样式组成，它存储在eMMC的特定区域，除了CMD21之外任何其他指令都访问不到，总共是8-bit模式下128字节（如果是4-bit模式则是64字节）

This data pattern is designed to creates “special” Signal Integrity corner cases on the bus like high SSO (simultaneous switching output) noise, DJ (deterministic jitter), ISI (intersymbol interference) and timing errors.

这个数据样式设计出来目的是为了在总线上造出特殊的极端的数据完整性场景，比如High SSO noise（高同步切换输出噪声），DJ（确定性抖动），ISI（码间干扰）和时序错误。

This tuning method works normally like this, host repeatedly send out CMD21, and device send back this same 128Byte tuning block on the data bus every time. Since Host already knows what it is based on the eMMC specification, then it knows finally which sampling point has largest margin, normally it is the center point of a series of continuous successful good sampling points.

这种tuning方式是这么工作的，host不断发出CMD21指令，eMMC在数据总线上每次回复这个一模一样的128字节的tuning块。因为主机根据eMMC协议规范已经很清楚这个数据是什么，所以主机会知道最终选取哪个采样点会有最大的余量，这个采样点是一串成功采样的最中间那个点。

Two more thing before ending, one is that device should be guaranteed to complete a sequence of 40 times CMD21 executions within 150ms based on the eMMC specification. The other one is that tuning block is for data bus, while Command/Response signal line might need host use other way for tuning, such as checking the CMD21 response CRC7.

结束之前还要说两件事，一是根据eMMC规范，eMMC需要保证在150毫秒完成40次的CMD21的应答。另一个是tuning块是针对数据总线的，Command/Response信号线要用其他的方式做tuning，比如检查CMD21的response应答的7位CRC校验。

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