NAND CE Pin Reduction

• 在大容量NAND 芯片封装中，一个 NAND 芯片中可能封装很多个NAND Package，每个package中一般有2~8根CE# PIN。使用CE# Reduction机制可以让host的单个CE#被多个NAND pacakge复用,这样使得Host所需要的CE#引脚大大减少。

• In higher density capacity implementations there may be a significant number of CE# pins required for a host to support where there are many NAND packages with two to four CE# pins per package. The CE# pin reduction mechanism enables a single CE# pin from the host to be shared by multiple NAND targets, thus enabling a significant reduction in the number of CE# pins required by the host system.

• CE Reduction机制在初始化过程中，为每一个NAND target 指定一个Volume地址。初始化后，host可以通过volume选择命令(E1h)在选择指定的Volume。

• In the CE# pin reduction mechanism, each NAND target is appointed a volume address during the initialization sequence. After initialization is complete, the host may address a particular volume (NAND target) by using the VOLUME SELECT (E1h) command.

• 在物理连接上，通过ENo和ENI依次串联起所有的NAND Package。第一个Patargetckage的ENI不会连接，后面的Package依次连接到前一个Package的ENo信号。

• ENi and ENo pins are added to each NAND package and a daisy chain is created between NAND packages. The first NAND package in the chain has the ENi pin as not connected. All other NAND packages have their ENi pin connected to the previous package's ENo pin in a daisy chain configuration. 

Tips:这里要区分 NAND 中 Package，Target，LUN 的概念划分。可以参考下图

• 在Power on的时候，ENo会被driver low。之后如果CE#被置成High状态，ENo对应为High-Z状态。如果CE#置成Low并set成对应的volume地址后，此时的ENo会被置高。
• At power-on, the ENo pins are driven LOW by the NAND device. ENo shall be High-Z when all CE# signals on the NAND package are HIGH. When a NAND target has had a volume address appointed with the SET FEATURES (EFh) command, then the ENo pin shall be pulled HIGH by the NAND target when the corresponding CE# is LOW. This enables the next NAND target on a subsequent package in the daisy chain to accept commands because the ENi pin pulls HIGH when ENo is no longer pulling LOW. After a volume address has been appointed to a volume (for example, NAND target), that volume shall become deselected and ignores the ENi pin until the next power cycle.

• ENi状态决定了NAND package是否接受cmd。 如果CE# LOW and ENi Pin is High,那么就可以接受cmd；反之如果CE# high或者ENi low，则不能接受cmd。

• The state of ENi determines whether the NAND package is able to accept commands. If the ENi pin is HIGH and CE# is LOW for the NAND target, then the NAND target shall accept commands. If the ENi pin is LOW or CE# is HIGH for the NAND target, then the NAND target shall not accept commands.

• To be selected to process a command, the VOLUME SELECT command shall be issued to the host target using the volume address that was previously appointed for a particular NAND target. After the CE# signal is pulled HIGH for t CEH time, all LUNs on a volume revert to their previous states.

• A HARD RESET (FDh) command will not undo CE# Pin Reduction or any previously set volume addressing assignments that were performed prior to the HARD RESET (FDh) command.

initial sequence

1. host power on the NAND.

2. host pulls CE# LOW.

3. 如果resetting all NAND Parallel，host 发送Reset(FFh) command.此时Reset CMD会被所有的NAND Device接受到。

4. 如果reseeting all NAND sequence

   (1). 只有ENi pin high的NAND device能接受到命令。

5. host发送set feature cmd 来设定 Volume Configure(58h)。每个 NAND target 的Volume address 应该都是独立的。发送set features 命令后，ENo被设置成 High 同时 Volume is deselected，直到 host 发送 Volume select(E1h) 来选中指定的 Volume。

常见的CE Reduction 拓扑结构

1. CE# Pin Reduction Topology with single channel

1. CE# Pin Reduction and Volume Addressing Topology with dual channel

Volume Appointment without CE# Pin Reduction

If CE# pin reduction is not used (for example, if ENi and ENo are not connected) and the host desires to have the terminator on a package that does not share a CE# with the selected NAND target, then each package that shares the CE# must have a volume appointed at initialization using the SET FEATURE (EFh) command using the volume configuration feature.

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